Functional programming gotcha

[Ed Schofield]

Hi all,

I find this strange:

>>> flist = []
>>> for i in range(3):
...     f = lambda x: x+i
...     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