Line-continuation "Anti-Idiom" and with statement

Terry Reedy tjreedy at udel.edu
Tue Nov 24 02:09:18 CET 2009


MRAB wrote:
> Neil Cerutti wrote:
>  > I installed Python 3.1 today, and I've been porting my small
>  > library of programs to the new system.
>  >
>  > I happened to read the interesting "Idioms and Anti-Idioms"
>  > HOWTO, and saw the '\' continuation character labeled an
>  > anti-idiom. I already generally avoided it, so I just nodded.
>  >
>  > Unfortunately, the new "nested" with statement (which I also read
>  > about today) forces me into this anti-idiom. When replacing an
>  > appearance of contextlib.nested with the 3K with statement, I
>  > ended up with an unexpected syntax error.
>  >
>  > with (open(roster_path, 'r') as roster_file,
>  >       open(disb_path, 'w') as out_file,
>  >       open(report_path, 'w') as report_file):
>  >
>  > The result was:
>  >
>  >   File "C:\project\codxml.py", line 184
>  >     with (open(roster_path, 'r') as roster_file,
>  >                                   ^
>  > SyntaxError: invalid syntax
>  >
>  > Unless I'm missing something, I have to subject my code to a bit
>  > of contintuation:
>  >
>  > with open(roster_path, 'r') as roster_file,\
>  >      open(disb_path, 'w') as out_file,\
>  >      open(report_path, 'w') as report_file:
>  >
>  > I was thinking about submitting a enhancement request to the
>  > HOWTO, explaining that continuation my be required in this
>  > context. Am I missing anything obvious?
>  >
> The relevant syntax is:
> 
>     with_stmt ::= "with" with_item ("," with_item)* ":" suite
> 
>     with_item ::= expression ["as" target]
> 
> Parentheses only work around an expression or when they are expected by
> the syntax, eg in function calls.
> 
> In this case it sees the '(' and thinks that it's the start of a
> parenthesised expression. It parses the expression, then sees 'as',
> hence a SyntaxError. I suppose it could've complained about a missing
> ')' instead.
> 
> I wonder whether an end-of-line could be ignored after a comma in a
> 'with' statement, and, for consistency, in any similar cases.

Then a line ending with ', ' would be different from one ending with 
',', useless space and tabs at ends of lines were *always* ignored. Not 
sure whether that would create problems though.




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