I have an array of functions and I'm trying to produce one function which consists of the composition of the elements in my array. My approach is:
def compose(list):
if len(list) == 1:
return lambda x:list[0](x)
list.reverse()
final=lambda x:x
for f in list:
final=lambda x:f(final(x))
return final
This method doesn't seems to be working, help will be appreciated.
(I'm reversing the list because this is the order of composition I want the functions to be)
It doesn't work because all the anonymous functions you create in the loop refer to the same loop variable and therefore share its final value.
As a quick fix, you can replace the assignment with:
final = lambda x, f=f, final=final: f(final(x))
Or, you can return the lambda from a function:
def wrap(accum, f):
return lambda x: f(accum(x))
...
final = wrap(final, f)
To understand what's going on, try this experiment:
>>> l = [lambda: n for n in xrange(10)]
>>> [f() for f in l]
[9, 9, 9, 9, 9, 9, 9, 9, 9, 9]
This result surprises many people, who expect the result to be [0, 1, 2, ...]. However, all the lambdas point to the same n variable, and all refer to its final value, which is 9. In your case, all the versions of final which are supposed to nest end up referring to the same f and, even worse, to the same final.
The topic of lambdas and for loops in Python has been already covered on SO.