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| 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] https://bugs.python.org/issue1355913 | |
| .. [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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