Re: GOTCHA with list comprehension

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Date	Wed, 5 Aug 2015 17:05:49 +1000
Subject	Re: GOTCHA with list comprehension
From	Chris Angelico <rosuav@gmail.com>
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On Wed, Aug 5, 2015 at 4:48 PM, Pavel S <pavel@schon.cz> 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 ...?

It certainly is. You can chain 'for' and 'if' clauses as much as you
like, and they behave exactly the way you'd expect. You might possibly
get a warning from a linter with your code, though, as it has an
always-true condition ("if not None" can never be false), so it's
possible something might hint at what's going on; but other than that,
all you can do is test stuff and see if it's giving the right result.

Incidentally, why Python 2.6? Python 2.7 has been out for a pretty
long time now, and if you can't move to version 3.x, I would at least
recommend using 2.7. Since the release of 2.6.9 back before Frozen
came out, that branch has been completely unmaintained. Grab yourself
a 2.7 and take advantage of some neat new features (for old values of
"new"), and improved compatibility with 3.x.

ChrisA

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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