How to create a list of functions depending on a parameter?

Paul Rudin paul.nospam at
Tue May 26 11:16:32 CEST 2009

"Diez B. Roggisch" <deets at> writes:

> enzo michelangeli schrieb:
>> Let's suppose I want to create a list of n functions of a single
>> argument, returning the sum between argument and index in the list, so
>> that e.g.:
>> f[0](10) will return 10
>> f[3](12) will return 15
>> ...and so on. I had naively though of coding:
>>  f = [lambda x: x+j for j in range(n)]
>> Unfortunately, each function in the list f[]() behaves as a closure,
>> and f[k](p) does NOT return p+k but p+j (for whatever value j has at
>> the moment: typically n, the last value assumed by j in the list
>> comprehension loop).
>> Is there a way of achieving my goal? (Of course, n is not a constant
>> known in advance, so I can't manually unroll the loop.)
> You need to capture n into the closure of the lambda:
> f = [lambda x, n=n: x+j for j in xrange(n)]

But be careful using such a technique with mutable arguments...

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