Why is calling a variable that contains a function invalid outside its scope?

In Python, the scope of a variable determines where it is accessible. If a variable contains a function, it can only be called within the scope where it is defined or where it is accessible through a closure or global scope. Calling it outside the scope of its definition results in a NameError, as the interpreter cannot see this variable.

Example:

def outer():
    def inner():
        print("Hello")
    inner()  # correct

outer()
inner()  # error, inner is not defined outside outer

Here, inner exists only inside outer, so calling it outside outer is impossible.