PEP 341 – Unifying try-except and try-finally
- Georg Brandl <georg at python.org>
- Standards Track
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>
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 )
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 .
However, according to Guido, it should be a piece of cake to implement  – at least for a core hacker.
This patch was committed 17 December 2005, SVN revision 41740 .
This document has been placed in the public domain.
Last modified: 2018-07-21 23:57:17 GMT