how does python assign value to inner function

Question

Can someone please explain how python assigned the value below to y. I understand how the function works but I do not understand how the function call a(10) assigns its value y.

def outside(x):
    def inside(y):
        return x ** y
    return inside

a = outside(2)

a(10)

C'mon guys, did you even read the question you've marked this one a duplicate of? — Ignacio Vazquez-Abrams, Jan 03 '15 at 11:54
@scopa, it is a function factory, outside is basically passed the exponent to raise to and then using a(10) you are calling the inner function where y is raised to a power of x. If you `print a.__closure__` you will see python creates a closure to refer to that object. — Padraic Cunningham, Jan 03 '15 at 12:12

NPE · Answer 1 · 2015-01-03T12:07:36.487

3

In this code a() is effectively an alias for inside(), with the value of x fixed at 2.

Thus a(10) is essentially inside(10), i.e. the value of y is 10.

Since x is 2 and y is 10, a(10) returns 2 ** 10 (1024).

This type of construct is used extensively in Python decorators. I'd recommend Understanding Python Decorators in 12 Easy Steps! as a nice introduction. Among other things it explains how Python functions are first-class objects and can be assigned to variables, passed as arguments to other functions etc.

edited Jan 03 '15 at 12:07

answered Jan 03 '15 at 12:01

NPE

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He called his function `inside` not `inner` – Duncan Jan 03 '15 at 12:02
Thanks I understand that y is being assigned the value 10. I am not clear on how python accepts a() as an alias. – scopa Jan 03 '15 at 12:07
@scopa: Have a read of the decorator tutorial I've linked to in the answer. Perhaps it'll answer some question. – NPE Jan 03 '15 at 12:08

BartoszKP · Answer 2 · 2015-01-03T12:09:58.907

What happens, step by step:

You're calling the outside function with x = 2.
outside returns the function inside defined in the context of x being 2.
So now, a contains the function: def inside(y): return 2 ** y
You're calling a(10), which is, recalling step 3, inside(10).

This feature is called Functions as First-Class Objects - it basically means that functions are like class instances, that can hold state, and can be passed in variables and arguments, just like anything else. Note the following example:

>>> def x():
...     pass
...
>>> x
<function x at 0x02F91A30>  # <-- function's address
>>> type(x)
<type 'function'>
>>>
>>> y = x
>>> type(y)
<type 'function'>
>>> y
<function x at 0x02F91A30>  # <-- the same address
>>>

how does python assign value to inner function

2 Answers2