How do nested functions work in Python?

Question

def maker(n):
    def action(x):
        return x ** n
    return action

f = maker(2)
print(f)
print(f(3))
print(f(4))

g = maker(3)
print(g(3))

print(f(3)) # stil         


        

Answer 1

People answered correctly about the closure, that is: the valid value for "n" inside action is the last value it had whenever "maker" was called.

One ease way to overcome this is to make your freevar (n) a variable inside the "action" function, which receives a copy of "n" in the moment it is run:

The easiest way to do this is to set "n" as a parameter whose default value is "n" at the moment of creation. This value for "n" stays fixed because default parameters for a function are stored in a tuple which is an attribute of the function itself (action.func_defaults in this case):

def maker(n):
    def action(x, k=n):
        return x ** k
    return action

Usage:

f = maker(2) # f is action(x, k=2)
f(3)   # returns 3^2 = 9
f(3,3) # returns 3^3 = 27