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Groups > comp.lang.python > #99947

Re: filter a list of strings

Path csiph.com!news.swapon.de!fu-berlin.de!uni-berlin.de!not-for-mail
From Chris Angelico <rosuav@gmail.com>
Newsgroups comp.lang.python
Subject Re: filter a list of strings
Date Thu, 3 Dec 2015 20:40:51 +1100
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On Thu, Dec 3, 2015 at 8:27 PM,  <c.buhtz@posteo.jp> wrote:
> Thank you for your suggestion. This will help a lot.
>
> On 2015-12-03 08:32 Jussi Piitulainen <harvesting@is.invalid> wrote:
>> list = [ item for item in list
>>          if ( 'Banana' not in item and
>>               'Car' not in item ) ]
>
> I often saw constructions like this
>   x for x in y if ...
> But I don't understand that combination of the Python keywords (for,
> in, if) I allready know. It is to complex to imagine what there really
> happen.
>
> I understand this
>   for x in y:
>     if ...
>
> But what is about the 'x' in front of all that?

It's called a *list comprehension*. The code Jussi posted is broadly
equivalent to this:

list = []
for item in list:
    if ( 'Banana' not in item and
            'Car' not in item ):
        list.append(item)

I recently came across this blog post, which visualizes comprehensions
fairly well.

http://treyhunner.com/2015/12/python-list-comprehensions-now-in-color/

The bit at the beginning (before the first 'for') goes inside a
list.append(...) call, and then everything else is basically the same.

ChrisA

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Thread

filter a list of strings <c.buhtz@posteo.jp> - 2015-12-03 02:15 +0100
  Re: filter a list of strings Jussi Piitulainen <harvesting@is.invalid> - 2015-12-03 08:32 +0200
    Re: filter a list of strings <c.buhtz@posteo.jp> - 2015-12-03 10:27 +0100
      Re: filter a list of strings Jussi Piitulainen <harvesting@is.invalid> - 2015-12-03 13:53 +0200
      Re: filter a list of strings Peter Pearson <pkpearson@nowhere.invalid> - 2015-12-05 19:42 +0000
    Re: filter a list of strings Chris Angelico <rosuav@gmail.com> - 2015-12-03 20:40 +1100
    Re: filter a list of strings Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de> - 2015-12-03 10:46 +0100
    Re: filter a list of strings Laura Creighton <lac@openend.se> - 2015-12-03 10:53 +0100
    Re: filter a list of strings jmp <jeanmichel@sequans.com> - 2015-12-03 11:03 +0100
    Re: filter a list of strings Peter Otten <__peter__@web.de> - 2015-12-03 11:13 +0100
    Re: filter a list of strings Denis McMahon <denismfmcmahon@gmail.com> - 2015-12-03 14:16 +0000
      Re: filter a list of strings Jussi Piitulainen <harvesting@is.invalid> - 2015-12-03 17:02 +0200
  Re: filter a list of strings Grobu <snailcoder@retrosite.invalid> - 2015-12-03 13:17 +0100

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