Generating families of lambda functions

Question

I would like to generate a family of lambda functions similar to this simplified example:

fns = [(lambda x: x == y) for y in range(10)]

The result I get from this is indeed a list of 10 functions. However, all 10 seem to have y bound to 9, which is the last value of the sequence. For example

[fns[i](9) for i in range(10)] --> [True, True, True, True, True, True, True, True, True, True]
fns[0](0) --> False

Why doesn't this work, and what's a clean work-around?

I've tried this in Python 2.7 and 3.3.

Change the lambda definition to `(lambda x, y = y: x == y)` – thefourtheye Feb 12 '14 at 06:43 — thefourtheye, Feb 12 '14 at 06:43

placeybordeaux · Accepted Answer · 2014-02-12T06:53:56.057

1

Edited due to comment.

>>> fns = [(lambda x,y = y: x == y) for y in range(10)]
>>> map(lambda x: x(1), fns)
[False, True, False, False, False, False, False, False, False, False]

edited Feb 12 '14 at 06:53

answered Feb 12 '14 at 06:46

placeybordeaux

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What if I had another for loop? Nest the lambda one more time? – thefourtheye Feb 12 '14 at 06:49
Nope, the one lambda is all the closure you need. Example included in edit. – placeybordeaux Feb 12 '14 at 06:52
We don't another lambda at all. Check my comment in the question. – thefourtheye Feb 12 '14 at 06:54

Generating families of lambda functions

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