7.4 The try statement
The try statement specifies exception handlers and/or cleanup code for a group of statements:
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try_stmt ::= try_exc_stmt | try_fin_stmt try_exc_stmt ::= "try" ":" suite ("except" [expression ["," target]] ":" suite)+["else" ":" suite]try_fin_stmt ::= "try" ":" suite "finally" ":" suite
There are two forms of try statement: try...except and try...finally. These forms cannot be mixed (but they can be nested in each other).
The try...except form specifies one or more exception handlers (the except clauses). 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 either the object that identifies the exception, or (for exceptions that are classes) it is a base class of the exception, or it is a tuple containing an item that is compatible with the exception. Note that the object identities must match, i.e. it must be the same object, not just an object 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's parameter 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 the
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 (see section 3.2)
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.7.1 Exceptions in the else clause are not handled by the preceding except clauses.
The try...finally form specifies a `cleanup' handler. The try clause is executed. When no exception occurs, the finally clause is executed. When an exception occurs in the try clause, the exception is temporarily saved, the finally clause is executed, and then the saved exception is re-raised. 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 section 4.2, and information on using the raise statement to generate exceptions may be found in section 6.9.
Footnotes
- Currently, control ``flows off the end'' except in the case of an exception or the execution of a return, continue, or break statement.
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Release 2.4, documentation updated on 29 November 2004.