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Date: Wed, 11 Jul 2012 09:36:14 +0200
Subject: Re: lambda in list comprehension acting funny
From: Daniel Fetchinson <fetchinson@googlemail.com>
To: Python <python-list@python.org>
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Xref: csiph.com comp.lang.python:25169

>>> funcs = [ lambda x: x**i for i in range( 5 ) ]
>>> print funcs[0]( 2 )
>>>
>>> This gives me
>>> 16
>>>
>>> When I was excepting
>>> 1
>>>
>>> Does anyone know why?
>
>    Just the way Python lambda expressions bind their variable
> references. Inner 'i' references the outer scope's 'i' variable and not
> its value 'at the time the lambda got defined'.
>
>
>> And more importantly, what's the simplest way to achieve the latter? :)
>
>    Try giving the lambda a default parameter (they get calculated and
> have their value stored at the time the lambda is defined) like this:
>    funcs = [ lambda x, i=i: x**i for i in range( 5 ) ]

Thanks a lot!
I worked around it by

def p(i):
    return lambda x: x**i
funcs = [ p(i) for i in range(5) ]

But your variant is nicer (matter of taste of course).

Cheers,
Daniel


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