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

[Diez B. Roggisch]

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)]

Also note the use of xrange instead of range - the latter creates a 
list-object while the former only returns a generator. Most of the 
times, xrange is thus faster, and also most of the times less memory 
consuming (exception to this rule occur for small n, and are neglible)

Diez