
Errors and Exceptions
*********************

Until now error messages haven't been more than mentioned, but if you
have tried out the examples you have probably seen some.  There are
(at least) two distinguishable kinds of errors: *syntax errors* and
*exceptions*.


Syntax Errors
=============

Syntax errors, also known as parsing errors, are perhaps the most
common kind of complaint you get while you are still learning Python:

   >>> while True print('Hello world')
     File "<stdin>", line 1, in ?
       while True print('Hello world')
                      ^
   SyntaxError: invalid syntax

The parser repeats the offending line and displays a little 'arrow'
pointing at the earliest point in the line where the error was
detected.  The error is caused by (or at least detected at) the token
*preceding* the arrow: in the example, the error is detected at the
function ``print()``, since a colon (``':'``) is missing before it.
File name and line number are printed so you know where to look in
case the input came from a script.


Exceptions
==========

Even if a statement or expression is syntactically correct, it may
cause an error when an attempt is made to execute it. Errors detected
during execution are called *exceptions* and are not unconditionally
fatal: you will soon learn how to handle them in Python programs.
Most exceptions are not handled by programs, however, and result in
error messages as shown here:

   >>> 10 * (1/0)
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   ZeroDivisionError: int division or modulo by zero
   >>> 4 + spam*3
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   NameError: name 'spam' is not defined
   >>> '2' + 2
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   TypeError: coercing to Unicode: need string or buffer, int found

The last line of the error message indicates what happened. Exceptions
come in different types, and the type is printed as part of the
message: the types in the example are ``ZeroDivisionError``,
``NameError`` and ``TypeError``. The string printed as the exception
type is the name of the built-in exception that occurred.  This is
true for all built-in exceptions, but need not be true for user-
defined exceptions (although it is a useful convention). Standard
exception names are built-in identifiers (not reserved keywords).

The rest of the line provides detail based on the type of exception
and what caused it.

The preceding part of the error message shows the context where the
exception happened, in the form of a stack traceback. In general it
contains a stack traceback listing source lines; however, it will not
display lines read from standard input.

*Built-in Exceptions* lists the built-in exceptions and their
meanings.


Handling Exceptions
===================

It is possible to write programs that handle selected exceptions. Look
at the following example, which asks the user for input until a valid
integer has been entered, but allows the user to interrupt the program
(using ``Control-C`` or whatever the operating system supports); note
that a user-generated interruption is signalled by raising the
``KeyboardInterrupt`` exception.

   >>> while True:
   ...     try:
   ...         x = int(input("Please enter a number: "))
   ...         break
   ...     except ValueError:
   ...         print("Oops!  That was no valid number.  Try again...")
   ...

The ``try`` statement works as follows.

* First, the *try clause* (the statement(s) between the ``try`` and
  ``except`` keywords) is executed.

* If no exception occurs, the *except clause* is skipped and execution
  of the ``try`` statement is finished.

* If an exception occurs during execution of the try clause, the rest
  of the clause is skipped.  Then if its type matches the exception
  named after the ``except`` keyword, the except clause is executed,
  and then execution continues after the ``try`` statement.

* If an exception occurs which does not match the exception named in
  the except clause, it is passed on to outer ``try`` statements; if
  no handler is found, it is an *unhandled exception* and execution
  stops with a message as shown above.

A ``try`` statement may have more than one except clause, to specify
handlers for different exceptions.  At most one handler will be
executed. Handlers only handle exceptions that occur in the
corresponding try clause, not in other handlers of the same ``try``
statement.  An except clause may name multiple exceptions as a
parenthesized tuple, for example:

   ... except (RuntimeError, TypeError, NameError):
   ...     pass

The last except clause may omit the exception name(s), to serve as a
wildcard. Use this with extreme caution, since it is easy to mask a
real programming error in this way!  It can also be used to print an
error message and then re-raise the exception (allowing a caller to
handle the exception as well):

   import sys

   try:
       f = open('myfile.txt')
       s = f.readline()
       i = int(s.strip())
   except IOError as err:
       print("I/O error: {0}".format(err))
   except ValueError:
       print("Could not convert data to an integer.")
   except:
       print("Unexpected error:", sys.exc_info()[0])
       raise

The ``try`` ... ``except`` statement has an optional *else clause*,
which, when present, must follow all except clauses.  It is useful for
code that must be executed if the try clause does not raise an
exception.  For example:

   for arg in sys.argv[1:]:
       try:
           f = open(arg, 'r')
       except IOError:
           print('cannot open', arg)
       else:
           print(arg, 'has', len(f.readlines()), 'lines')
           f.close()

The use of the ``else`` clause is better than adding additional code
to the ``try`` clause because it avoids accidentally catching an
exception that wasn't raised by the code being protected by the
``try`` ... ``except`` statement.

When an exception occurs, it may have an associated value, also known
as the exception's *argument*. The presence and type of the argument
depend on the exception type.

The except clause may specify a variable after the exception name.
The variable is bound to an exception instance with the arguments
stored in ``instance.args``.  For convenience, the exception instance
defines ``__str__()`` so the arguments can be printed directly without
having to reference ``.args``.  One may also instantiate an exception
first before raising it and add any attributes to it as desired.

   >>> try:
   ...    raise Exception('spam', 'eggs')
   ... except Exception as inst:
   ...    print(type(inst))    # the exception instance
   ...    print(inst.args)     # arguments stored in .args
   ...    print(inst)          # __str__ allows args to be printed directly,
   ...                         # but may be overridden in exception subclasses
   ...    x, y = inst.args     # unpack args
   ...    print('x =', x)
   ...    print('y =', y)
   ...
   <class 'Exception'>
   ('spam', 'eggs')
   ('spam', 'eggs')
   x = spam
   y = eggs

If an exception has arguments, they are printed as the last part
('detail') of the message for unhandled exceptions.

Exception handlers don't just handle exceptions if they occur
immediately in the try clause, but also if they occur inside functions
that are called (even indirectly) in the try clause. For example:

   >>> def this_fails():
   ...     x = 1/0
   ...
   >>> try:
   ...     this_fails()
   ... except ZeroDivisionError as err:
   ...     print('Handling run-time error:', err)
   ...
   Handling run-time error: int division or modulo by zero


Raising Exceptions
==================

The ``raise`` statement allows the programmer to force a specified
exception to occur. For example:

   >>> raise NameError('HiThere')
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   NameError: HiThere

The sole argument to ``raise`` indicates the exception to be raised.
This must be either an exception instance or an exception class (a
class that derives from ``Exception``).

If you need to determine whether an exception was raised but don't
intend to handle it, a simpler form of the ``raise`` statement allows
you to re-raise the exception:

   >>> try:
   ...     raise NameError('HiThere')
   ... except NameError:
   ...     print('An exception flew by!')
   ...     raise
   ...
   An exception flew by!
   Traceback (most recent call last):
     File "<stdin>", line 2, in ?
   NameError: HiThere


User-defined Exceptions
=======================

Programs may name their own exceptions by creating a new exception
class. Exceptions should typically be derived from the ``Exception``
class, either directly or indirectly.  For example:

   >>> class MyError(Exception):
   ...     def __init__(self, value):
   ...         self.value = value
   ...     def __str__(self):
   ...         return repr(self.value)
   ...
   >>> try:
   ...     raise MyError(2*2)
   ... except MyError as e:
   ...     print('My exception occurred, value:', e.value)
   ...
   My exception occurred, value: 4
   >>> raise MyError('oops!')
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
   __main__.MyError: 'oops!'

In this example, the default ``__init__()`` of ``Exception`` has been
overridden.  The new behavior simply creates the *value* attribute.
This replaces the default behavior of creating the *args* attribute.

Exception classes can be defined which do anything any other class can
do, but are usually kept simple, often only offering a number of
attributes that allow information about the error to be extracted by
handlers for the exception.  When creating a module that can raise
several distinct errors, a common practice is to create a base class
for exceptions defined by that module, and subclass that to create
specific exception classes for different error conditions:

   class Error(Exception):
       """Base class for exceptions in this module."""
       pass

   class InputError(Error):
       """Exception raised for errors in the input.

       Attributes:
           expression -- input expression in which the error occurred
           message -- explanation of the error
       """

       def __init__(self, expression, message):
           self.expression = expression
           self.message = message

   class TransitionError(Error):
       """Raised when an operation attempts a state transition that's not
       allowed.

       Attributes:
           previous -- state at beginning of transition
           next -- attempted new state
           message -- explanation of why the specific transition is not allowed
       """

       def __init__(self, previous, next, message):
           self.previous = previous
           self.next = next
           self.message = message

Most exceptions are defined with names that end in "Error," similar to
the naming of the standard exceptions.

Many standard modules define their own exceptions to report errors
that may occur in functions they define.  More information on classes
is presented in chapter *Classes*.


Defining Clean-up Actions
=========================

The ``try`` statement has another optional clause which is intended to
define clean-up actions that must be executed under all circumstances.
For example:

   >>> try:
   ...     raise KeyboardInterrupt
   ... finally:
   ...     print('Goodbye, world!')
   ...
   Goodbye, world!
   Traceback (most recent call last):
     File "<stdin>", line 2, in ?
   KeyboardInterrupt

A *finally clause* is always executed before leaving the ``try``
statement, whether an exception has occurred or not. When an exception
has occurred in the ``try`` clause and has not been handled by an
``except`` clause (or it has occurred in a ``except`` or ``else``
clause), it is re-raised after the ``finally`` clause has been
executed.  The ``finally`` clause is also executed "on the way out"
when any other clause of the ``try`` statement is left via a
``break``, ``continue`` or ``return`` statement.  A more complicated
example:

   >>> def divide(x, y):
   ...     try:
   ...         result = x / y
   ...     except ZeroDivisionError:
   ...         print("division by zero!")
   ...     else:
   ...         print("result is", result)
   ...     finally:
   ...         print("executing finally clause")
   ...
   >>> divide(2, 1)
   result is 2
   executing finally clause
   >>> divide(2, 0)
   division by zero!
   executing finally clause
   >>> divide("2", "1")
   executing finally clause
   Traceback (most recent call last):
     File "<stdin>", line 1, in ?
     File "<stdin>", line 3, in divide
   TypeError: unsupported operand type(s) for /: 'str' and 'str'

As you can see, the ``finally`` clause is executed in any event.  The
``TypeError`` raised by dividing two strings is not handled by the
``except`` clause and therefore re-raised after the ``finally`` clause
has been executed.

In real world applications, the ``finally`` clause is useful for
releasing external resources (such as files or network connections),
regardless of whether the use of the resource was successful.


Predefined Clean-up Actions
===========================

Some objects define standard clean-up actions to be undertaken when
the object is no longer needed, regardless of whether or not the
operation using the object succeeded or failed. Look at the following
example, which tries to open a file and print its contents to the
screen.

   for line in open("myfile.txt"):
       print(line)

The problem with this code is that it leaves the file open for an
indeterminate amount of time after this part of the code has finished
executing. This is not an issue in simple scripts, but can be a
problem for larger applications. The ``with`` statement allows objects
like files to be used in a way that ensures they are always cleaned up
promptly and correctly.

   with open("myfile.txt") as f:
       for line in f:
           print(line)

After the statement is executed, the file *f* is always closed, even
if a problem was encountered while processing the lines. Objects
which, like files, provide predefined clean-up actions will indicate
this in their documentation.
