Is nested function a good approach when required by only one function? [closed]

Let's say that a function A is required only by function B, should A be defined inside B?

Simple example. Two methods, one called from another:

def method_a(arg):
    some_data = method_b(arg)

def method_b(arg):
    return some_data

In Python we can declare def inside another def. So, if method_b is required for and called only from method_a, should I declare method_b inside method_a? like this :

def method_a(arg):
    
    def method_b(arg):
        return some_data

    some_data = method_b(arg)

Or should I avoid doing this?

>>> def sum(x, y):
...     def do_it():
...             return x + y
...     return do_it
... 
>>> a = sum(1, 3)
>>> a
<function do_it at 0xb772b304>
>>> a()
4

Is this what you were looking for? It's called a closure.

You don't really gain much by doing this, in fact it slows method_a down because it'll define and recompile the other function every time it's called. Given that, it would probably be better to just prefix the function name with underscore to indicate it's a private method -- i.e. _method_b.

I suppose you might want to do this if the nested function's definition varied each time for some reason, but that may indicate a flaw in your design. That said, there is a valid reason to do this to allow the nested function to use arguments that were passed to the outer function but not explicitly passed on to them, which sometimes occurs when writing function decorators, for example. It's what is being shown in the accepted answer although a decorator is not being defined or used.

Update:

Here's proof that nesting them is slower (using Python 3.6.1), although admittedly not by much in this trivial case:

setup = """
class Test(object):
    def separate(self, arg):
        some_data = self._method_b(arg)

    def _method_b(self, arg):
        return arg+1

    def nested(self, arg):

        def method_b2(self, arg):
            return arg+1

        some_data = method_b2(self, arg)

obj = Test()
"""
from timeit import Timer
print(min(Timer(stmt='obj.separate(42)', setup=setup).repeat()))  # -> 0.24479823284461724
print(min(Timer(stmt='obj.nested(42)', setup=setup).repeat()))    # -> 0.26553459700452575

Note I added some self arguments to your sample functions to make them more like real methods (although method_b2 still isn't technically a method of the Test class). Also the nested function is actually called in that version, unlike yours.

A function inside of a function is commonly used for closures.

(There is a lot of contention over what exactly makes a closure a closure.)

Here's an example using the built-in sum(). It defines start once and uses it from then on:

def sum_partial(start):
    def sum_start(iterable):
        return sum(iterable, start)
    return sum_start

In use:

>>> sum_with_1 = sum_partial(1)
>>> sum_with_3 = sum_partial(3)
>>> 
>>> sum_with_1
<function sum_start at 0x7f3726e70b90>
>>> sum_with_3
<function sum_start at 0x7f3726e70c08>
>>> sum_with_1((1,2,3))
7
>>> sum_with_3((1,2,3))
9

Built-in python closure

functools.partial is an example of a closure.

From the python docs, it's roughly equivalent to:

def partial(func, *args, **keywords):
    def newfunc(*fargs, **fkeywords):
        newkeywords = keywords.copy()
        newkeywords.update(fkeywords)
        return func(*(args + fargs), **newkeywords)
    newfunc.func = func
    newfunc.args = args
    newfunc.keywords = keywords
    return newfunc

(Kudos to @user225312 below for the answer. I find this example easier to figure out, and hopefully will help answer @mango's comment.)