A function inside a function Python

Question

This is a problem asked before but I can't really understand the other explain of this kind of problem so I'm here to re-write it in more details. While studying I have encountered this kind of code that I am not at all familiar .. I can not understand how to interpret this g() function in f() function ! Why the piece of code inside g() where x = 10 and y = z*w does not run ? It's only print me the value of y I gave, calling f() with 5 !

x = 99

def f(y):
    w = x + y
    def g():
        x = 10
        y = z * w
    print y

f(5)

Answer 1

In Python, def is a statement: takes a function name, possibly arguments, then an indented function body -- compiles it all into a function object, which it binds to the given name in the scope where def appeared (here, locally to f).

So you ask "Why the piece of code inside g() where x = 10 and y = z*w does not run" -- very simply, because you never call g!

The fact that g is local to f (or as is also known "nested in f") is not germane.

Whether local or global, anytime you def g but then never call g, the code in g's body will not execute.

Incidentally, this is a detail in which Python coincides with every other language I've ever heard about. If a function is defined (some languages call that "declared") and never called, then the function's body code never runs. Have you ever heard of any language doing otherwise -- i.e, executing the code body of a function that's defined but never called?!