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The try statement

The try statement specifies exception handlers and/or cleanup code for a group of statements:

Syntax (simplified):

try:
    suite
[except [expression [, target]] :
    suite ]…
[else:
    suite ]
[finally:
    suite ]

Changed in version 2.5: In previous versions of Python, try…except…finally did not work. try…except had to be nested in try…finally.

The except clause(s) specify one or more exception handlers. When no exception occurs in the try clause, no exception handler is executed. When an exception occurs in the try suite, a search for an exception handler is started. This search inspects the except clauses in turn until one is found that matches the exception. An expression-less except clause, if present, must be last; it matches any exception. For an except clause with an expression, that expression is evaluated, and the clause matches the exception if the resulting object is compatible with the exception. An object is compatible with an exception if it is the class or a base class of the exception object, a tuple containing an item compatible with the exception, or, in the (deprecated) case of string exceptions, is the raised string itself (note that the object identities must match, i.e. it must be the same string object, not just a string with the same value).

If no except clause matches the exception, the search for an exception handler continues in the surrounding code and on the invocation stack.

If the evaluation of an expression in the header of an except clause raises an exception, the original search for a handler is canceled and a search starts for the new exception in the surrounding code and on the call stack (it is treated as if the entire try statement raised the exception).

When a matching except clause is found, the exception is assigned to the target specified in that except clause, if present, and the except clause’s suite is executed. All except clauses must have an executable block. When the end of this block is reached, execution continues normally after the entire try statement. (This means that if two nested handlers exist for the same exception, and the exception occurs in the try clause of the inner handler, the outer handler will not handle the exception.)

Before an except clause’s suite is executed, details about te exception are assigned to three variables in the sys module: sys.exc_type receives the object identifying the exception; sys.exc_value receives the exception’s parameter; sys.exc_traceback receives a traceback object identifying the point in the program where the exception occurred. These details are also available through the sys.exc_info function, which returns a tuple (exc_type, exc_value, exc_traceback). Use of the corresponding variables is deprecated in favor of this function, since their use is unsafe in a threaded program. As of Python 1.5, the variables are restored to their previous values (before the call) when returning from a function that handled an exception.

The optional else clause is executed if and when control flows off the end of the try clause. Exceptions in the else clause are not handled by the preceding except clauses.

If finally is present, it specifies a cleanup handler. The try clause is executed, including any except and else clauses. If an exception occurs in any of the clauses and is not handled, the exception is temporarily saved. The finally clause is executed. If there is a saved exception, it is re-raised at the end of the finally clause. If the finally clause raises another exception or executes a return or break statement, the saved exception is lost. A continue statement is illegal in the finally clause. (The reason is a problem with the current implementation - this restriction may be lifted in the future). The exception information is not available to the program during execution of the finally clause.

When a return, break or continue statement is executed in the try suite of a try-finally statement, the finally clause is also executed on the way out. A continue statement is illegal in the finally clause. (The reason is a problem with the current implementation — this restriction may be lifted in the future).

Additional information on exceptions can be found in exceptions, and information on using the raise statement to generate exceptions may be found in raise.


Footnotes

… stack.7.1 The exception is propogated to the invocation stack only if there is no finally clause that negates the exception. … clause.7.2 Currently, control “flows off the end” except in the case of an exception or the execution of a return, continue, or break statement.