Lambda evaluation

Jp Calderone exarkun at divmod.com
Thu Oct 6 22:27:00 CEST 2005


On Thu, 06 Oct 2005 16:18:15 -0400, Joshua Ginsberg <listspam at flowtheory.net> wrote:
>So this part makes total sense to me:
>
>>>> d = {}
>>>> for x in [1,2,3]:
>...     d[x] = lambda y: y*x
>...
>>>> d[1](3)
>9
>
>Because x in the lambda definition isn't evaluated until the lambda is
>executed, at which point x is 3.
>
>Is there a way to specifically hard code into that lambda definition the
>contemporary value of an external variable? In other words, is there a
>way to rewrite the line "d[x] = lambda y: y*x" so that it is always the
>case that d[1](3) = 3?

There are several ways, but this one involves the least additional typing:

    >>> d = {}
    >>> for x in 1, 2, 3:
    ...     d[x] = lambda y, x=x: y * x
    ... 
    >>> d[1](3)
    3

Who needs closures, anyway? :)

Jp



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