[Patches] [ python-Patches-1492509 ] Unification of list-comp and for syntax

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Patches item #1492509, was opened at 2006-05-21 17:06
Message generated for change (Comment added) made by hwundram
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Category: Core (C code)
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Submitted By: Heiko Wundram (hwundram)
Assigned to: Nobody/Anonymous (nobody)
Summary: Unification of list-comp and for syntax

Initial Comment:
The following patch adds the ability for:

for <expr> in <expr> if <expr>:
    <do something>

to the Python core. This unifies the syntax of
list/generator comprehensions and the for statement
somewhat, because both now accept conditions which
produce an immediate continue.

I've posted a PEP to python-dev, which details the
changes this patch makes (which are all
backwards-compatible).

The patch doesn't try to address more than the actual
code required to make this feature work yet (except for
changes to Modules/parsermodule.c and Doc/ref/ref7.tex,
which details the for statement). If there's consensus
on this feature, I'll gladly produce more documentation.

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>Comment By: Heiko Wundram (hwundram)
Date: 2006-05-24 08:10

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Sure, you can wrap the iterable, or you can even do:

if x in y:
    if not x:
        continue
    ...

or

if x in y:
    if x:
        ...

without using any form of "iterator magic". Read my PEP-xxx
on py-dev, and my explanation there of why I think this is a
"good thing"(TM), but I won't go explain it here again,
because generally people have told be to drop it.

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Comment By: Jim Jewett (jimjjewett)
Date: 2006-05-23 22:14

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It seems I misread what the intent was -- I was thinking of 
the if as guarding the entire for loop, not just a single 
iteration.

Because of this confusion, I have to be -1.  

Is there a reason you can't just wrap your iterable 
sequence with another iterator?

for x in (candidate for candidate in fullseq if test):



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Comment By: Jim Jewett (jimjjewett)
Date: 2006-05-23 21:53

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I'm not loving the interaction with conditional expressions.

for x in (1,2,3) if test else (3,2,1):

I suppose this techically isn't ambiguous because else is a 
keyword.

On the other hand, you could do it now using he if-else

for x in real_seq if test else ():



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