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 string objects or class instances. Selection of a matching except clause is based on object identity (i.e., two different string objects with the same value represent different exceptions!) For string exceptions, the except clause must reference the same string object. For class exceptions, the except clause must reference the same class or a base class of it.

When an exception is raised, an object (maybe None) is passed as the exception's ``parameter'' or ``value''; this object does not affect the selection of an exception handler, but is passed to the selected exception handler as additional information. For class exceptions, this object must be an instance of the exception class being raised.

See also the description of the try statement in section 7.4 and raise statement in section 6.8.

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