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

Copyright

This document has been placed in the public domain.