Re: What does a list comprehension do

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From	Marko Rauhamaa <marko@pacujo.net>
Newsgroups	comp.lang.python
Subject	Re: What does a list comprehension do
Date	Thu, 26 Nov 2015 14:56:58 +0200
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Antoon Pardon <antoon.pardon@rece.vub.ac.be>:

> Personnaly I would prefer:
>
>>>> q = [(lambda i: lambda x: i * x)(i) for i in range(4)]
>>>> q[0](1), q[3](1)
> (0, 3)
>
> And this is where I ask whether it would be worth the effort to change
> the behaviour of python.

Don't go there.

Consider:

    q = []
    n = 0
    x = "hello"

    for i in range(4):
        def stepper():
            global n
            n += 1
            return i * x
        q.append(stepper)

    print(n)
    print(q[1]())
    print(n)
    x = "there"
    print(q[3]())
    print(n)

which prints:

    0
    hellohellohello
    1
    theretherethere
    2

after your change, you'd get:

    0
    hello
    0
    hellohellohello
    0

> It also seems that people who try this for the first time are
> surprised with what they get and seem to expect there list
> comprehension to act as if they had written the second version.

I might trip over that one, too. Still, nothing should be changed.


Marko

Thread

What does a list comprehension do (was: Late-binding of function defaults (was Re: What is a function parameter =[] for?)) Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-11-25 14:51 +0100
  Re: What does a list comprehension do (was: Late-binding of function defaults (was Re: What is a function parameter =[] for?)) Nobody <nobody@nowhere.invalid> - 2015-11-26 11:13 +0000
    Re: What does a list comprehension do Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-11-26 12:52 +0100
      Re: What does a list comprehension do Marko Rauhamaa <marko@pacujo.net> - 2015-11-26 14:56 +0200
        Re: What does a list comprehension do Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-11-26 14:33 +0100
          Re: What does a list comprehension do Marko Rauhamaa <marko@pacujo.net> - 2015-11-26 15:56 +0200
            Re: What does a list comprehension do Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-11-26 15:26 +0100
              Re: What does a list comprehension do Marko Rauhamaa <marko@pacujo.net> - 2015-11-26 17:36 +0200
                Re: What does a list comprehension do Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-11-26 20:18 +0100
              Re: What does a list comprehension do Jussi Piitulainen <harvest@is.invalid> - 2015-11-26 18:11 +0200

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