Line-continuation "Anti-Idiom" and with statement

Terry Reedy tjreedy at udel.edu
Tue Nov 24 00:40:33 CET 2009


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

Right. The first open paren is illegal.

> 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:

with open(roster_path, 'r') as roster_file, open(
     disb_path, 'w') as out_file, open(report_path, 'w') as report_file:

would work ;-)

> 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?

I would suggest replacing
"It is usually much better to use the implicit continuation inside 
parenthesis:", <should be parentheses>
which says both too much and too little, with something like
"When the desired linebreak is within an expression, it is usually much 
better to use implicit continuation inside parentheses, brackets, or 
braces. If necessary, the whole expression can be surrounded by a new 
pair of parentheses."

I would not mention 'with' specifically since there are many other 
non-expression contexts where one cannot randomly add parentheses.

I believe that '\ \n' would always be harmless or a SyntexError outside 
of expressons. I believe 'subtly wrong' only applies within expressions. 
  So I would not call \ continuation an anti-pattern outside 
expressions. So you might suggest that the whole entry specify 
expression context to begin with. To me, your example shows why blanket 
condemnation is wrong.

You might separately request that with-item sequences be optionally 
surrounded by parens, but I have a suspicion that this was considered 
and rejected on either technical grounds and/or the grounds that \ 
continuation was purposefully left in the language in 3.x to be used 
when implicit continuation is not appropriate, as in this situation.

The HOWTOs are not scripture.

Terry Jan Reedy







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