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Re: lambda in list comprehension acting funny

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From Ian Kelly <ian.g.kelly@gmail.com>
Date Thu, 12 Jul 2012 10:53:25 -0600
Subject Re: lambda in list comprehension acting funny
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Xref csiph.com comp.lang.python:25225

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On Wed, Jul 11, 2012 at 9:59 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> On Wed, 11 Jul 2012 08:41:57 +0200, Daniel Fetchinson wrote:
>
>> funcs = [ lambda x: x**i for i in range( 5 ) ]
>
> Here's another solution:
>
> from functools import partial
> funcs = [partial(lambda i, x: x**i, i) for i in range(5)]
>
>
> Notice that the arguments i and x are defined in the opposite order.
> That's because partial only applies positional arguments from the left.
> If there was a "right partial" that applies from the right, we could use
> the built-in instead:
>
> funcs = [rpartial(pow, i) for i in range(5)]

You know, partial does handle keyword arguments correctly:

funcs = [partial(lambda x, i: x ** i, i=i) for i in range(5)]

This might be bad practice though if there are other arguments to the
right of the keywords that the eventual caller might want to specify,
as those arguments would then effectively be keyword-only.  The error
message in that case is not particularly illuminating, either:

>>> def f(a, b, c):
...   return (a, b, c)
...
>>> g = partial(f, b=42)
>>> g(3, 14)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: f() got multiple values for keyword argument 'b'

It would be a bit clearer if it instead noted that 'c' were
keyword-only in the 'g' argspec.

Cheers,
Ian

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lambda in list comprehension acting funny Daniel Fetchinson <fetchinson@googlemail.com> - 2012-07-11 08:41 +0200
  Re: lambda in list comprehension acting funny "Colin J. Williams" <cjw@ncf.ca> - 2012-07-11 06:28 -0400
    Re: lambda in list comprehension acting funny Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-11 10:34 -0600
      Re: lambda in list comprehension acting funny 88888 Dihedral <dihedral88888@googlemail.com> - 2012-07-11 20:39 -0700
        Re: lambda in list comprehension acting funny Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-12 03:51 +0000
          Re: lambda in list comprehension acting funny 88888 Dihedral <dihedral88888@googlemail.com> - 2012-07-11 22:04 -0700
            Re: lambda in list comprehension acting funny Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-12 06:18 +0000
              Re: lambda in list comprehension acting funny Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-07-12 08:40 +0100
      Re: lambda in list comprehension acting funny 88888 Dihedral <dihedral88888@googlemail.com> - 2012-07-11 20:39 -0700
  Re: lambda in list comprehension acting funny woooee <woooee@gmail.com> - 2012-07-11 11:38 -0700
    Re: lambda in list comprehension acting funny John Ladasky <john_ladasky@sbcglobal.net> - 2012-07-11 13:21 -0700
      Re: lambda in list comprehension acting funny Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-12 00:52 +0000
      Re: lambda in list comprehension acting funny Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-07-11 21:05 -0400
        Re: lambda in list comprehension acting funny Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-12 03:53 +0000
          Re: lambda in list comprehension acting funny Terry Reedy <tjreedy@udel.edu> - 2012-07-12 00:24 -0400
          Re: lambda in list comprehension acting funny Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-07-12 00:39 -0400
            Re: lambda in list comprehension acting funny Robert Miles <robertmiles@teranews.com> - 2012-08-15 19:26 -0500
      Re: lambda in list comprehension acting funny John O'Hagan <research@johnohagan.com> - 2012-07-12 15:29 +1000
      Re: lambda in list comprehension acting funny Robert Kern <robert.kern@gmail.com> - 2012-07-12 10:06 +0100
    Re: lambda in list comprehension acting funny Hans Mulder <hansmu@xs4all.nl> - 2012-07-12 00:22 +0200
    Re: lambda in list comprehension acting funny Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-11 23:47 +0000
      Re: lambda in list comprehension acting funny Daniel Fetchinson <fetchinson@googlemail.com> - 2012-07-12 04:54 +0200
  Re: lambda in list comprehension acting funny Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-12 03:59 +0000
    Re: lambda in list comprehension acting funny Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-12 10:53 -0600
    Re: lambda in list comprehension acting funny Rotwang <sg552@hotmail.co.uk> - 2012-07-12 18:23 +0100
      Re: lambda in list comprehension acting funny Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-13 02:20 +0000
  Re: lambda in list comprehension acting funny rusi <rustompmody@gmail.com> - 2012-07-12 21:33 -0700
    Re: lambda in list comprehension acting funny Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-13 06:36 +0000
      Re: lambda in list comprehension acting funny rusi <rustompmody@gmail.com> - 2012-07-13 06:44 -0700
        Re: lambda in list comprehension acting funny rusi <rustompmody@gmail.com> - 2012-07-13 07:45 -0700
        RE: lambda in list comprehension acting funny "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-07-13 16:12 +0000
          Re: lambda in list comprehension acting funny rusi <rustompmody@gmail.com> - 2012-07-13 09:46 -0700
            Re: lambda in list comprehension acting funny Chris Angelico <rosuav@gmail.com> - 2012-07-14 03:20 +1000
          Re: lambda in list comprehension acting funny Hans Mulder <hansmu@xs4all.nl> - 2012-07-13 19:53 +0200
            RE: lambda in list comprehension acting funny "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-07-13 18:06 +0000
            Re: lambda in list comprehension acting funny Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-13 12:54 -0600
              Re: lambda in list comprehension acting funny Hans Mulder <hansmu@xs4all.nl> - 2012-07-13 21:26 +0200
              Re: lambda in list comprehension acting funny Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-14 23:29 +0000
                Re: lambda in list comprehension acting funny Chris Angelico <rosuav@gmail.com> - 2012-07-15 10:49 +1000
                Re: lambda in list comprehension acting funny Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-15 08:32 +0000
                Re: lambda in list comprehension acting funny Chris Angelico <rosuav@gmail.com> - 2012-07-15 18:44 +1000
                Re: lambda in list comprehension acting funny Hans Mulder <hansmu@xs4all.nl> - 2012-07-15 21:25 +0200
                Re: lambda in list comprehension acting funny Terry Reedy <tjreedy@udel.edu> - 2012-07-15 06:27 -0400
            Re: lambda in list comprehension acting funny rusi <rustompmody@gmail.com> - 2012-07-13 19:31 -0700
              Re: lambda in list comprehension acting funny Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-14 03:43 +0000
                Re: lambda in list comprehension acting funny rusi <rustompmody@gmail.com> - 2012-07-13 21:53 -0700
                Re: lambda in list comprehension acting funny Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-14 07:46 +0000
        Re: lambda in list comprehension acting funny Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-07-13 13:47 -0400

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