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Functional programming gotcha

Robin Becker robin at jessikat.fsnet.co.uk
Fri Oct 24 15:34:02 EDT 2003 
In article <Pine.LNX.4.33.0310242013060.3754-100000 at tethys.ftw.at>, Ed
Schofield <schofield at ftw.at> writes
>
>Hi all,
>
>I find this strange:
>
>>>> flist = []
>>>> for i in range(3):
>...     f = lambda x: x+i

try using f = lambda x,i=i: x+i ie bind at the definition point

>...     flist.append(f)
>...
>>>> [f(1) for f in flist]
>[3, 3, 3]
>>>>
>
>What I expect is:
>
>>>> [f(1) for f in flist]
>[1,2,3]
>>>>
>
>Is this a bug in Python?  It happens on my builds of
>Python 2.3 and 2.2.3.
>
>Replacing the lambda function by a named function, as in
>
>flist = []
>for i in range(3):
>    def f(x):
>        return x + i
>    flist.append(f)
>
>[f(1) for f in flist]
>
>gives the same result.
>
>I have a workaround (of sorts) adapted from the Python tutorial:
>
>>>> def make_incrementor(n):
>...     return lambda x: x + n
>...
>>>> flist = [make_incrementor(i) for i in range(3)]
>>>>
>>>> [f(1) for f in flist]
>[1, 2, 3]
>>>>
>
>but I'd prefer the flexibility of the first approach.  Any ideas?
>
>Any explanations for why Python does this?  Any justifications for why
>Python _should_ do this?  Who believes this is a bug?
>
>Kind regards,
>Ed Schofield
>

-- 
Robin Becker
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