[Python-bugs-list] [ python-Bugs-431886 ] listcomp syntax too confusing (tuples)

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Bugs item #431886, was opened at 2001-06-08 23:34
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Category: Parser/Compiler
Group: None
Status: Open
Resolution: None
>Priority: 7
Submitted By: Tim Peters (tim_one)
>Assigned to: Guido van Rossum (gvanrossum)
Summary: listcomp syntax too confusing (tuples)

Initial Comment:
We were careful to make sure that tuple targets in 
listcomps required parens, i.e.

[x, x+1 for x in s]   # rejected
[(x, x+1) for x in s] # OK

but we didn't anticipate other "surprise tuple" 
cases.  Most recently from c.l.py,

"""
I tried the one-line command in a interaction mode:
[x for x in [1, 2, 3], y for y in [4, 5, 6]]
and the result surprised me, that is:
[[1,2,3],[1,2,3],[1,2,3],9,9,9]
Who can explain the behavior?
Since I expected the result should be:
[[1,4],[1,5],[1,6],[2,4],...]
"""

This is too surprising; we should require that the 
listcomp be spelled

[x for x in ([1, 2, 3], y) for y in [4, 5, 6]]

instead if that's really what they want (which it 
almost certainly isn't!).

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>Comment By: Tim Peters (tim_one)
Date: 2001-10-14 15:27

Message:
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user_id=31435

Guido, this is easy to fix:

diff -u -r1.44 Grammar
--- Grammar     2001/08/08 05:00:17     1.44
+++ Grammar     2001/10/14 21:57:12
@@ -97,5 +97,5 @@
 argument: [test '='] test      # Really [keyword '='] test

 list_iter: list_for | list_if
-list_for: 'for' exprlist 'in' testlist [list_iter]
+list_for: 'for' exprlist 'in' test [list_iter]
 list_if: 'if' test [list_iter]

test_parser fails then, and parsermodule.c also needs a 
patch.  A combined context diff is attached.

I'm assigning this to you because it's not wholly backward 
compatible; e.g.,

[i**2 for i in 1, 2]

is no longer accepted -- but then that's the point of the 
exercise.  I don't know of any real code that breaks.  Risk 
it?  Forget it?  PEP?  YAFS (yet another future stmt)?

Bumped the priority, because if we *are* going to change 
it, we should do so for 2.2.

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