how to write function that returns function

[Kragen Sitaker]

spam at bugbear.com (Paul Graham) writes:
> def addn(x):
>   return lambda y,z=y: x+z
(assuming you mean lambda y,z=x: x+z)
> 
> but I don't think this is exactly the same thing,
> because it returns a function that takes a second
> optional argument.  That is a substantial difference.

This is embarrassing, which I guess is why we have nested scopes in
2.1+ so we don't have to do this any more; but here's a solution for
1.5.2:

class addn:
    def __init__(self, x): self.x = x
    def __call__(self, y): return self.x + y

This is a class instead of a function, but that is not a substantial
difference.

Unlike the nested-scopes case, this lets you modify the state of the
object in a straightforward way; a "counter" closure in Python is
ugly:

def counter(startvalue):
    state = [startvalue]
    def counter_internal():
        state[0] += 1
        return state[0]
    return counter_internal

The class equivalent is a little better:
class counter:
    def __init__(self, startvalue): self.state = startvalue
    def __call__(self):
        self.state = self.state + 1
        return self.state

(I didn't use += because this class version will work in 1.5.2 and
earlier Pythons, which don't have +=.  The nested-scopes version
doesn't.)