Python nested function and decorator - How to use a function that returns another function object

A Python function can return the function.

def f():
    def g():
        print('g() is called.')

    return g()


f()
# g() is called.

f() returns g() so g() is called. is printed.

Return the function object

def f():
    def g():
        print('g() is called.')

    return g


f()

Nothing is printed, that is g() is not called. For, g is a function object and f() simply returns it. In fact, f() returns the function object g.

def f():
    def g():
        print('g() is called.')

    return g


a = f()

print(a)  # <function f.<locals>.g at 0x103d53e50>
print(type(a))  # <class 'function'>

a is a function so a() prints g() is called..

def f():
    def g():
        print('g() is called.')

    return g


a = f()

print(a)  # <function f.<locals>.g at 0x103d53e50>
print(type(a))  # <class 'function'>

a()
# g() is called.

The argument can be a function object

def f(other_function):
    def g():
        print('g() is called.')
        other_function()

    return g


def agree():
    print('I agree.')


a = f(agree)

a()
# g() is called.
# I agree.

a is after all g so g() is called. is printed first.

Python decorator

In the above code, a = f(agree) and a() seems redundant so Python creates the "decorator".

def f(other_function):
    def g():
        print('g() is called.')
        other_function()

    return g


@f
def agree():
    print('I agree.')


agree()
# g() is called.
# I agree.