[Python-Dev] Pre-PEP: Unifying try-except and try-finally

[Reinhold Birkenfeld]

Hello,

after proposing this here (albeit deep in the PEP 340 thread) and
getting a somewhat affirmatory response from Guido, I have written
something that could become a PEP if sufficiently hatched...

---------------------------

PEP: XXX
Title: Unifying try-except and try-finally
Version: $Revision: $
Last-Modified: $Date: $
Author: Reinhold Birkenfeld <reinhold-birkenfeld-nospam at wolke7.net>
Status: Draft
Type: Standards Track
Content-Type: text/plain
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 occured'
        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] |
                   'try' ':' suite (except_clause ':' suite)*
                   ['else' ':' 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.


References

    None yet.


Copyright

    This document has been placed in the public domain.


-----------------------
Reinhold


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