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(python2.1-ref.info)try statement

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The `try' statement
===================

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

     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 Note: standard type
hierarchy) 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.(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).

---------- Footnotes ----------

(1)  Currently, control "flows off the end" except in the case of an
exception or the execution of a `return', `continue', or `break'
statement.