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# 20.21. `http.server` — HTTP servers

This module defines classes for implementing HTTP servers (Web servers).

One class, `HTTPServer`, is a `socketserver.TCPServer` subclass. It creates and listens at the HTTP socket, dispatching the requests to a handler. Code to create and run the server looks like this:

    def run(server_class=HTTPServer, handler_class=BaseHTTPRequestHandler):
        server_address = ('', 8000)
        httpd = server_class(server_address, handler_class)
        httpd.serve_forever()

class `http.server.``HTTPServer`(server_address, RequestHandlerClass)
  This class builds on the `TCPServer` class by storing the server address as instance variables named `server_name` and `server_port`. The server is accessible by the handler, typically through the handler’s `server` instance variable.

The `HTTPServer` must be given a RequestHandlerClass on instantiation, of which this module provides three different variants:

class `http.server.``BaseHTTPRequestHandler`(request, client_address, server)

  This class is used to handle the HTTP requests that arrive at the server. By itself, it cannot respond to any actual HTTP requests; it must be subclassed to handle each request method (e.g. GET or POST). `BaseHTTPRequestHandler` provides a number of class and instance variables, and methods for use by subclasses.

  The handler will parse the request and the headers, then call a method specific to the request type. The method name is constructed from the request. For example, for the request method `SPAM`, the `do_SPAM()` method will be called with no arguments. All of the relevant information is stored in instance variables of the handler. Subclasses should not need to override or extend the `__init__()` method.

  `BaseHTTPRequestHandler` has the following instance variables:

  `client_address`
    Contains a tuple of the form `(host, port)` referring to the client’s address.
  `server`
    Contains the server instance.
  `command`
    Contains the command (request type). For example, `'GET'`.
  `path`
    Contains the request path.
  `request_version`
    Contains the version string from the request. For example, `'HTTP/1.0'`.
  `headers`
    Holds an instance of the class specified by the `MessageClass` class variable. This instance parses and manages the headers in the HTTP request.
  `rfile`
    Contains an input stream, positioned at the start of the optional input data.
  `wfile`
    Contains the output stream for writing a response back to the client. Proper adherence to the HTTP protocol must be used when writing to this stream.

  `BaseHTTPRequestHandler` has the following class variables:

  `server_version`
    Specifies the server software version. You may want to override this. The format is multiple whitespace-separated strings, where each string is of the form name[/version]. For example, `'BaseHTTP/0.2'`.
  `sys_version`
    Contains the Python system version, in a form usable by the `version_string` method and the `server_version` class variable. For example, `'Python/1.4'`.
  `error_message_format`
    Specifies a format string for building an error response to the client. It uses parenthesized, keyed format specifiers, so the format operand must be a dictionary. The code key should be an integer, specifying the numeric HTTP error code value. message should be a string containing a (detailed) error message of what occurred, and explain should be an explanation of the error code number. Default message and explain values can found in the responses class variable.
  `error_content_type`
    Specifies the Content-Type HTTP header of error responses sent to the client. The default value is `'text/html'`.
  `protocol_version`
    This specifies the HTTP protocol version used in responses. If set to `'HTTP/1.1'`, the server will permit HTTP persistent connections; however, your server must then include an accurate `Content-Length` header (using `send_header()`) in all of its responses to clients. For backwards compatibility, the setting defaults to `'HTTP/1.0'`.
  `MessageClass`
    Specifies an `email.message.Message`-like class to parse HTTP headers. Typically, this is not overridden, and it defaults to `http.client.HTTPMessage`.
  `responses`
    This variable contains a mapping of error code integers to two-element tuples containing a short and long message. For example, `{code: (shortmessage, longmessage)}`. The shortmessage is usually used as the message key in an error response, and longmessage as the explain key (see the `error_message_format` class variable).

  A `BaseHTTPRequestHandler` instance has the following methods:

  `handle`()
    Calls `handle_one_request()` once (or, if persistent connections are enabled, multiple times) to handle incoming HTTP requests. You should never need to override it; instead, implement appropriate `do_*()` methods.
  `handle_one_request`()
    This method will parse and dispatch the request to the appropriate `do_*()` method. You should never need to override it.
  `send_error`(code[, message])
    Sends and logs a complete error reply to the client. The numeric code specifies the HTTP error code, with message as optional, more specific text. A complete set of headers is sent, followed by text composed using the `error_message_format` class variable.
  `send_response`(code[, message])
    Sends a response header and logs the accepted request. The HTTP response line is sent, followed by Server and Date headers. The values for these two headers are picked up from the `version_string()` and `date_time_string()` methods, respectively.
  `send_header`(keyword, value)
    Writes a specific HTTP header to the output stream. keyword should specify the header keyword, with value specifying its value.
  `end_headers`()
    Sends a blank line, indicating the end of the HTTP headers in the response.
  `log_request`([code[, size]])
    Logs an accepted (successful) request. code should specify the numeric HTTP code associated with the response. If a size of the response is available, then it should be passed as the size parameter.
  `log_error`(...)
    Logs an error when a request cannot be fulfilled. By default, it passes the message to `log_message()`, so it takes the same arguments (format and additional values).
  `log_message`(format, ...)
    Logs an arbitrary message to `sys.stderr`. This is typically overridden to create custom error logging mechanisms. The format argument is a standard printf-style format string, where the additional arguments to `log_message()` are applied as inputs to the formatting. The client address and current date and time are prefixed to every message logged.
  `version_string`()
    Returns the server software’s version string. This is a combination of the `server_version` and `sys_version` class variables.
  `date_time_string`([timestamp])

    Returns the date and time given by timestamp (which must be in the format returned by `time.time()`), formatted for a message header. If timestamp is omitted, it uses the current date and time.

    The result looks like `'Sun, 06 Nov 1994 08:49:37 GMT'`.

  `log_date_time_string`()
    Returns the current date and time, formatted for logging.
  `address_string`()
    Returns the client address, formatted for logging. A name lookup is performed on the client’s IP address.

class `http.server.``SimpleHTTPRequestHandler`(request, client_address, server)

  This class serves files from the current directory and below, directly mapping the directory structure to HTTP requests.

  A lot of the work, such as parsing the request, is done by the base class `BaseHTTPRequestHandler`. This class implements the `do_GET()` and `do_HEAD()` functions.

  The following are defined as class-level attributes of `SimpleHTTPRequestHandler`:

  `server_version`
    This will be `"SimpleHTTP/" + __version__`, where `__version__` is defined at the module level.
  `extensions_map`
    A dictionary mapping suffixes into MIME types. The default is signified by an empty string, and is considered to be `application/octet-stream`. The mapping is used case-insensitively, and so should contain only lower-cased keys.

  The `SimpleHTTPRequestHandler` class defines the following methods:

  `do_HEAD`()
    This method serves the `'HEAD'` request type: it sends the headers it would send for the equivalent `GET` request. See the `do_GET()` method for a more complete explanation of the possible headers.
  `do_GET`()

    The request is mapped to a local file by interpreting the request as a path relative to the current working directory.

    If the request was mapped to a directory, the directory is checked for a file named `index.html` or `index.htm` (in that order). If found, the file’s contents are returned; otherwise a directory listing is generated by calling the `list_directory()` method. This method uses `os.listdir()` to scan the directory, and returns a `404` error response if the `listdir()` fails.

    If the request was mapped to a file, it is opened and the contents are returned. Any `IOError` exception in opening the requested file is mapped to a `404`, `'File not found'` error. Otherwise, the content type is guessed by calling the `guess_type()` method, which in turn uses the extensions_map variable.

    A `'Content-type:'` header with the guessed content type is output, followed by a `'Content-Length:'` header with the file’s size and a `'Last-Modified:'` header with the file’s modification time.

    Then follows a blank line signifying the end of the headers, and then the contents of the file are output. If the file’s MIME type starts with `text/` the file is opened in text mode; otherwise binary mode is used.

    For example usage, see the implementation of the `test()` function.

class `http.server.``CGIHTTPRequestHandler`(request, client_address, server)

  This class is used to serve either files or output of CGI scripts from the current directory and below. Note that mapping HTTP hierarchic structure to local directory structure is exactly as in `SimpleHTTPRequestHandler`.

  Note

  CGI scripts run by the `CGIHTTPRequestHandler` class cannot execute redirects (HTTP code 302), because code 200 (script output follows) is sent prior to execution of the CGI script. This pre-empts the status code.

  The class will however, run the CGI script, instead of serving it as a file, if it guesses it to be a CGI script. Only directory-based CGI are used — the other common server configuration is to treat special extensions as denoting CGI scripts.

  The `do_GET()` and `do_HEAD()` functions are modified to run CGI scripts and serve the output, instead of serving files, if the request leads to somewhere below the `cgi_directories` path.

  The `CGIHTTPRequestHandler` defines the following data member:

  `cgi_directories`
    This defaults to `['/cgi-bin', '/htbin']` and describes directories to treat as containing CGI scripts.

  The `CGIHTTPRequestHandler` defines the following method:

  `do_POST`()
    This method serves the `'POST'` request type, only allowed for CGI scripts. Error 501, “Can only POST to CGI scripts”, is output when trying to POST to a non-CGI url.

  Note that CGI scripts will be run with UID of user nobody, for security reasons. Problems with the CGI script will be translated to error 403.

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