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# `logging` — Logging facility for Python

This module defines functions and classes which implement a flexible error logging system for applications.

Logging is performed by calling methods on instances of the `Logger` class (hereafter called loggers). Each instance has a name, and they are conceptually arranged in a namespace hierarchy using dots (periods) as separators. For example, a logger named “scan” is the parent of loggers “scan.text”, “scan.html” and “scan.pdf”. Logger names can be anything you want, and indicate the area of an application in which a logged message originates.

Logged messages also have levels of importance associated with them. The default levels provided are `DEBUG`, `INFO`, `WARNING`, `ERROR` and `CRITICAL`. As a convenience, you indicate the importance of a logged message by calling an appropriate method of `Logger`. The methods are `debug()`, `info()`, `warning()`, `error()` and `critical()`, which mirror the default levels. You are not constrained to use these levels: you can specify your own and use a more general `Logger` method, `log()`, which takes an explicit level argument.

## Logging tutorial

The key benefit of having the logging API provided by a standard library module is that all Python modules can participate in logging, so your application log can include messages from third-party modules.

It is, of course, possible to log messages with different verbosity levels or to different destinations. Support for writing log messages to files, HTTP GET/POST locations, email via SMTP, generic sockets, or OS-specific logging mechanisms are all supported by the standard module. You can also create your own log destination class if you have special requirements not met by any of the built-in classes.

### Simple examples

Most applications are probably going to want to log to a file, so let’s start with that case. Using the `basicConfig()` function, we can set up the default handler so that debug messages are written to a file:

    import logging
    LOG_FILENAME = '/tmp/logging_example.out'
    logging.basicConfig(filename=LOG_FILENAME,level=logging.DEBUG,)

    logging.debug('This message should go to the log file')

And now if we open the file and look at what we have, we should find the log message:

    DEBUG:root:This message should go to the log file

If you run the script repeatedly, the additional log messages are appended to the file. To create a new file each time, you can pass a filemode argument to `basicConfig()` with a value of `'w'`. Rather than managing the file size yourself, though, it is simpler to use a `RotatingFileHandler`:

    import glob
    import logging
    import logging.handlers

    LOG_FILENAME = '/tmp/logging_rotatingfile_example.out'

    # Set up a specific logger with our desired output level
    my_logger = logging.getLogger('MyLogger')
    my_logger.setLevel(logging.DEBUG)

    # Add the log message handler to the logger
    handler = logging.handlers.RotatingFileHandler(
                  LOG_FILENAME, maxBytes=20, backupCount=5)

    my_logger.addHandler(handler)

    # Log some messages
    for i in range(20):
        my_logger.debug('i = %d' % i)

    # See what files are created
    logfiles = glob.glob('%s*' % LOG_FILENAME)

    for filename in logfiles:
        print(filename)

The result should be 6 separate files, each with part of the log history for the application:

    /tmp/logging_rotatingfile_example.out
    /tmp/logging_rotatingfile_example.out.1
    /tmp/logging_rotatingfile_example.out.2
    /tmp/logging_rotatingfile_example.out.3
    /tmp/logging_rotatingfile_example.out.4
    /tmp/logging_rotatingfile_example.out.5

The most current file is always `/tmp/logging_rotatingfile_example.out`, and each time it reaches the size limit it is renamed with the suffix `.1`. Each of the existing backup files is renamed to increment the suffix (`.1` becomes `.2`, etc.) and the `.5` file is erased.

Obviously this example sets the log length much much too small as an extreme example. You would want to set maxBytes to an appropriate value.

Another useful feature of the logging API is the ability to produce different messages at different log levels. This allows you to instrument your code with debug messages, for example, but turning the log level down so that those debug messages are not written for your production system. The default levels are `CRITICAL`, `ERROR`, `WARNING`, `INFO`, `DEBUG` and `NOTSET`.

The logger, handler, and log message call each specify a level. The log message is only emitted if the handler and logger are configured to emit messages of that level or lower. For example, if a message is `CRITICAL`, and the logger is set to `ERROR`, the message is emitted. If a message is a `WARNING`, and the logger is set to produce only `ERROR`s, the message is not emitted:

    import logging
    import sys

    LEVELS = {'debug': logging.DEBUG,
              'info': logging.INFO,
              'warning': logging.WARNING,
              'error': logging.ERROR,
              'critical': logging.CRITICAL}

    if len(sys.argv) > 1:
        level_name = sys.argv[1]
        level = LEVELS.get(level_name, logging.NOTSET)
        logging.basicConfig(level=level)

    logging.debug('This is a debug message')
    logging.info('This is an info message')
    logging.warning('This is a warning message')
    logging.error('This is an error message')
    logging.critical('This is a critical error message')

Run the script with an argument like ‘debug’ or ‘warning’ to see which messages show up at different levels:

    $ python logging_level_example.py debug
    DEBUG:root:This is a debug message
    INFO:root:This is an info message
    WARNING:root:This is a warning message
    ERROR:root:This is an error message
    CRITICAL:root:This is a critical error message

    $ python logging_level_example.py info
    INFO:root:This is an info message
    WARNING:root:This is a warning message
    ERROR:root:This is an error message
    CRITICAL:root:This is a critical error message

You will notice that these log messages all have `root` embedded in them. The logging module supports a hierarchy of loggers with different names. An easy way to tell where a specific log message comes from is to use a separate logger object for each of your modules. Each new logger “inherits” the configuration of its parent, and log messages sent to a logger include the name of that logger. Optionally, each logger can be configured differently, so that messages from different modules are handled in different ways. Let’s look at a simple example of how to log from different modules so it is easy to trace the source of the message:

    import logging

    logging.basicConfig(level=logging.WARNING)

    logger1 = logging.getLogger('package1.module1')
    logger2 = logging.getLogger('package2.module2')

    logger1.warning('This message comes from one module')
    logger2.warning('And this message comes from another module')

And the output:

    $ python logging_modules_example.py
    WARNING:package1.module1:This message comes from one module
    WARNING:package2.module2:And this message comes from another module

There are many more options for configuring logging, including different log message formatting options, having messages delivered to multiple destinations, and changing the configuration of a long-running application on the fly using a socket interface. All of these options are covered in depth in the library module documentation.

### Loggers

The logging library takes a modular approach and offers the several categories of components: loggers, handlers, filters, and formatters. Loggers expose the interface that application code directly uses. Handlers send the log records to the appropriate destination. Filters provide a finer grained facility for determining which log records to send on to a handler. Formatters specify the layout of the resultant log record.

`Logger` objects have a threefold job. First, they expose several methods to application code so that applications can log messages at runtime. Second, logger objects determine which log messages to act upon based upon severity (the default filtering facility) or filter objects. Third, logger objects pass along relevant log messages to all interested log handlers.

The most widely used methods on logger objects fall into two categories: configuration and message sending.

- `Logger.setLevel()` specifies the lowest-severity log message a logger will handle, where debug is the lowest built-in severity level and critical is the highest built-in severity. For example, if the severity level is info, the logger will handle only info, warning, error, and critical messages and will ignore debug messages.
- `Logger.addFilter()` and `Logger.removeFilter()` add and remove filter objects from the logger object. This tutorial does not address filters.

With the logger object configured, the following methods create log messages:

- `Logger.debug()`, `Logger.info()`, `Logger.warning()`, `Logger.error()`, and `Logger.critical()` all create log records with a message and a level that corresponds to their respective method names. The message is actually a format string, which may contain the standard string substitution syntax of `%s`, `%d`, `%f`, and so on. The rest of their arguments is a list of objects that correspond with the substitution fields in the message. With regard to `**kwargs`, the logging methods care only about a keyword of `exc_info` and use it to determine whether to log exception information.
- `Logger.exception()` creates a log message similar to `Logger.error()`. The difference is that `Logger.exception()` dumps a stack trace along with it. Call this method only from an exception handler.
- `Logger.log()` takes a log level as an explicit argument. This is a little more verbose for logging messages than using the log level convenience methods listed above, but this is how to log at custom log levels.

`getLogger()` returns a reference to a logger instance with the specified if it it is provided, or `root` if not. The names are period-separated hierarchical structures. Multiple calls to `getLogger()` with the same name will return a reference to the same logger object. Loggers that are further down in the hierarchical list are children of loggers higher up in the list. For example, given a logger with a name of `foo`, loggers with names of `foo.bar`, `foo.bar.baz`, and `foo.bam` are all children of `foo`. Child loggers propagate messages up to their parent loggers. Because of this, it is unnecessary to define and configure all the loggers an application uses. It is sufficient to configure a top-level logger and create child loggers as needed.

### Handlers

`Handler` objects are responsible for dispatching the appropriate log messages (based on the log messages’ severity) to the handler’s specified destination. Logger objects can add zero or more handler objects to themselves with an `addHandler()` method. As an example scenario, an application may want to send all log messages to a log file, all log messages of error or higher to stdout, and all messages of critical to an email address. This scenario requires three individual handlers where each handler is responsible for sending messages of a specific severity to a specific location.

The standard library includes quite a few handler types; this tutorial uses only `StreamHandler` and `FileHandler` in its examples.

There are very few methods in a handler for application developers to concern themselves with. The only handler methods that seem relevant for application developers who are using the built-in handler objects (that is, not creating custom handlers) are the following configuration methods:

- The `Handler.setLevel()` method, just as in logger objects, specifies the lowest severity that will be dispatched to the appropriate destination. Why are there two `setLevel()` methods? The level set in the logger determines which severity of messages it will pass to its handlers. The level set in each handler determines which messages that handler will send on. `setFormatter()` selects a Formatter object for this handler to use.
- `addFilter()` and `removeFilter()` respectively configure and deconfigure filter objects on handlers.

Application code should not directly instantiate and use handlers. Instead, the `Handler` class is a base class that defines the interface that all Handlers should have and establishes some default behavior that child classes can use (or override).

### Formatters

Formatter objects configure the final order, structure, and contents of the log message. Unlike the base `logging.Handler` class, application code may instantiate formatter classes, although you could likely subclass the formatter if your application needs special behavior. The constructor takes two optional arguments: a message format string and a date format string. If there is no message format string, the default is to use the raw message. If there is no date format string, the default date format is:

    %Y-%m-%d %H:%M:%S

with the milliseconds tacked on at the end.

The message format string uses `%(<dictionary key>)s` styled string substitution; the possible keys are documented in Formatter Objects.

The following message format string will log the time in a human-readable format, the severity of the message, and the contents of the message, in that order:

    "%(asctime)s - %(levelname)s - %(message)s"

### Configuring Logging

Programmers can configure logging either by creating loggers, handlers, and formatters explicitly in a main module with the configuration methods listed above (using Python code), or by creating a logging config file. The following code is an example of configuring a very simple logger, a console handler, and a simple formatter in a Python module:

    import logging

    # create logger
    logger = logging.getLogger("simple_example")
    logger.setLevel(logging.DEBUG)
    # create console handler and set level to debug
    ch = logging.StreamHandler()
    ch.setLevel(logging.DEBUG)
    # create formatter
    formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
    # add formatter to ch
    ch.setFormatter(formatter)
    # add ch to logger
    logger.addHandler(ch)

    # "application" code
    logger.debug("debug message")
    logger.info("info message")
    logger.warn("warn message")
    logger.error("error message")
    logger.critical("critical message")

Running this module from the command line produces the following output:

    $ python simple_logging_module.py
    2005-03-19 15:10:26,618 - simple_example - DEBUG - debug message
    2005-03-19 15:10:26,620 - simple_example - INFO - info message
    2005-03-19 15:10:26,695 - simple_example - WARNING - warn message
    2005-03-19 15:10:26,697 - simple_example - ERROR - error message
    2005-03-19 15:10:26,773 - simple_example - CRITICAL - critical message

The following Python module creates a logger, handler, and formatter nearly identical to those in the example listed above, with the only difference being the names of the objects:

    import logging
    import logging.config

    logging.config.fileConfig("logging.conf")

    # create logger
    logger = logging.getLogger("simpleExample")

    # "application" code
    logger.debug("debug message")
    logger.info("info message")
    logger.warn("warn message")
    logger.error("error message")
    logger.critical("critical message")

Here is the logging.conf file:

    [loggers]
    keys=root,simpleExample

    [handlers]
    keys=consoleHandler

    [formatters]
    keys=simpleFormatter

    [logger_root]
    level=DEBUG
    handlers=consoleHandler

    [logger_simpleExample]
    level=DEBUG
    handlers=consoleHandler
    qualname=simpleExample
    propagate=0

    [handler_consoleHandler]
    class=StreamHandler
    level=DEBUG
    formatter=simpleFormatter
    args=(sys.stdout,)

    [formatter_simpleFormatter]
    format=%(asctime)s - %(name)s - %(levelname)s - %(message)s
    datefmt=

The output is nearly identical to that of the non-config-file-based example:

    $ python simple_logging_config.py
    2005-03-19 15:38:55,977 - simpleExample - DEBUG - debug message
    2005-03-19 15:38:55,979 - simpleExample - INFO - info message
    2005-03-19 15:38:56,054 - simpleExample - WARNING - warn message
    2005-03-19 15:38:56,055 - simpleExample - ERROR - error message
    2005-03-19 15:38:56,130 - simpleExample - CRITICAL - critical message

You can see that the config file approach has a few advantages over the Python code approach, mainly separation of configuration and code and the ability of noncoders to easily modify the logging properties.

### Configuring Logging for a Library

When developing a library which uses logging, some consideration needs to be given to its configuration. If the using application does not use logging, and library code makes logging calls, then a one-off message “No handlers could be found for logger X.Y.Z” is printed to the console. This message is intended to catch mistakes in logging configuration, but will confuse an application developer who is not aware of logging by the library.

In addition to documenting how a library uses logging, a good way to configure library logging so that it does not cause a spurious message is to add a handler which does nothing. This avoids the message being printed, since a handler will be found: it just doesn’t produce any output. If the library user configures logging for application use, presumably that configuration will add some handlers, and if levels are suitably configured then logging calls made in library code will send output to those handlers, as normal.

A do-nothing handler can be simply defined as follows:

    import logging

    class NullHandler(logging.Handler):
        def emit(self, record):
            pass

An instance of this handler should be added to the top-level logger of the logging namespace used by the library. If all logging by a library foo is done using loggers with names matching “foo.x.y”, then the code:

    import logging

    h = NullHandler()
    logging.getLogger("foo").addHandler(h)

should have the desired effect. If an organisation produces a number of libraries, then the logger name specified can be “orgname.foo” rather than just “foo”.

New in version 3.1.

The `NullHandler` class was not present in previous versions, but is now included, so that it need not be defined in library code.

## Logging Levels

The numeric values of logging levels are given in the following table. These are primarily of interest if you want to define your own levels, and need them to have specific values relative to the predefined levels. If you define a level with the same numeric value, it overwrites the predefined value; the predefined name is lost.

Level | Numeric value
--- | ---
`CRITICAL` | 50
`ERROR` | 40
`WARNING` | 30
`INFO` | 20
`DEBUG` | 10
`NOTSET` | 0

Levels can also be associated with loggers, being set either by the developer or through loading a saved logging configuration. When a logging method is called on a logger, the logger compares its own level with the level associated with the method call. If the logger’s level is higher than the method call’s, no logging message is actually generated. This is the basic mechanism controlling the verbosity of logging output.

Logging messages are encoded as instances of the `LogRecord` class. When a logger decides to actually log an event, a `LogRecord` instance is created from the logging message.

Logging messages are subjected to a dispatch mechanism through the use of handlers, which are instances of subclasses of the `Handler` class. Handlers are responsible for ensuring that a logged message (in the form of a `LogRecord`) ends up in a particular location (or set of locations) which is useful for the target audience for that message (such as end users, support desk staff, system administrators, developers). Handlers are passed `LogRecord` instances intended for particular destinations. Each logger can have zero, one or more handlers associated with it (via the `addHandler()` method of `Logger`). In addition to any handlers directly associated with a logger, all handlers associated with all ancestors of the logger are called to dispatch the message.

Just as for loggers, handlers can have levels associated with them. A handler’s level acts as a filter in the same way as a logger’s level does. If a handler decides to actually dispatch an event, the `emit()` method is used to send the message to its destination. Most user-defined subclasses of `Handler` will need to override this `emit()`.

## Useful Handlers

In addition to the base `Handler` class, many useful subclasses are provided:

1. `StreamHandler` instances send error messages to streams (file-like objects).
2. `FileHandler` instances send error messages to disk files.

1. `BaseRotatingHandler` is the base class for handlers that rotate log files at a certain point. It is not meant to be instantiated directly. Instead, use `RotatingFileHandler` or `TimedRotatingFileHandler`.
2. `RotatingFileHandler` instances send error messages to disk files, with support for maximum log file sizes and log file rotation.
3. `TimedRotatingFileHandler` instances send error messages to disk files, rotating the log file at certain timed intervals.
4. `SocketHandler` instances send error messages to TCP/IP sockets.
5. `DatagramHandler` instances send error messages to UDP sockets.
6. `SMTPHandler` instances send error messages to a designated email address.
7. `SysLogHandler` instances send error messages to a Unix syslog daemon, possibly on a remote machine.
8. `NTEventLogHandler` instances send error messages to a Windows NT/2000/XP event log.
9. `MemoryHandler` instances send error messages to a buffer in memory, which is flushed whenever specific criteria are met.
10. `HTTPHandler` instances send error messages to an HTTP server using either `GET` or `POST` semantics.
11. `WatchedFileHandler` instances watch the file they are logging to. If the file changes, it is closed and reopened using the file name. This handler is only useful on Unix-like systems; Windows does not support the underlying mechanism used.

1. `NullHandler` instances do nothing with error messages. They are used by library developers who want to use logging, but want to avoid the “No handlers could be found for logger XXX” message which can be displayed if the library user has not configured logging. See Configuring Logging for a Library for more information.

New in version 3.1.

The `NullHandler` class was not present in previous versions.

The `NullHandler`, `StreamHandler` and `FileHandler` classes are defined in the core logging package. The other handlers are defined in a sub- module, `logging.handlers`. (There is also another sub-module, `logging.config`, for configuration functionality.)

Logged messages are formatted for presentation through instances of the `Formatter` class. They are initialized with a format string suitable for use with the % operator and a dictionary.

For formatting multiple messages in a batch, instances of `BufferingFormatter` can be used. In addition to the format string (which is applied to each message in the batch), there is provision for header and trailer format strings.

When filtering based on logger level and/or handler level is not enough, instances of `Filter` can be added to both `Logger` and `Handler` instances (through their `addFilter()` method). Before deciding to process a message further, both loggers and handlers consult all their filters for permission. If any filter returns a false value, the message is not processed further.

The basic `Filter` functionality allows filtering by specific logger name. If this feature is used, messages sent to the named logger and its children are allowed through the filter, and all others dropped.

## Module-Level Functions

In addition to the classes described above, there are a number of module- level functions.

`logging.``getLogger`([name])

  Return a logger with the specified name or, if no name is specified, return a logger which is the root logger of the hierarchy. If specified, the name is typically a dot-separated hierarchical name like “a”, “a.b” or “a.b.c.d”. Choice of these names is entirely up to the developer who is using logging.

  All calls to this function with a given name return the same logger instance. This means that logger instances never need to be passed between different parts of an application.

`logging.``getLoggerClass`()

  Return either the standard `Logger` class, or the last class passed to `setLoggerClass()`. This function may be called from within a new class definition, to ensure that installing a customised `Logger` class will not undo customisations already applied by other code. For example:

      class MyLogger(logging.getLoggerClass()):
          # ... override behaviour here

`logging.``debug`(msg[, *args[, **kwargs]])

  Logs a message with level `DEBUG` on the root logger. The msg is the message format string, and the args are the arguments which are merged into msg using the string formatting operator. (Note that this means that you can use keywords in the format string, together with a single dictionary argument.)

  There are two keyword arguments in kwargs which are inspected: exc_info which, if it does not evaluate as false, causes exception information to be added to the logging message. If an exception tuple (in the format returned by `sys.exc_info()`) is provided, it is used; otherwise, `sys.exc_info()` is called to get the exception information.

  The other optional keyword argument is extra which can be used to pass a dictionary which is used to populate the __dict__ of the LogRecord created for the logging event with user-defined attributes. These custom attributes can then be used as you like. For example, they could be incorporated into logged messages. For example:

      FORMAT = "%(asctime)-15s %(clientip)s %(user)-8s %(message)s"
      logging.basicConfig(format=FORMAT)
      d = {'clientip': '192.168.0.1', 'user': 'fbloggs'}
      logging.warning("Protocol problem: %s", "connection reset", extra=d)

  would print something like

      2006-02-08 22:20:02,165 192.168.0.1 fbloggs  Protocol problem: connection reset

  The keys in the dictionary passed in extra should not clash with the keys used by the logging system. (See the `Formatter` documentation for more information on which keys are used by the logging system.)

  If you choose to use these attributes in logged messages, you need to exercise some care. In the above example, for instance, the `Formatter` has been set up with a format string which expects ‘clientip’ and ‘user’ in the attribute dictionary of the LogRecord. If these are missing, the message will not be logged because a string formatting exception will occur. So in this case, you always need to pass the extra dictionary with these keys.

  While this might be annoying, this feature is intended for use in specialized circumstances, such as multi-threaded servers where the same code executes in many contexts, and interesting conditions which arise are dependent on this context (such as remote client IP address and authenticated user name, in the above example). In such circumstances, it is likely that specialized `Formatter`s would be used with particular `Handler`s.

`logging.``info`(msg[, *args[, **kwargs]])
  Logs a message with level `INFO` on the root logger. The arguments are interpreted as for `debug()`.

`logging.``warning`(msg[, *args[, **kwargs]])
  Logs a message with level `WARNING` on the root logger. The arguments are interpreted as for `debug()`.

`logging.``error`(msg[, *args[, **kwargs]])
  Logs a message with level `ERROR` on the root logger. The arguments are interpreted as for `debug()`.

`logging.``critical`(msg[, *args[, **kwargs]])
  Logs a message with level `CRITICAL` on the root logger. The arguments are interpreted as for `debug()`.

`logging.``exception`(msg[, *args])
  Logs a message with level `ERROR` on the root logger. The arguments are interpreted as for `debug()`. Exception info is added to the logging message. This function should only be called from an exception handler.

`logging.``log`(level, msg[, *args[, **kwargs]])
  Logs a message with level level on the root logger. The other arguments are interpreted as for `debug()`.

`logging.``disable`(lvl)
  Provides an overriding level lvl for all loggers which takes precedence over the logger’s own level. When the need arises to temporarily throttle logging output down across the whole application, this function can be useful.

`logging.``addLevelName`(lvl, levelName)
  Associates level lvl with text levelName in an internal dictionary, which is used to map numeric levels to a textual representation, for example when a `Formatter` formats a message. This function can also be used to define your own levels. The only constraints are that all levels used must be registered using this function, levels should be positive integers and they should increase in increasing order of severity.

`logging.``getLevelName`(lvl)
  Returns the textual representation of logging level lvl. If the level is one of the predefined levels `CRITICAL`, `ERROR`, `WARNING`, `INFO` or `DEBUG` then you get the corresponding string. If you have associated levels with names using `addLevelName()` then the name you have associated with lvl is returned. If a numeric value corresponding to one of the defined levels is passed in, the corresponding string representation is returned. Otherwise, the string “Level %s” % lvl is returned.

`logging.``makeLogRecord`(attrdict)
  Creates and returns a new `LogRecord` instance whose attributes are defined by attrdict. This function is useful for taking a pickled `LogRecord` attribute dictionary, sent over a socket, and reconstituting it as a `LogRecord` instance at the receiving end.

`logging.``basicConfig`([**kwargs])

  Does basic configuration for the logging system by creating a `StreamHandler` with a default `Formatter` and adding it to the root logger. The function does nothing if any handlers have been defined for the root logger. The functions `debug()`, `info()`, `warning()`, `error()` and `critical()` will call `basicConfig()` automatically if no handlers are defined for the root logger.

  The following keyword arguments are supported.

  Format | Description
  --- | ---
  `filename` | Specifies that a FileHandler be created, using the specified filename, rather than a StreamHandler.
  `filemode` | Specifies the mode to open the file, if filename is specified (if filemode is unspecified, it defaults to ‘a’).
  `format` | Use the specified format string for the handler.
  `datefmt` | Use the specified date/time format.
  `level` | Set the root logger level to the specified level.
  `stream` | Use the specified stream to initialize the StreamHandler. Note that this argument is incompatible with ‘filename’ - if both are present, ‘stream’ is ignored.

`logging.``shutdown`()
  Informs the logging system to perform an orderly shutdown by flushing and closing all handlers. This should be called at application exit and no further use of the logging system should be made after this call.

`logging.``setLoggerClass`(klass)
  Tells the logging system to use the class klass when instantiating a logger. The class should define `__init__()` such that only a name argument is required, and the `__init__()` should call `Logger.__init__()`. This function is typically called before any loggers are instantiated by applications which need to use custom logger behavior.

See also

PEP 282 - A Logging System
  The proposal which described this feature for inclusion in the Python standard library.
Original Python logging package
  This is the original source for the `logging` package. The version of the package available from this site is suitable for use with Python 1.5.2, 2.1.x and 2.2.x, which do not include the `logging` package in the standard library.

## Logger Objects

Loggers have the following attributes and methods. Note that Loggers are never instantiated directly, but always through the module-level function `logging.getLogger(name)`.

`Logger.``propagate`
  If this evaluates to false, logging messages are not passed by this logger or by child loggers to higher level (ancestor) loggers. The constructor sets this attribute to 1.

`Logger.``setLevel`(lvl)

  Sets the threshold for this logger to lvl. Logging messages which are less severe than lvl will be ignored. When a logger is created, the level is set to `NOTSET` (which causes all messages to be processed when the logger is the root logger, or delegation to the parent when the logger is a non-root logger). Note that the root logger is created with level `WARNING`.

  The term “delegation to the parent” means that if a logger has a level of NOTSET, its chain of ancestor loggers is traversed until either an ancestor with a level other than NOTSET is found, or the root is reached.

  If an ancestor is found with a level other than NOTSET, then that ancestor’s level is treated as the effective level of the logger where the ancestor search began, and is used to determine how a logging event is handled.

  If the root is reached, and it has a level of NOTSET, then all messages will be processed. Otherwise, the root’s level will be used as the effective level.

`Logger.``isEnabledFor`(lvl)
  Indicates if a message of severity lvl would be processed by this logger. This method checks first the module-level level set by `logging.disable(lvl)` and then the logger’s effective level as determined by `getEffectiveLevel()`.

`Logger.``getEffectiveLevel`()
  Indicates the effective level for this logger. If a value other than `NOTSET` has been set using `setLevel()`, it is returned. Otherwise, the hierarchy is traversed towards the root until a value other than `NOTSET` is found, and that value is returned.

`Logger.``debug`(msg[, *args[, **kwargs]])

  Logs a message with level `DEBUG` on this logger. The msg is the message format string, and the args are the arguments which are merged into msg using the string formatting operator. (Note that this means that you can use keywords in the format string, together with a single dictionary argument.)

  There are two keyword arguments in kwargs which are inspected: exc_info which, if it does not evaluate as false, causes exception information to be added to the logging message. If an exception tuple (in the format returned by `sys.exc_info()`) is provided, it is used; otherwise, `sys.exc_info()` is called to get the exception information.

  The other optional keyword argument is extra which can be used to pass a dictionary which is used to populate the __dict__ of the LogRecord created for the logging event with user-defined attributes. These custom attributes can then be used as you like. For example, they could be incorporated into logged messages. For example:

      FORMAT = "%(asctime)-15s %(clientip)s %(user)-8s %(message)s"
      logging.basicConfig(format=FORMAT)
      d = { 'clientip' : '192.168.0.1', 'user' : 'fbloggs' }
      logger = logging.getLogger("tcpserver")
      logger.warning("Protocol problem: %s", "connection reset", extra=d)

  would print something like

      2006-02-08 22:20:02,165 192.168.0.1 fbloggs  Protocol problem: connection reset

  The keys in the dictionary passed in extra should not clash with the keys used by the logging system. (See the `Formatter` documentation for more information on which keys are used by the logging system.)

  If you choose to use these attributes in logged messages, you need to exercise some care. In the above example, for instance, the `Formatter` has been set up with a format string which expects ‘clientip’ and ‘user’ in the attribute dictionary of the LogRecord. If these are missing, the message will not be logged because a string formatting exception will occur. So in this case, you always need to pass the extra dictionary with these keys.

  While this might be annoying, this feature is intended for use in specialized circumstances, such as multi-threaded servers where the same code executes in many contexts, and interesting conditions which arise are dependent on this context (such as remote client IP address and authenticated user name, in the above example). In such circumstances, it is likely that specialized `Formatter`s would be used with particular `Handler`s.

`Logger.``info`(msg[, *args[, **kwargs]])
  Logs a message with level `INFO` on this logger. The arguments are interpreted as for `debug()`.

`Logger.``warning`(msg[, *args[, **kwargs]])
  Logs a message with level `WARNING` on this logger. The arguments are interpreted as for `debug()`.

`Logger.``error`(msg[, *args[, **kwargs]])
  Logs a message with level `ERROR` on this logger. The arguments are interpreted as for `debug()`.

`Logger.``critical`(msg[, *args[, **kwargs]])
  Logs a message with level `CRITICAL` on this logger. The arguments are interpreted as for `debug()`.

`Logger.``log`(lvl, msg[, *args[, **kwargs]])
  Logs a message with integer level lvl on this logger. The other arguments are interpreted as for `debug()`.

`Logger.``exception`(msg[, *args])
  Logs a message with level `ERROR` on this logger. The arguments are interpreted as for `debug()`. Exception info is added to the logging message. This method should only be called from an exception handler.

`Logger.``addFilter`(filt)
  Adds the specified filter filt to this logger.

`Logger.``removeFilter`(filt)
  Removes the specified filter filt from this logger.

`Logger.``filter`(record)
  Applies this logger’s filters to the record and returns a true value if the record is to be processed.

`Logger.``addHandler`(hdlr)
  Adds the specified handler hdlr to this logger.

`Logger.``removeHandler`(hdlr)
  Removes the specified handler hdlr from this logger.

`Logger.``findCaller`()
  Finds the caller’s source filename and line number. Returns the filename, line number and function name as a 3-element tuple.

`Logger.``handle`(record)
  Handles a record by passing it to all handlers associated with this logger and its ancestors (until a false value of propagate is found). This method is used for unpickled records received from a socket, as well as those created locally. Logger-level filtering is applied using `filter()`.

`Logger.``makeRecord`(name, lvl, fn, lno, msg, args, exc_info[, func, extra])
  This is a factory method which can be overridden in subclasses to create specialized `LogRecord` instances.

## Basic example

The `logging` package provides a lot of flexibility, and its configuration can appear daunting. This section demonstrates that simple use of the logging package is possible.

The simplest example shows logging to the console:

    import logging

    logging.debug('A debug message')
    logging.info('Some information')
    logging.warning('A shot across the bows')

If you run the above script, you’ll see this:

    WARNING:root:A shot across the bows

Because no particular logger was specified, the system used the root logger. The debug and info messages didn’t appear because by default, the root logger is configured to only handle messages with a severity of WARNING or above. The message format is also a configuration default, as is the output destination of the messages - `sys.stderr`. The severity level, the message format and destination can be easily changed, as shown in the example below:

    import logging

    logging.basicConfig(level=logging.DEBUG,
                        format='%(asctime)s %(levelname)s %(message)s',
                        filename='/tmp/myapp.log',
                        filemode='w')
    logging.debug('A debug message')
    logging.info('Some information')
    logging.warning('A shot across the bows')

The `basicConfig()` method is used to change the configuration defaults, which results in output (written to `/tmp/myapp.log`) which should look something like the following:

    2004-07-02 13:00:08,743 DEBUG A debug message
    2004-07-02 13:00:08,743 INFO Some information
    2004-07-02 13:00:08,743 WARNING A shot across the bows

This time, all messages with a severity of DEBUG or above were handled, and the format of the messages was also changed, and output went to the specified file rather than the console.

Formatting uses the old Python string formatting - see section Old String Formatting Operations. The format string takes the following common specifiers. For a complete list of specifiers, consult the `Formatter` documentation.

Format | Description
--- | ---
`%(name)s` | Name of the logger (logging channel).
`%(levelname)s` | Text logging level for the message (`'DEBUG'`, `'INFO'`, `'WARNING'`, `'ERROR'`, `'CRITICAL'`).
`%(asctime)s` | Human-readable time when the `LogRecord` was created. By default this is of the form “2003-07-08 16:49:45,896” (the numbers after the comma are millisecond portion of the time).
`%(message)s` | The logged message.

To change the date/time format, you can pass an additional keyword parameter, datefmt, as in the following:

    import logging

    logging.basicConfig(level=logging.DEBUG,
                        format='%(asctime)s %(levelname)-8s %(message)s',
                        datefmt='%a, %d %b %Y %H:%M:%S',
                        filename='/temp/myapp.log',
                        filemode='w')
    logging.debug('A debug message')
    logging.info('Some information')
    logging.warning('A shot across the bows')

which would result in output like

    Fri, 02 Jul 2004 13:06:18 DEBUG    A debug message
    Fri, 02 Jul 2004 13:06:18 INFO     Some information
    Fri, 02 Jul 2004 13:06:18 WARNING  A shot across the bows

The date format string follows the requirements of `strftime()` - see the documentation for the `time` module.

If, instead of sending logging output to the console or a file, you’d rather use a file-like object which you have created separately, you can pass it to `basicConfig()` using the stream keyword argument. Note that if both stream and filename keyword arguments are passed, the stream argument is ignored.

Of course, you can put variable information in your output. To do this, simply have the message be a format string and pass in additional arguments containing the variable information, as in the following example:

    import logging

    logging.basicConfig(level=logging.DEBUG,
                        format='%(asctime)s %(levelname)-8s %(message)s',
                        datefmt='%a, %d %b %Y %H:%M:%S',
                        filename='/temp/myapp.log',
                        filemode='w')
    logging.error('Pack my box with %d dozen %s', 5, 'liquor jugs')

which would result in

    Wed, 21 Jul 2004 15:35:16 ERROR    Pack my box with 5 dozen liquor jugs

## Logging to multiple destinations

Let’s say you want to log to console and file with different message formats and in differing circumstances. Say you want to log messages with levels of DEBUG and higher to file, and those messages at level INFO and higher to the console. Let’s also assume that the file should contain timestamps, but the console messages should not. Here’s how you can achieve this:

    import logging

    # set up logging to file - see previous section for more details
    logging.basicConfig(level=logging.DEBUG,
                        format='%(asctime)s %(name)-12s %(levelname)-8s %(message)s',
                        datefmt='%m-%d %H:%M',
                        filename='/temp/myapp.log',
                        filemode='w')
    # define a Handler which writes INFO messages or higher to the sys.stderr
    console = logging.StreamHandler()
    console.setLevel(logging.INFO)
    # set a format which is simpler for console use
    formatter = logging.Formatter('%(name)-12s: %(levelname)-8s %(message)s')
    # tell the handler to use this format
    console.setFormatter(formatter)
    # add the handler to the root logger
    logging.getLogger('').addHandler(console)

    # Now, we can log to the root logger, or any other logger. First the root...
    logging.info('Jackdaws love my big sphinx of quartz.')

    # Now, define a couple of other loggers which might represent areas in your
    # application:

    logger1 = logging.getLogger('myapp.area1')
    logger2 = logging.getLogger('myapp.area2')

    logger1.debug('Quick zephyrs blow, vexing daft Jim.')
    logger1.info('How quickly daft jumping zebras vex.')
    logger2.warning('Jail zesty vixen who grabbed pay from quack.')
    logger2.error('The five boxing wizards jump quickly.')

When you run this, on the console you will see

    root        : INFO     Jackdaws love my big sphinx of quartz.
    myapp.area1 : INFO     How quickly daft jumping zebras vex.
    myapp.area2 : WARNING  Jail zesty vixen who grabbed pay from quack.
    myapp.area2 : ERROR    The five boxing wizards jump quickly.

and in the file you will see something like

    10-22 22:19 root         INFO     Jackdaws love my big sphinx of quartz.
    10-22 22:19 myapp.area1  DEBUG    Quick zephyrs blow, vexing daft Jim.
    10-22 22:19 myapp.area1  INFO     How quickly daft jumping zebras vex.
    10-22 22:19 myapp.area2  WARNING  Jail zesty vixen who grabbed pay from quack.
    10-22 22:19 myapp.area2  ERROR    The five boxing wizards jump quickly.

As you can see, the DEBUG message only shows up in the file. The other messages are sent to both destinations.

This example uses console and file handlers, but you can use any number and combination of handlers you choose.

## Adding contextual information to your logging output

Sometimes you want logging output to contain contextual information in addition to the parameters passed to the logging call. For example, in a networked application, it may be desirable to log client-specific information in the log (e.g. remote client’s username, or IP address). Although you could use the extra parameter to achieve this, it’s not always convenient to pass the information in this way. While it might be tempting to create `Logger` instances on a per-connection basis, this is not a good idea because these instances are not garbage collected. While this is not a problem in practice, when the number of `Logger` instances is dependent on the level of granularity you want to use in logging an application, it could be hard to manage if the number of `Logger` instances becomes effectively unbounded.

An easy way in which you can pass contextual information to be output along with logging event information is to use the `LoggerAdapter` class. This class is designed to look like a `Logger`, so that you can call `debug()`, `info()`, `warning()`, `error()`, `exception()`, `critical()` and `log()`. These methods have the same signatures as their counterparts in `Logger`, so you can use the two types of instances interchangeably.

When you create an instance of `LoggerAdapter`, you pass it a `Logger` instance and a dict-like object which contains your contextual information. When you call one of the logging methods on an instance of `LoggerAdapter`, it delegates the call to the underlying instance of `Logger` passed to its constructor, and arranges to pass the contextual information in the delegated call. Here’s a snippet from the code of `LoggerAdapter`:

    def debug(self, msg, *args, **kwargs):
        """
        Delegate a debug call to the underlying logger, after adding
        contextual information from this adapter instance.
        """
        msg, kwargs = self.process(msg, kwargs)
        self.logger.debug(msg, *args, **kwargs)

The `process()` method of `LoggerAdapter` is where the contextual information is added to the logging output. It’s passed the message and keyword arguments of the logging call, and it passes back (potentially) modified versions of these to use in the call to the underlying logger. The default implementation of this method leaves the message alone, but inserts an “extra” key in the keyword argument whose value is the dict-like object passed to the constructor. Of course, if you had passed an “extra” keyword argument in the call to the adapter, it will be silently overwritten.

The advantage of using “extra” is that the values in the dict-like object are merged into the `LogRecord` instance’s __dict__, allowing you to use customized strings with your `Formatter` instances which know about the keys of the dict-like object. If you need a different method, e.g. if you want to prepend or append the contextual information to the message string, you just need to subclass `LoggerAdapter` and override `process()` to do what you need. Here’s an example script which uses this class, which also illustrates what dict-like behaviour is needed from an arbitrary “dict-like” object for use in the constructor:

    import logging

    class ConnInfo:
        """
        An example class which shows how an arbitrary class can be used as
        the 'extra' context information repository passed to a LoggerAdapter.
        """

        def __getitem__(self, name):
            """
            To allow this instance to look like a dict.
            """
            from random import choice
            if name == "ip":
                result = choice(["127.0.0.1", "192.168.0.1"])
            elif name == "user":
                result = choice(["jim", "fred", "sheila"])
            else:
                result = self.__dict__.get(name, "?")
            return result

        def __iter__(self):
            """
            To allow iteration over keys, which will be merged into
            the LogRecord dict before formatting and output.
            """
            keys = ["ip", "user"]
            keys.extend(self.__dict__.keys())
            return keys.__iter__()

    if __name__ == "__main__":
        from random import choice
        levels = (logging.DEBUG, logging.INFO, logging.WARNING, logging.ERROR, logging.CRITICAL)
        a1 = logging.LoggerAdapter(logging.getLogger("a.b.c"),
                                   { "ip" : "123.231.231.123", "user" : "sheila" })
        logging.basicConfig(level=logging.DEBUG,
                            format="%(asctime)-15s %(name)-5s %(levelname)-8s IP: %(ip)-15s User: %(user)-8s %(message)s")
        a1.debug("A debug message")
        a1.info("An info message with %s", "some parameters")
        a2 = logging.LoggerAdapter(logging.getLogger("d.e.f"), ConnInfo())
        for x in range(10):
            lvl = choice(levels)
            lvlname = logging.getLevelName(lvl)
            a2.log(lvl, "A message at %s level with %d %s", lvlname, 2, "parameters")

When this script is run, the output should look something like this:

    2008-01-18 14:49:54,023 a.b.c DEBUG    IP: 123.231.231.123 User: sheila   A debug message
    2008-01-18 14:49:54,023 a.b.c INFO     IP: 123.231.231.123 User: sheila   An info message with some parameters
    2008-01-18 14:49:54,023 d.e.f CRITICAL IP: 192.168.0.1     User: jim      A message at CRITICAL level with 2 parameters
    2008-01-18 14:49:54,033 d.e.f INFO     IP: 192.168.0.1     User: jim      A message at INFO level with 2 parameters
    2008-01-18 14:49:54,033 d.e.f WARNING  IP: 192.168.0.1     User: sheila   A message at WARNING level with 2 parameters
    2008-01-18 14:49:54,033 d.e.f ERROR    IP: 127.0.0.1       User: fred     A message at ERROR level with 2 parameters
    2008-01-18 14:49:54,033 d.e.f ERROR    IP: 127.0.0.1       User: sheila   A message at ERROR level with 2 parameters
    2008-01-18 14:49:54,033 d.e.f WARNING  IP: 192.168.0.1     User: sheila   A message at WARNING level with 2 parameters
    2008-01-18 14:49:54,033 d.e.f WARNING  IP: 192.168.0.1     User: jim      A message at WARNING level with 2 parameters
    2008-01-18 14:49:54,033 d.e.f INFO     IP: 192.168.0.1     User: fred     A message at INFO level with 2 parameters
    2008-01-18 14:49:54,033 d.e.f WARNING  IP: 192.168.0.1     User: sheila   A message at WARNING level with 2 parameters
    2008-01-18 14:49:54,033 d.e.f WARNING  IP: 127.0.0.1       User: jim      A message at WARNING level with 2 parameters

## Sending and receiving logging events across a network

Let’s say you want to send logging events across a network, and handle them at the receiving end. A simple way of doing this is attaching a `SocketHandler` instance to the root logger at the sending end:

    import logging, logging.handlers

    rootLogger = logging.getLogger('')
    rootLogger.setLevel(logging.DEBUG)
    socketHandler = logging.handlers.SocketHandler('localhost',
                        logging.handlers.DEFAULT_TCP_LOGGING_PORT)
    # don't bother with a formatter, since a socket handler sends the event as
    # an unformatted pickle
    rootLogger.addHandler(socketHandler)

    # Now, we can log to the root logger, or any other logger. First the root...
    logging.info('Jackdaws love my big sphinx of quartz.')

    # Now, define a couple of other loggers which might represent areas in your
    # application:

    logger1 = logging.getLogger('myapp.area1')
    logger2 = logging.getLogger('myapp.area2')

    logger1.debug('Quick zephyrs blow, vexing daft Jim.')
    logger1.info('How quickly daft jumping zebras vex.')
    logger2.warning('Jail zesty vixen who grabbed pay from quack.')
    logger2.error('The five boxing wizards jump quickly.')

At the receiving end, you can set up a receiver using the `socketserver` module. Here is a basic working example:

    import cPickle
    import logging
    import logging.handlers
    import socketserver
    import struct

    class LogRecordStreamHandler(socketserver.StreamRequestHandler):
        """Handler for a streaming logging request.

        This basically logs the record using whatever logging policy is
        configured locally.
        """

        def handle(self):
            """
            Handle multiple requests - each expected to be a 4-byte length,
            followed by the LogRecord in pickle format. Logs the record
            according to whatever policy is configured locally.
            """
            while True:
                chunk = self.connection.recv(4)
                if len(chunk) < 4:
                    break
                slen = struct.unpack(">L", chunk)[0]
                chunk = self.connection.recv(slen)
                while len(chunk) < slen:
                    chunk = chunk + self.connection.recv(slen - len(chunk))
                obj = self.unPickle(chunk)
                record = logging.makeLogRecord(obj)
                self.handleLogRecord(record)

        def unPickle(self, data):
            return cPickle.loads(data)

        def handleLogRecord(self, record):
            # if a name is specified, we use the named logger rather than the one
            # implied by the record.
            if self.server.logname is not None:
                name = self.server.logname
            else:
                name = record.name
            logger = logging.getLogger(name)
            # N.B. EVERY record gets logged. This is because Logger.handle
            # is normally called AFTER logger-level filtering. If you want
            # to do filtering, do it at the client end to save wasting
            # cycles and network bandwidth!
            logger.handle(record)

    class LogRecordSocketReceiver(socketserver.ThreadingTCPServer):
        """simple TCP socket-based logging receiver suitable for testing.
        """

        allow_reuse_address = 1

        def __init__(self, host='localhost',
                     port=logging.handlers.DEFAULT_TCP_LOGGING_PORT,
                     handler=LogRecordStreamHandler):
            socketserver.ThreadingTCPServer.__init__(self, (host, port), handler)
            self.abort = 0
            self.timeout = 1
            self.logname = None

        def serve_until_stopped(self):
            import select
            abort = 0
            while not abort:
                rd, wr, ex = select.select([self.socket.fileno()],
                                           [], [],
                                           self.timeout)
                if rd:
                    self.handle_request()
                abort = self.abort

    def main():
        logging.basicConfig(
            format="%(relativeCreated)5d %(name)-15s %(levelname)-8s %(message)s")
        tcpserver = LogRecordSocketReceiver()
        print("About to start TCP server...")
        tcpserver.serve_until_stopped()

    if __name__ == "__main__":
        main()

First run the server, and then the client. On the client side, nothing is printed on the console; on the server side, you should see something like:

    About to start TCP server...
       59 root            INFO     Jackdaws love my big sphinx of quartz.
       59 myapp.area1     DEBUG    Quick zephyrs blow, vexing daft Jim.
       69 myapp.area1     INFO     How quickly daft jumping zebras vex.
       69 myapp.area2     WARNING  Jail zesty vixen who grabbed pay from quack.
       69 myapp.area2     ERROR    The five boxing wizards jump quickly.

## Handler Objects

Handlers have the following attributes and methods. Note that `Handler` is never instantiated directly; this class acts as a base for more useful subclasses. However, the `__init__()` method in subclasses needs to call `Handler.__init__()`.

`Handler.``__init__`(level=NOTSET)
  Initializes the `Handler` instance by setting its level, setting the list of filters to the empty list and creating a lock (using `createLock()`) for serializing access to an I/O mechanism.

`Handler.``createLock`()
  Initializes a thread lock which can be used to serialize access to underlying I/O functionality which may not be threadsafe.

`Handler.``acquire`()
  Acquires the thread lock created with `createLock()`.

`Handler.``release`()
  Releases the thread lock acquired with `acquire()`.

`Handler.``setLevel`(lvl)
  Sets the threshold for this handler to lvl. Logging messages which are less severe than lvl will be ignored. When a handler is created, the level is set to `NOTSET` (which causes all messages to be processed).

`Handler.``setFormatter`(form)
  Sets the `Formatter` for this handler to form.

`Handler.``addFilter`(filt)
  Adds the specified filter filt to this handler.

`Handler.``removeFilter`(filt)
  Removes the specified filter filt from this handler.

`Handler.``filter`(record)
  Applies this handler’s filters to the record and returns a true value if the record is to be processed.

`Handler.``flush`()
  Ensure all logging output has been flushed. This version does nothing and is intended to be implemented by subclasses.

`Handler.``close`()
  Tidy up any resources used by the handler. This version does no output but removes the handler from an internal list of handlers which is closed when `shutdown()` is called. Subclasses should ensure that this gets called from overridden `close()` methods.

`Handler.``handle`(record)
  Conditionally emits the specified logging record, depending on filters which may have been added to the handler. Wraps the actual emission of the record with acquisition/release of the I/O thread lock.

`Handler.``handleError`(record)
  This method should be called from handlers when an exception is encountered during an `emit()` call. By default it does nothing, which means that exceptions get silently ignored. This is what is mostly wanted for a logging system - most users will not care about errors in the logging system, they are more interested in application errors. You could, however, replace this with a custom handler if you wish. The specified record is the one which was being processed when the exception occurred.

`Handler.``format`(record)
  Do formatting for a record - if a formatter is set, use it. Otherwise, use the default formatter for the module.

`Handler.``emit`(record)
  Do whatever it takes to actually log the specified logging record. This version is intended to be implemented by subclasses and so raises a `NotImplementedError`.

### StreamHandler

The `StreamHandler` class, located in the core `logging` package, sends logging output to streams such as sys.stdout, sys.stderr or any file-like object (or, more precisely, any object which supports `write()` and `flush()` methods).

class `logging.``StreamHandler`([strm])

  Returns a new instance of the `StreamHandler` class. If strm is specified, the instance will use it for logging output; otherwise, sys.stderr will be used.

  `emit`(record)
    If a formatter is specified, it is used to format the record. The record is then written to the stream with a trailing newline. If exception information is present, it is formatted using `traceback.print_exception()` and appended to the stream.
  `flush`()
    Flushes the stream by calling its `flush()` method. Note that the `close()` method is inherited from `Handler` and so does no output, so an explicit `flush()` call may be needed at times.

### FileHandler

The `FileHandler` class, located in the core `logging` package, sends logging output to a disk file. It inherits the output functionality from `StreamHandler`.

class `logging.``FileHandler`(filename[, mode[, encoding[, delay]]])

  Returns a new instance of the `FileHandler` class. The specified file is opened and used as the stream for logging. If mode is not specified, `'a'` is used. If encoding is not None, it is used to open the file with that encoding. If delay is true, then file opening is deferred until the first call to `emit()`. By default, the file grows indefinitely.

  `close`()
    Closes the file.
  `emit`(record)
    Outputs the record to the file.

### NullHandler

New in version 3.1.

The `NullHandler` class, located in the core `logging` package, does not do any formatting or output. It is essentially a “no-op” handler for use by library developers.

class `logging.``NullHandler`

  Returns a new instance of the `NullHandler` class.

  `emit`(record)
    This method does nothing.

See Configuring Logging for a Library for more information on how to use `NullHandler`.

### WatchedFileHandler

The `WatchedFileHandler` class, located in the `logging.handlers` module, is a `FileHandler` which watches the file it is logging to. If the file changes, it is closed and reopened using the file name.

A file change can happen because of usage of programs such as newsyslog and logrotate which perform log file rotation. This handler, intended for use under Unix/Linux, watches the file to see if it has changed since the last emit. (A file is deemed to have changed if its device or inode have changed.) If the file has changed, the old file stream is closed, and the file opened to get a new stream.

This handler is not appropriate for use under Windows, because under Windows open log files cannot be moved or renamed - logging opens the files with exclusive locks - and so there is no need for such a handler. Furthermore, ST_INO is not supported under Windows; `stat()` always returns zero for this value.

class `logging.handlers.``WatchedFileHandler`(filename[, mode[, encoding[, delay]]])

  Returns a new instance of the `WatchedFileHandler` class. The specified file is opened and used as the stream for logging. If mode is not specified, `'a'` is used. If encoding is not None, it is used to open the file with that encoding. If delay is true, then file opening is deferred until the first call to `emit()`. By default, the file grows indefinitely.

  `emit`(record)
    Outputs the record to the file, but first checks to see if the file has changed. If it has, the existing stream is flushed and closed and the file opened again, before outputting the record to the file.

### RotatingFileHandler

The `RotatingFileHandler` class, located in the `logging.handlers` module, supports rotation of disk log files.

class `logging.handlers.``RotatingFileHandler`(filename[, mode[, maxBytes[, backupCount[, encoding[, delay]]]]])

  Returns a new instance of the `RotatingFileHandler` class. The specified file is opened and used as the stream for logging. If mode is not specified, `'a'` is used. If encoding is not None, it is used to open the file with that encoding. If delay is true, then file opening is deferred until the first call to `emit()`. By default, the file grows indefinitely.

  You can use the maxBytes and backupCount values to allow the file to rollover at a predetermined size. When the size is about to be exceeded, the file is closed and a new file is silently opened for output. Rollover occurs whenever the current log file is nearly maxBytes in length; if maxBytes is zero, rollover never occurs. If backupCount is non-zero, the system will save old log files by appending the extensions “.1”, “.2” etc., to the filename. For example, with a backupCount of 5 and a base file name of `app.log`, you would get `app.log`, `app.log.1`, `app.log.2`, up to `app.log.5`. The file being written to is always `app.log`. When this file is filled, it is closed and renamed to `app.log.1`, and if files `app.log.1`, `app.log.2`, etc. exist, then they are renamed to `app.log.2`, `app.log.3` etc. respectively.

  `doRollover`()
    Does a rollover, as described above.
  `emit`(record)
    Outputs the record to the file, catering for rollover as described previously.

### TimedRotatingFileHandler

The `TimedRotatingFileHandler` class, located in the `logging.handlers` module, supports rotation of disk log files at certain timed intervals.

class `logging.handlers.``TimedRotatingFileHandler`(filename[, when[, interval[, backupCount[, encoding[, delay[, utc]]]]]])

  Returns a new instance of the `TimedRotatingFileHandler` class. The specified file is opened and used as the stream for logging. On rotating it also sets the filename suffix. Rotating happens based on the product of when and interval.

  You can use the when to specify the type of interval. The list of possible values is below. Note that they are not case sensitive.

  Value | Type of interval
  --- | ---
  `'S'` | Seconds
  `'M'` | Minutes
  `'H'` | Hours
  `'D'` | Days
  `'W'` | Week day (0=Monday)
  `'midnight'` | Roll over at midnight

  The system will save old log files by appending extensions to the filename. The extensions are date-and-time based, using the strftime format `%Y-%m-%d_%H-%M-%S` or a leading portion thereof, depending on the rollover interval. If the utc argument is true, times in UTC will be used; otherwise local time is used.

  If backupCount is nonzero, at most backupCount files will be kept, and if more would be created when rollover occurs, the oldest one is deleted. The deletion logic uses the interval to determine which files to delete, so changing the interval may leave old files lying around.

  `doRollover`()
    Does a rollover, as described above.
  `emit`(record)
    Outputs the record to the file, catering for rollover as described above.

### SocketHandler

The `SocketHandler` class, located in the `logging.handlers` module, sends logging output to a network socket. The base class uses a TCP socket.

class `logging.handlers.``SocketHandler`(host, port)

  Returns a new instance of the `SocketHandler` class intended to communicate with a remote machine whose address is given by host and port.

  `close`()
    Closes the socket.
  `emit`()
    Pickles the record’s attribute dictionary and writes it to the socket in binary format. If there is an error with the socket, silently drops the packet. If the connection was previously lost, re-establishes the connection. To unpickle the record at the receiving end into a `LogRecord`, use the `makeLogRecord()` function.
  `handleError`()
    Handles an error which has occurred during `emit()`. The most likely cause is a lost connection. Closes the socket so that we can retry on the next event.
  `makeSocket`()
    This is a factory method which allows subclasses to define the precise type of socket they want. The default implementation creates a TCP socket (`socket.SOCK_STREAM`).
  `makePickle`(record)
    Pickles the record’s attribute dictionary in binary format with a length prefix, and returns it ready for transmission across the socket.
  `send`(packet)
    Send a pickled string packet to the socket. This function allows for partial sends which can happen when the network is busy.

### DatagramHandler

The `DatagramHandler` class, located in the `logging.handlers` module, inherits from `SocketHandler` to support sending logging messages over UDP sockets.

class `logging.handlers.``DatagramHandler`(host, port)

  Returns a new instance of the `DatagramHandler` class intended to communicate with a remote machine whose address is given by host and port.

  `emit`()
    Pickles the record’s attribute dictionary and writes it to the socket in binary format. If there is an error with the socket, silently drops the packet. To unpickle the record at the receiving end into a `LogRecord`, use the `makeLogRecord()` function.
  `makeSocket`()
    The factory method of `SocketHandler` is here overridden to create a UDP socket (`socket.SOCK_DGRAM`).
  `send`(s)
    Send a pickled string to a socket.

### SysLogHandler

The `SysLogHandler` class, located in the `logging.handlers` module, supports sending logging messages to a remote or local Unix syslog.

class `logging.handlers.``SysLogHandler`([address[, facility]])

  Returns a new instance of the `SysLogHandler` class intended to communicate with a remote Unix machine whose address is given by address in the form of a `(host, port)` tuple. If address is not specified, `('localhost', 514)` is used. The address is used to open a UDP socket. An alternative to providing a `(host, port)` tuple is providing an address as a string, for example “/dev/log”. In this case, a Unix domain socket is used to send the message to the syslog. If facility is not specified, `LOG_USER` is used.

  `close`()
    Closes the socket to the remote host.
  `emit`(record)
    The record is formatted, and then sent to the syslog server. If exception information is present, it is not sent to the server.
  `encodePriority`(facility, priority)
    Encodes the facility and priority into an integer. You can pass in strings or integers - if strings are passed, internal mapping dictionaries are used to convert them to integers.

### NTEventLogHandler

The `NTEventLogHandler` class, located in the `logging.handlers` module, supports sending logging messages to a local Windows NT, Windows 2000 or Windows XP event log. Before you can use it, you need Mark Hammond’s Win32 extensions for Python installed.

class `logging.handlers.``NTEventLogHandler`(appname[, dllname[, logtype]])

  Returns a new instance of the `NTEventLogHandler` class. The appname is used to define the application name as it appears in the event log. An appropriate registry entry is created using this name. The dllname should give the fully qualified pathname of a .dll or .exe which contains message definitions to hold in the log (if not specified, `'win32service.pyd'` is used - this is installed with the Win32 extensions and contains some basic placeholder message definitions. Note that use of these placeholders will make your event logs big, as the entire message source is held in the log. If you want slimmer logs, you have to pass in the name of your own .dll or .exe which contains the message definitions you want to use in the event log). The logtype is one of `'Application'`, `'System'` or `'Security'`, and defaults to `'Application'`.

  `close`()
    At this point, you can remove the application name from the registry as a source of event log entries. However, if you do this, you will not be able to see the events as you intended in the Event Log Viewer - it needs to be able to access the registry to get the .dll name. The current version does not do this.
  `emit`(record)
    Determines the message ID, event category and event type, and then logs the message in the NT event log.
  `getEventCategory`(record)
    Returns the event category for the record. Override this if you want to specify your own categories. This version returns 0.
  `getEventType`(record)
    Returns the event type for the record. Override this if you want to specify your own types. This version does a mapping using the handler’s typemap attribute, which is set up in `__init__()` to a dictionary which contains mappings for `DEBUG`, `INFO`, `WARNING`, `ERROR` and `CRITICAL`. If you are using your own levels, you will either need to override this method or place a suitable dictionary in the handler’s typemap attribute.
  `getMessageID`(record)
    Returns the message ID for the record. If you are using your own messages, you could do this by having the msg passed to the logger being an ID rather than a format string. Then, in here, you could use a dictionary lookup to get the message ID. This version returns 1, which is the base message ID in `win32service.pyd`.

### SMTPHandler

The `SMTPHandler` class, located in the `logging.handlers` module, supports sending logging messages to an email address via SMTP.

class `logging.handlers.``SMTPHandler`(mailhost, fromaddr, toaddrs, subject[, credentials])

  Returns a new instance of the `SMTPHandler` class. The instance is initialized with the from and to addresses and subject line of the email. The toaddrs should be a list of strings. To specify a non-standard SMTP port, use the (host, port) tuple format for the mailhost argument. If you use a string, the standard SMTP port is used. If your SMTP server requires authentication, you can specify a (username, password) tuple for the credentials argument.

  `emit`(record)
    Formats the record and sends it to the specified addressees.
  `getSubject`(record)
    If you want to specify a subject line which is record-dependent, override this method.

### MemoryHandler

The `MemoryHandler` class, located in the `logging.handlers` module, supports buffering of logging records in memory, periodically flushing them to a target handler. Flushing occurs whenever the buffer is full, or when an event of a certain severity or greater is seen.

`MemoryHandler` is a subclass of the more general `BufferingHandler`, which is an abstract class. This buffers logging records in memory. Whenever each record is added to the buffer, a check is made by calling `shouldFlush()` to see if the buffer should be flushed. If it should, then `flush()` is expected to do the needful.

class `logging.handlers.``BufferingHandler`(capacity)

  Initializes the handler with a buffer of the specified capacity.

  `emit`(record)
    Appends the record to the buffer. If `shouldFlush()` returns true, calls `flush()` to process the buffer.
  `flush`()
    You can override this to implement custom flushing behavior. This version just zaps the buffer to empty.
  `shouldFlush`(record)
    Returns true if the buffer is up to capacity. This method can be overridden to implement custom flushing strategies.

class `logging.handlers.``MemoryHandler`(capacity[, flushLevel[, target]])

  Returns a new instance of the `MemoryHandler` class. The instance is initialized with a buffer size of capacity. If flushLevel is not specified, `ERROR` is used. If no target is specified, the target will need to be set using `setTarget()` before this handler does anything useful.

  `close`()
    Calls `flush()`, sets the target to `None` and clears the buffer.
  `flush`()
    For a `MemoryHandler`, flushing means just sending the buffered records to the target, if there is one. Override if you want different behavior.
  `setTarget`(target)
    Sets the target handler for this handler.
  `shouldFlush`(record)
    Checks for buffer full or a record at the flushLevel or higher.

### HTTPHandler

The `HTTPHandler` class, located in the `logging.handlers` module, supports sending logging messages to a Web server, using either `GET` or `POST` semantics.

class `logging.handlers.``HTTPHandler`(host, url[, method])

  Returns a new instance of the `HTTPHandler` class. The instance is initialized with a host address, url and HTTP method. The host can be of the form `host:port`, should you need to use a specific port number. If no method is specified, `GET` is used.

  `emit`(record)
    Sends the record to the Web server as an URL-encoded dictionary.

## Formatter Objects

`Formatter`s have the following attributes and methods. They are responsible for converting a `LogRecord` to (usually) a string which can be interpreted by either a human or an external system. The base `Formatter` allows a formatting string to be specified. If none is supplied, the default value of `'%(message)s'` is used.

A Formatter can be initialized with a format string which makes use of knowledge of the `LogRecord` attributes - such as the default value mentioned above making use of the fact that the user’s message and arguments are pre-formatted into a `LogRecord`‘s message attribute. This format string contains standard python %-style mapping keys. See section Old String Formatting Operations for more information on string formatting.

Currently, the useful mapping keys in a `LogRecord` are:

Format | Description
--- | ---
`%(name)s` | Name of the logger (logging channel).
`%(levelno)s` | Numeric logging level for the message (`DEBUG`, `INFO`, `WARNING`, `ERROR`, `CRITICAL`).
`%(levelname)s` | Text logging level for the message (`'DEBUG'`, `'INFO'`, `'WARNING'`, `'ERROR'`, `'CRITICAL'`).
`%(pathname)s` | Full pathname of the source file where the logging call was issued (if available).
`%(filename)s` | Filename portion of pathname.
`%(module)s` | Module (name portion of filename).
`%(funcName)s` | Name of function containing the logging call.
`%(lineno)d` | Source line number where the logging call was issued (if available).
`%(created)f` | Time when the `LogRecord` was created (as returned by `time.time()`).
`%(relativeCreated)d` | Time in milliseconds when the LogRecord was created, relative to the time the logging module was loaded.
`%(asctime)s` | Human-readable time when the `LogRecord` was created. By default this is of the form “2003-07-08 16:49:45,896” (the numbers after the comma are millisecond portion of the time).
`%(msecs)d` | Millisecond portion of the time when the `LogRecord` was created.
`%(thread)d` | Thread ID (if available).
`%(threadName)s` | Thread name (if available).
`%(process)d` | Process ID (if available).
`%(message)s` | The logged message, computed as `msg % args`.

class `logging.``Formatter`([fmt[, datefmt]])

  Returns a new instance of the `Formatter` class. The instance is initialized with a format string for the message as a whole, as well as a format string for the date/time portion of a message. If no fmt is specified, `'%(message)s'` is used. If no datefmt is specified, the ISO8601 date format is used.

  `format`(record)
    The record’s attribute dictionary is used as the operand to a string formatting operation. Returns the resulting string. Before formatting the dictionary, a couple of preparatory steps are carried out. The message attribute of the record is computed using msg % args. If the formatting string contains `'(asctime)'`, `formatTime()` is called to format the event time. If there is exception information, it is formatted using `formatException()` and appended to the message. Note that the formatted exception information is cached in attribute exc_text. This is useful because the exception information can be pickled and sent across the wire, but you should be careful if you have more than one `Formatter` subclass which customizes the formatting of exception information. In this case, you will have to clear the cached value after a formatter has done its formatting, so that the next formatter to handle the event doesn’t use the cached value but recalculates it afresh.
  `formatTime`(record[, datefmt])
    This method should be called from `format()` by a formatter which wants to make use of a formatted time. This method can be overridden in formatters to provide for any specific requirement, but the basic behavior is as follows: if datefmt (a string) is specified, it is used with `time.strftime()` to format the creation time of the record. Otherwise, the ISO8601 format is used. The resulting string is returned.
  `formatException`(exc_info)
    Formats the specified exception information (a standard exception tuple as returned by `sys.exc_info()`) as a string. This default implementation just uses `traceback.print_exception()`. The resulting string is returned.

## Filter Objects

`Filter`s can be used by `Handler`s and `Logger`s for more sophisticated filtering than is provided by levels. The base filter class only allows events which are below a certain point in the logger hierarchy. For example, a filter initialized with “A.B” will allow events logged by loggers “A.B”, “A.B.C”, “A.B.C.D”, “A.B.D” etc. but not “A.BB”, “B.A.B” etc. If initialized with the empty string, all events are passed.

class `logging.``Filter`([name])

  Returns an instance of the `Filter` class. If name is specified, it names a logger which, together with its children, will have its events allowed through the filter. If no name is specified, allows every event.

  `filter`(record)
    Is the specified record to be logged? Returns zero for no, nonzero for yes. If deemed appropriate, the record may be modified in-place by this method.

## LogRecord Objects

`LogRecord` instances are created every time something is logged. They contain all the information pertinent to the event being logged. The main information passed in is in msg and args, which are combined using msg % args to create the message field of the record. The record also includes information such as when the record was created, the source line where the logging call was made, and any exception information to be logged.

class `logging.``LogRecord`(name, lvl, pathname, lineno, msg, args, exc_info[, func])

  Returns an instance of `LogRecord` initialized with interesting information. The name is the logger name; lvl is the numeric level; pathname is the absolute pathname of the source file in which the logging call was made; lineno is the line number in that file where the logging call is found; msg is the user-supplied message (a format string); args is the tuple which, together with msg, makes up the user message; and exc_info is the exception tuple obtained by calling `sys.exc_info()` (or `None`, if no exception information is available). The func is the name of the function from which the logging call was made. If not specified, it defaults to `None`.

  `getMessage`()
    Returns the message for this `LogRecord` instance after merging any user-supplied arguments with the message.

## LoggerAdapter Objects

`LoggerAdapter` instances are used to conveniently pass contextual information into logging calls. For a usage example , see the section on adding contextual information to your logging output.

class `logging.``LoggerAdapter`(logger, extra)

  Returns an instance of `LoggerAdapter` initialized with an underlying `Logger` instance and a dict-like object.

  `process`(msg, kwargs)
    Modifies the message and/or keyword arguments passed to a logging call in order to insert contextual information. This implementation takes the object passed as extra to the constructor and adds it to kwargs using key ‘extra’. The return value is a (msg, kwargs) tuple which has the (possibly modified) versions of the arguments passed in.

In addition to the above, `LoggerAdapter` supports all the logging methods of `Logger`, i.e. `debug()`, `info()`, `warning()`, `error()`, `exception()`, `critical()` and `log()`. These methods have the same signatures as their counterparts in `Logger`, so you can use the two types of instances interchangeably.

## Thread Safety

The logging module is intended to be thread-safe without any special work needing to be done by its clients. It achieves this though using threading locks; there is one lock to serialize access to the module’s shared data, and each handler also creates a lock to serialize access to its underlying I/O.

## Configuration

### Configuration functions

The following functions configure the logging module. They are located in the `logging.config` module. Their use is optional — you can configure the logging module using these functions or by making calls to the main API (defined in `logging` itself) and defining handlers which are declared either in `logging` or `logging.handlers`.

`logging.``fileConfig`(fname[, defaults])
  Reads the logging configuration from a `configparser`-format file named fname. This function can be called several times from an application, allowing an end user the ability to select from various pre-canned configurations (if the developer provides a mechanism to present the choices and load the chosen configuration). Defaults to be passed to the ConfigParser can be specified in the defaults argument.

`logging.``listen`([port])

  Starts up a socket server on the specified port, and listens for new configurations. If no port is specified, the module’s default `DEFAULT_LOGGING_CONFIG_PORT` is used. Logging configurations will be sent as a file suitable for processing by `fileConfig()`. Returns a `Thread` instance on which you can call `start()` to start the server, and which you can `join()` when appropriate. To stop the server, call `stopListening()`.

  To send a configuration to the socket, read in the configuration file and send it to the socket as a string of bytes preceded by a four-byte length string packed in binary using `struct.pack('>L', n)`.

`logging.``stopListening`()
  Stops the listening server which was created with a call to `listen()`. This is typically called before calling `join()` on the return value from `listen()`.

### Configuration file format

The configuration file format understood by `fileConfig()` is based on `configparser` functionality. The file must contain sections called `[loggers]`, `[handlers]` and `[formatters]` which identify by name the entities of each type which are defined in the file. For each such entity, there is a separate section which identifies how that entity is configured. Thus, for a logger named `log01` in the `[loggers]` section, the relevant configuration details are held in a section `[logger_log01]`. Similarly, a handler called `hand01` in the `[handlers]` section will have its configuration held in a section called `[handler_hand01]`, while a formatter called `form01` in the `[formatters]` section will have its configuration specified in a section called `[formatter_form01]`. The root logger configuration must be specified in a section called `[logger_root]`.

Examples of these sections in the file are given below.

    [loggers]
    keys=root,log02,log03,log04,log05,log06,log07

    [handlers]
    keys=hand01,hand02,hand03,hand04,hand05,hand06,hand07,hand08,hand09

    [formatters]
    keys=form01,form02,form03,form04,form05,form06,form07,form08,form09

The root logger must specify a level and a list of handlers. An example of a root logger section is given below.

    [logger_root]
    level=NOTSET
    handlers=hand01

The `level` entry can be one of `DEBUG, INFO, WARNING, ERROR, CRITICAL` or `NOTSET`. For the root logger only, `NOTSET` means that all messages will be logged. Level values are `eval()`uated in the context of the `logging` package’s namespace.

The `handlers` entry is a comma-separated list of handler names, which must appear in the `[handlers]` section. These names must appear in the `[handlers]` section and have corresponding sections in the configuration file.

For loggers other than the root logger, some additional information is required. This is illustrated by the following example.

    [logger_parser]
    level=DEBUG
    handlers=hand01
    propagate=1
    qualname=compiler.parser

The `level` and `handlers` entries are interpreted as for the root logger, except that if a non-root logger’s level is specified as `NOTSET`, the system consults loggers higher up the hierarchy to determine the effective level of the logger. The `propagate` entry is set to 1 to indicate that messages must propagate to handlers higher up the logger hierarchy from this logger, or 0 to indicate that messages are not propagated to handlers up the hierarchy. The `qualname` entry is the hierarchical channel name of the logger, that is to say the name used by the application to get the logger.

Sections which specify handler configuration are exemplified by the following.

    [handler_hand01]
    class=StreamHandler
    level=NOTSET
    formatter=form01
    args=(sys.stdout,)

The `class` entry indicates the handler’s class (as determined by `eval()` in the `logging` package’s namespace). The `level` is interpreted as for loggers, and `NOTSET` is taken to mean “log everything”.

The `formatter` entry indicates the key name of the formatter for this handler. If blank, a default formatter (`logging._defaultFormatter`) is used. If a name is specified, it must appear in the `[formatters]` section and have a corresponding section in the configuration file.

The `args` entry, when `eval()`uated in the context of the `logging` package’s namespace, is the list of arguments to the constructor for the handler class. Refer to the constructors for the relevant handlers, or to the examples below, to see how typical entries are constructed.

    [handler_hand02]
    class=FileHandler
    level=DEBUG
    formatter=form02
    args=('python.log', 'w')

    [handler_hand03]
    class=handlers.SocketHandler
    level=INFO
    formatter=form03
    args=('localhost', handlers.DEFAULT_TCP_LOGGING_PORT)

    [handler_hand04]
    class=handlers.DatagramHandler
    level=WARN
    formatter=form04
    args=('localhost', handlers.DEFAULT_UDP_LOGGING_PORT)

    [handler_hand05]
    class=handlers.SysLogHandler
    level=ERROR
    formatter=form05
    args=(('localhost', handlers.SYSLOG_UDP_PORT), handlers.SysLogHandler.LOG_USER)

    [handler_hand06]
    class=handlers.NTEventLogHandler
    level=CRITICAL
    formatter=form06
    args=('Python Application', '', 'Application')

    [handler_hand07]
    class=handlers.SMTPHandler
    level=WARN
    formatter=form07
    args=('localhost', 'from@abc', ['user1@abc', 'user2@xyz'], 'Logger Subject')

    [handler_hand08]
    class=handlers.MemoryHandler
    level=NOTSET
    formatter=form08
    target=
    args=(10, ERROR)

    [handler_hand09]
    class=handlers.HTTPHandler
    level=NOTSET
    formatter=form09
    args=('localhost:9022', '/log', 'GET')

Sections which specify formatter configuration are typified by the following.

    [formatter_form01]
    format=F1 %(asctime)s %(levelname)s %(message)s
    datefmt=
    class=logging.Formatter

The `format` entry is the overall format string, and the `datefmt` entry is the `strftime()`-compatible date/time format string. If empty, the package substitutes ISO8601 format date/times, which is almost equivalent to specifying the date format string `"%Y-%m-%d %H:%M:%S"`. The ISO8601 format also specifies milliseconds, which are appended to the result of using the above format string, with a comma separator. An example time in ISO8601 format is `2003-01-23 00:29:50,411`.

The `class` entry is optional. It indicates the name of the formatter’s class (as a dotted module and class name.) This option is useful for instantiating a `Formatter` subclass. Subclasses of `Formatter` can present exception tracebacks in an expanded or condensed format.

### Configuration server example

Here is an example of a module using the logging configuration server:

    import logging
    import logging.config
    import time
    import os

    # read initial config file
    logging.config.fileConfig("logging.conf")

    # create and start listener on port 9999
    t = logging.config.listen(9999)
    t.start()

    logger = logging.getLogger("simpleExample")

    try:
        # loop through logging calls to see the difference
        # new configurations make, until Ctrl+C is pressed
        while True:
            logger.debug("debug message")
            logger.info("info message")
            logger.warn("warn message")
            logger.error("error message")
            logger.critical("critical message")
            time.sleep(5)
    except KeyboardInterrupt:
        # cleanup
        logging.config.stopListening()
        t.join()

And here is a script that takes a filename and sends that file to the server, properly preceded with the binary-encoded length, as the new logging configuration:

    #!/usr/bin/env python
    import socket, sys, struct

    data_to_send = open(sys.argv[1], "r").read()

    HOST = 'localhost'
    PORT = 9999
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    print("connecting...")
    s.connect((HOST, PORT))
    print("sending config...")
    s.send(struct.pack(">L", len(data_to_send)))
    s.send(data_to_send)
    s.close()
    print("complete")

## More examples

### Multiple handlers and formatters

Loggers are plain Python objects. The `addHandler()` method has no minimum or maximum quota for the number of handlers you may add. Sometimes it will be beneficial for an application to log all messages of all severities to a text file while simultaneously logging errors or above to the console. To set this up, simply configure the appropriate handlers. The logging calls in the application code will remain unchanged. Here is a slight modification to the previous simple module-based configuration example:

    import logging

    logger = logging.getLogger("simple_example")
    logger.setLevel(logging.DEBUG)
    # create file handler which logs even debug messages
    fh = logging.FileHandler("spam.log")
    fh.setLevel(logging.DEBUG)
    # create console handler with a higher log level
    ch = logging.StreamHandler()
    ch.setLevel(logging.ERROR)
    # create formatter and add it to the handlers
    formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
    ch.setFormatter(formatter)
    fh.setFormatter(formatter)
    # add the handlers to logger
    logger.addHandler(ch)
    logger.addHandler(fh)

    # "application" code
    logger.debug("debug message")
    logger.info("info message")
    logger.warn("warn message")
    logger.error("error message")
    logger.critical("critical message")

Notice that the “application” code does not care about multiple handlers. All that changed was the addition and configuration of a new handler named fh.

The ability to create new handlers with higher- or lower-severity filters can be very helpful when writing and testing an application. Instead of using many `print` statements for debugging, use `logger.debug`: Unlike the print statements, which you will have to delete or comment out later, the logger.debug statements can remain intact in the source code and remain dormant until you need them again. At that time, the only change that needs to happen is to modify the severity level of the logger and/or handler to debug.

### Using logging in multiple modules

It was mentioned above that multiple calls to `logging.getLogger('someLogger')` return a reference to the same logger object. This is true not only within the same module, but also across modules as long as it is in the same Python interpreter process. It is true for references to the same object; additionally, application code can define and configure a parent logger in one module and create (but not configure) a child logger in a separate module, and all logger calls to the child will pass up to the parent. Here is a main module:

    import logging
    import auxiliary_module

    # create logger with "spam_application"
    logger = logging.getLogger("spam_application")
    logger.setLevel(logging.DEBUG)
    # create file handler which logs even debug messages
    fh = logging.FileHandler("spam.log")
    fh.setLevel(logging.DEBUG)
    # create console handler with a higher log level
    ch = logging.StreamHandler()
    ch.setLevel(logging.ERROR)
    # create formatter and add it to the handlers
    formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
    fh.setFormatter(formatter)
    ch.setFormatter(formatter)
    # add the handlers to the logger
    logger.addHandler(fh)
    logger.addHandler(ch)

    logger.info("creating an instance of auxiliary_module.Auxiliary")
    a = auxiliary_module.Auxiliary()
    logger.info("created an instance of auxiliary_module.Auxiliary")
    logger.info("calling auxiliary_module.Auxiliary.do_something")
    a.do_something()
    logger.info("finished auxiliary_module.Auxiliary.do_something")
    logger.info("calling auxiliary_module.some_function()")
    auxiliary_module.some_function()
    logger.info("done with auxiliary_module.some_function()")

Here is the auxiliary module:

    import logging

    # create logger
    module_logger = logging.getLogger("spam_application.auxiliary")

    class Auxiliary:
        def __init__(self):
            self.logger = logging.getLogger("spam_application.auxiliary.Auxiliary")
            self.logger.info("creating an instance of Auxiliary")
        def do_something(self):
            self.logger.info("doing something")
            a = 1 + 1
            self.logger.info("done doing something")

    def some_function():
        module_logger.info("received a call to \"some_function\"")

The output looks like this:

    2005-03-23 23:47:11,663 - spam_application - INFO -
       creating an instance of auxiliary_module.Auxiliary
    2005-03-23 23:47:11,665 - spam_application.auxiliary.Auxiliary - INFO -
       creating an instance of Auxiliary
    2005-03-23 23:47:11,665 - spam_application - INFO -
       created an instance of auxiliary_module.Auxiliary
    2005-03-23 23:47:11,668 - spam_application - INFO -
       calling auxiliary_module.Auxiliary.do_something
    2005-03-23 23:47:11,668 - spam_application.auxiliary.Auxiliary - INFO -
       doing something
    2005-03-23 23:47:11,669 - spam_application.auxiliary.Auxiliary - INFO -
       done doing something
    2005-03-23 23:47:11,670 - spam_application - INFO -
       finished auxiliary_module.Auxiliary.do_something
    2005-03-23 23:47:11,671 - spam_application - INFO -
       calling auxiliary_module.some_function()
    2005-03-23 23:47:11,672 - spam_application.auxiliary - INFO -
       received a call to "some_function"
    2005-03-23 23:47:11,673 - spam_application - INFO -
       done with auxiliary_module.some_function()

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