Nested Function in Python

python nested-function
Hosam Aly picture Hosam Aly · Oct 19, 2009 · Viewed 91.4k times · Source

What benefit or implications could we get with Python code like this:

class some_class(parent_class):
    def doOp(self, x, y):
        def add(x, y):
            return x + y
        return add(x, y)

I found this in an open-source project, doing something useful inside the nested function, but doing absolutely nothing outside it except calling it. (The actual code can be found here.) Why might someone code it like this? Is there some benefit or side effect for writing the code inside the nested function rather than in the outer, normal function?

Answer

Adam Rosenfield picture Adam Rosenfield · Oct 19, 2009

Normally you do it to make closures:

def make_adder(x):
    def add(y):
        return x + y
    return add

plus5 = make_adder(5)
print(plus5(12))  # prints 17

Inner functions can access variables from the enclosing scope (in this case, the local variable x). If you're not accessing any variables from the enclosing scope, they're really just ordinary functions with a different scope.

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