lambda - Python create function in a loop capturing the loop variable

Question

What's going on here? I'm trying to create a list of functions:

def f(a,b):
    return a*b

funcs = []

for i in range(0,10):
    funcs.append(lambda x:f(i,x))

This isn't doing what I expect. I would expect the list to act like this:

funcs[3](3) = 9
funcs[0](5) = 0

But all the functions in the list seem to be identical, and be setting the fixed value to be 9:

funcs[3](3) = 27
funcs[3](1) = 9

funcs[2](6) = 54

Any ideas?

Answer 1

lambdas in python are closures.... the arguments you give it aren't going to be evaluated until the lambda is evaluated. At that time, i=9 regardless, because your iteration is finished.

The behavior you're looking for can be achieved with functools.partial

import functools

def f(a,b):
    return a*b

funcs = []

for i in range(0,10):
    funcs.append(functools.partial(f,i))