124 lines
2.9 KiB
Plaintext
124 lines
2.9 KiB
Plaintext
PEP: 341
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Title: Unifying try-except and try-finally
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Version: $Revision$
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Last-Modified: $Date$
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Author: Georg Brandl <g.brandl@gmx.net>
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Status: Final
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Type: Standards Track
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Content-Type: text/plain
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Created: 04-May-2005
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Post-History:
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Abstract
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This PEP proposes a change in the syntax and semantics of try
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statements to allow combined try-except-finally blocks. This
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means in short that it would be valid to write
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try:
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<do something>
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except Exception:
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<handle the error>
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finally:
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<cleanup>
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Rationale/Proposal
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There are many use cases for the try-except statement and
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for the try-finally statement per se; however, often one needs
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to catch exceptions and execute some cleanup code afterwards.
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It is slightly annoying and not very intelligible that
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one has to write
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f = None
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try:
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try:
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f = open(filename)
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text = f.read()
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except IOError:
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print 'An error occured'
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finally:
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if f:
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f.close()
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So it is proposed that a construction like this
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try:
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<suite 1>
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except Ex1:
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<suite 2>
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<more except: clauses>
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else:
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<suite 3>
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finally:
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<suite 4>
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be exactly the same as the legacy
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try:
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try:
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<suite 1>
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except Ex1:
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<suite 2>
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<more except: clauses>
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else:
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<suite 3>
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finally:
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<suite 4>
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This is backwards compatible, and every try statement that is
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legal today would continue to work.
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Changes to the grammar
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The grammar for the try statement, which is currently
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try_stmt: ('try' ':' suite (except_clause ':' suite)+
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['else' ':' suite] | 'try' ':' suite 'finally' ':' suite)
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would have to become
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try_stmt: 'try' ':' suite
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(
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(except_clause ':' suite)+
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['else' ':' suite]
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['finally' ':' suite]
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'finally' ':' suite
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)
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Implementation
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As the PEP author currently does not have sufficient knowledge
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of the CPython implementation, he is unfortunately not able
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to deliver one. Thomas Lee has submitted a patch[2].
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However, according to Guido, it should be a piece of cake to
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implement[1] -- at least for a core hacker.
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This patch was commited 17 December 2005, SVN revision 41740 [3].
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References
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[1] http://mail.python.org/pipermail/python-dev/2005-May/053319.html
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[2] http://python.org/sf/1355913
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[3] http://mail.python.org/pipermail/python-checkins/2005-December/048457.html
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Copyright
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This document has been placed in the public domain.
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