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Re: Implicit conversion to boolean in if and while statements

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From Serhiy Storchaka <storchaka@gmail.com>
Subject Re: Implicit conversion to boolean in if and while statements
Date Fri, 15 Feb 2013 21:09:42 +0200
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On 10.02.13 13:37, Steven D'Aprano wrote:
> Unfortunately, Python has a minor design flaw. One of the most common
> use-cases for sets is for membership testing of literal sets:
> 
> def example(arg):
>      if arg in {'spam', 'ham', 'eggs', 'cheese'}:
>          ...
> 
> Unfortunately, set literals create *mutable* sets, not frozensets. That
> means that the compiler wastes time and memory creating am over-allocated
> mutable set object. If set literals created immutable frozensets, there
> would be some nice opportunities for the compiler to optimize this
> use-case.

CPython 3.2 optimizes this special case and creates a constant frozenset.

$ echo "arg in {'spam', 'ham', 'eggs', 'cheese'}" | python3.2 -m dis -
  1           0 LOAD_NAME                0 (arg) 
              3 LOAD_CONST               5 (frozenset({'cheese', 'eggs', 'ham', 'spam'})) 
              6 COMPARE_OP               6 (in) 
              9 POP_TOP              
             10 LOAD_CONST               4 (None) 
             13 RETURN_VALUE

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Thread

Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-07 22:16 -0800
  Re: Implicit conversion to boolean in if and while statements Michael Torrie <torriem@gmail.com> - 2013-02-07 23:27 -0700
    Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-07 22:32 -0800
    Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-07 22:32 -0800
  Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-09 02:16 +1100
    Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-08 09:48 -0800
      Re: Implicit conversion to boolean in if and while statements Roy Smith <roy@panix.com> - 2013-02-08 12:58 -0500
      Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-08 11:58 -0800
        Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2013-02-09 11:05 +1100
          Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-08 16:49 -0800
            Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2013-02-09 12:17 +1100
              Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-09 20:54 -0800
                Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2013-02-10 16:04 +1100
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-11 04:28 -0800
                Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2013-02-11 23:50 +1100
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-11 05:28 -0800
                Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2013-02-12 00:52 +1100
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-11 20:24 -0800
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-11 20:24 -0800
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-11 05:28 -0800
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-11 04:28 -0800
              Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-09 20:54 -0800
            Re: Implicit conversion to boolean in if and while statements Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-08 18:29 -0700
            Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2013-02-09 16:01 +1100
              Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-09 20:26 -0800
                Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2013-02-10 15:50 +1100
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-10 18:09 -0800
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-10 18:09 -0800
                Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-10 22:37 +1100
                Re: Implicit conversion to boolean in if and while statements Roy Smith <roy@panix.com> - 2013-02-10 09:45 -0500
                Re: Implicit conversion to boolean in if and while statements Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2013-02-10 18:06 +0100
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-11 04:18 -0800
                Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2013-02-11 23:40 +1100
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-11 05:13 -0800
                Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2013-02-12 00:35 +1100
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-11 20:00 -0800
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-11 20:00 -0800
                Re: Implicit conversion to boolean in if and while statements 88888 Dihedral <dihedral88888@googlemail.com> - 2013-02-11 17:06 -0800
                Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2013-02-12 16:55 +1100
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-12 09:48 -0800
                Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2013-02-13 08:24 +1100
                Re: Implicit conversion to boolean in if and while statements 88888 Dihedral <dihedral88888@googlemail.com> - 2013-02-12 18:43 -0800
                Re: Implicit conversion to boolean in if and while statements 88888 Dihedral <dihedral88888@googlemail.com> - 2013-02-12 18:43 -0800
                Re: Implicit conversion to boolean in if and while statements Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-15 13:12 -0700
                Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2013-02-16 11:12 +1100
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-12 09:48 -0800
                Re: Implicit conversion to boolean in if and while statements 88888 Dihedral <dihedral88888@googlemail.com> - 2013-02-11 17:06 -0800
                Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-11 05:13 -0800
                Re: Implicit conversion to boolean in if and while statements Serhiy Storchaka <storchaka@gmail.com> - 2013-02-15 21:09 +0200
              Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-09 20:26 -0800
          Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-08 16:49 -0800
        Re: Implicit conversion to boolean in if and while statements Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-08 18:06 -0700
          Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-09 22:25 -0800
          Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2013-02-09 22:25 -0800
      Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-09 14:16 +1100
        Re: Implicit conversion to boolean in if and while statements Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-02-09 15:36 -0500
      Re: Implicit conversion to boolean in if and while statements Mark Janssen <dreamingforward@gmail.com> - 2013-02-11 21:12 -0800

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