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Re: GOTCHA with list comprehension

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Pavel S wrote:

> 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 ...?

I think it's just that a condition may be a constant expression. Python 
evaluates (not None) for every item in values. Other variants:

>>> if 42:
...     print("branch always taken")
... 
branch always taken
>>> always_yes = "yes" if True else "no"
>>> always_yes
'yes'
>>> [c for c in "foo" if "true in a boolean context"]
['f', 'o', 'o']

An optimizer might detect that (not None) is always True in a boolean 
context, but that would be an implementation detail.

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Thread

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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