python-peps/pep-0341.txt

PEP: 341
Title: Unifying try-except and try-finally
Version: $Revision$
Last-Modified: $Date$
Author: Georg Brandl <georg@python.org>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 04-May-2005
Post-History:


Abstract
========

This PEP proposes a change in the syntax and semantics of try
statements to allow combined try-except-finally blocks. This
means in short that it would be valid to write::

    try:
        <do something>
    except Exception:
        <handle the error>
    finally:
        <cleanup>


Rationale/Proposal
==================

There are many use cases for the try-except statement and
for the try-finally statement per se; however, often one needs
to catch exceptions and execute some cleanup code afterwards.
It is slightly annoying and not very intelligible that
one has to write::

    f = None
    try:
        try:
            f = open(filename)
            text = f.read()
        except IOError:
            print 'An error occurred'
    finally:
        if f:
            f.close()

So it is proposed that a construction like this::

    try:
        <suite 1>
    except Ex1:
        <suite 2>
    <more except: clauses>
    else:
        <suite 3>
    finally:
        <suite 4>

be exactly the same as the legacy::

    try:
        try:
            <suite 1>
        except Ex1:
            <suite 2>
        <more except: clauses>
        else:
            <suite 3>
    finally:
        <suite 4>

This is backwards compatible, and every try statement that is
legal today would continue to work.


Changes to the grammar
======================

The grammar for the try statement, which is currently::

    try_stmt: ('try' ':' suite (except_clause ':' suite)+
            ['else' ':' suite] | 'try' ':' suite 'finally' ':' suite)

would have to become::

    try_stmt: 'try' ':' suite
            (
                (except_clause ':' suite)+
                ['else' ':' suite]
                ['finally' ':' suite]
            |
                'finally' ':' suite
            )


Implementation
==============

As the PEP author currently does not have sufficient knowledge
of the CPython implementation, he is unfortunately not able
to deliver one.  Thomas Lee has submitted a patch [2]_.

However, according to Guido, it should be a piece of cake to
implement [1]_ -- at least for a core hacker.

This patch was committed 17 December 2005, SVN revision 41740 [3]_.


References
==========

.. [1] https://mail.python.org/pipermail/python-dev/2005-May/053319.html
.. [2] http://python.org/sf/1355913
.. [3] https://mail.python.org/pipermail/python-checkins/2005-December/048457.html


Copyright
=========

This document has been placed in the public domain.



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