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Groups > comp.lang.python > #94997
| Newsgroups | comp.lang.python |
|---|---|
| Date | 2015-08-05 00:03 -0700 |
| References | <ebf0a31e-1021-41d2-affb-8318838a464b@googlegroups.com> |
| Message-ID | <631b480d-497a-4451-9577-2788f9d4c85b@googlegroups.com> (permalink) |
| Subject | Re: GOTCHA with list comprehension |
| From | Pavel S <pavel@schon.cz> |
It seems this is allowed by the grammar:
list_display ::= "[" [expression_list | list_comprehension] "]"
list_comprehension ::= expression list_for
list_for ::= "for" target_list "in" old_expression_list [list_iter]
old_expression_list ::= old_expression [("," old_expression)+ [","]]
old_expression ::= or_test | old_lambda_expr
list_iter ::= list_for | list_if
list_if ::= "if" old_expression [list_iter]
So chaining multiple ifs is fine:
[ i for i in range(10) if True if True if True if True ]
Dne středa 5. srpna 2015 8:49:20 UTC+2 Pavel S napsal(a):
> Hi,
>
> I recently found interesting GOTCHA while doing list comprehension in python 2.6:
>
> >>> values = ( True, False, 1, 2, 3, None )
> >>> [ value for value in values if value if not None ]
> [True, 1, 2, 3]
>
> I was wondering why this list comprehension returns incorrect results and finally found a typo in the condition. The typo wasn't visible at the first look.
>
> My intention was: if value is not None
> But I wrote: if value if not None
>
> Is that a language feature of list comprehension that it accepts conditions like: if A if B if C if D ...?
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GOTCHA with list comprehension Pavel S <pavel@schon.cz> - 2015-08-04 23:48 -0700
Re: GOTCHA with list comprehension Pavel S <pavel@schon.cz> - 2015-08-05 00:03 -0700
Re: GOTCHA with list comprehension Chris Angelico <rosuav@gmail.com> - 2015-08-05 17:20 +1000
Re: GOTCHA with list comprehension Peter Otten <__peter__@web.de> - 2015-08-05 09:04 +0200
Re: GOTCHA with list comprehension Chris Angelico <rosuav@gmail.com> - 2015-08-05 17:05 +1000
Re: GOTCHA with list comprehension Pavel S <pavel@schon.cz> - 2015-08-05 00:10 -0700
Re: GOTCHA with list comprehension Chris Angelico <rosuav@gmail.com> - 2015-08-05 17:21 +1000
Re: GOTCHA with list comprehension Pavel S <pavel@schon.cz> - 2015-08-05 00:33 -0700
Re: GOTCHA with list comprehension Marko Rauhamaa <marko@pacujo.net> - 2015-08-05 12:01 +0300
Re: GOTCHA with list comprehension Chris Angelico <rosuav@gmail.com> - 2015-08-05 19:52 +1000
Re: GOTCHA with list comprehension Marko Rauhamaa <marko@pacujo.net> - 2015-08-05 14:10 +0300
Re: GOTCHA with list comprehension Saran Ahluwalia <ahlusar.ahluwalia@gmail.com> - 2015-08-07 09:08 -0400
Re: GOTCHA with list comprehension Laura Creighton <lac@openend.se> - 2015-08-06 06:39 +0200
Re: GOTCHA with list comprehension Chris Angelico <rosuav@gmail.com> - 2015-08-06 14:44 +1000
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