Re: Could you explain why the following generates 4 same elements list?

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From	dieter <dieter@handshake.de>
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Subject	Re: Could you explain why the following generates 4 same elements list?
Date	Thu, 19 Nov 2015 08:27:52 +0100
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fl <rxjwg98@gmail.com> writes:

> Hi,
>
> I cannot reason out why the code:
> ////////
> def mpl():
>     return [lambda x : i * x for i in range(4)]
>     
> print [m(2) for m in mpl()]
> /////////
>
> has result:
>
> [6, 6, 6, 6]

The "i" in your lambda definition is a variable reference which
is not dereferenced (i.e. name replaced by the value) at definition
but only at call time. Thus, all your "lambda"s are in fact equal;
they all look up the current value of "i" when they are called
(which happens to be "3").

To avoid this, you must force the dereferencing at definition time.
This could look like:

     return [lambda x, i=i: i * x for i in range(4)]

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Could you explain why the following generates 4 same elements list? fl <rxjwg98@gmail.com> - 2015-11-18 16:05 -0800
  Re: Could you explain why the following generates 4 same elements list? Ian Kelly <ian.g.kelly@gmail.com> - 2015-11-18 17:47 -0700
  Re: Could you explain why the following generates 4 same elements list? dieter <dieter@handshake.de> - 2015-11-19 08:27 +0100

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