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4.2 Exceptions

Exceptions are a means of breaking out of the normal flow of control of a code block in order to handle errors or other exceptional conditions. An exception is raised at the point where the error is detected; it may be handled by the surrounding code block or by any code block that directly or indirectly invoked the code block where the error occurred.

The Python interpreter raises an exception when it detects a run-time error (such as division by zero). A Python program can also explicitly raise an exception with the `raise` statement. Exception handlers are specified with the `try` ... `except` statement. The `try` ... `finally` statement specifies cleanup code which does not handle the exception, but is executed whether an exception occurred or not in the preceding code.

Python uses the ``termination'' model of error handling: an exception handler can find out what happened and continue execution at an outer level, but it cannot repair the cause of the error and retry the failing operation (except by re-entering the offending piece of code from the top).

When an exception is not handled at all, the interpreter terminates execution of the program, or returns to its interactive main loop. In either case, it prints a stack backtrace, except when the exception is `SystemExit`.

Exceptions are identified by class instances. The `except` clause is selected depending on the class of the instance: it must reference the class of the instance or a base class thereof. The instance can be received by the handler and can carry additional information about the exceptional condition.

Exceptions can also be identified by strings, in which case the `except` clause is selected by object identity. An arbitrary value can be raised along with the identifying string which can be passed to the handler.

Deprecated since release 2.5. String exceptions should not be used in new code. They will not be supported in a future version of Python. Old code should be rewritten to use class exceptions instead.

Warning: Messages to exceptions are not part of the Python API. Their contents may change from one version of Python to the next without warning and should not be relied on by code which will run under multiple versions of the interpreter.

See also the description of the `try` statement in section 7.4 and `raise` statement in section 6.9.