Nested Function in Python

Normally you do it to make closures:

def make_adder(x):    def add(y):        return x + y    return addplus5 = 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.

Aside from function generators, where internal function creation is almost the definition of a function generator, the reason I create nested functions is to improve readability. If I have a tiny function that will only be invoked by the outer function, then I inline the definition so you don't have to skip around to determine what that function is doing. I can always move the inner method outside of the encapsulating method if I find a need to reuse the function at a later date.

Toy example:

import sysdef Foo():    def e(s):        sys.stderr.write('ERROR: ')        sys.stderr.write(s)        sys.stderr.write('\n')    e('I regret to inform you')    e('that a shameful thing has happened.')    e('Thus, I must issue this desultory message')    e('across numerous lines.')Foo()

One potential benefit of using inner methods is that it allows you to use outer method local variables without passing them as arguments.

def helper(feature, resultBuffer):  resultBuffer.print(feature)  resultBuffer.printLine()  resultBuffer.flush()def save(item, resultBuffer):  helper(item.description, resultBuffer)  helper(item.size, resultBuffer)  helper(item.type, resultBuffer)

can be written as follows, which arguably reads better

def save(item, resultBuffer):  def helper(feature):    resultBuffer.print(feature)    resultBuffer.printLine()    resultBuffer.flush()  helper(item.description)  helper(item.size)  helper(item.type)