Everything in a Python program is an object, a function is also an object

def func():
    print('Hello World!')

f = func

print(id(f), id(func)) # 140232159555440 140232159555440
print(type(f)) # <class 'function'>
print(f) # <function func at 0x7f81700e6f70>
f() # Hello World!

			

import inspect

# function comment
def func():
    """Function Doc
    """
    print('Hello World!')

if __name__ == '__main__':
    f = func

    print(inspect.isfunction(f)) # check if an object is a function
    print(inspect.getdoc(f)) # get the object doc string
    print(inspect.getcomments(f)) # get comments before function

    print(inspect.getfile(f)) # get the file name in which the function is defined
    print(inspect.getsource(f)) # get the source code
            

To take another function as an argument

The inner function is referred to as a callback, because a call back to the inner function can modify the outer function’s behavior

def inner():
    print('Hello World!')

def outer(f):
    f()

outer(inner) # Hello World!
            

def inner(a, b):
    return a+b

def outer(f, x, y):
    return f(x, y)

print(outer(inner, 1, 2)) # 3
            

To return another function to its caller

def outer():
    def inner():
        print('Hello World!')

    return inner

f = outer()
f() # Hello World!
            

lambda [parameter_list]: [expression], return one single expression

add = lambda a, b: a+b

print(add(1, 2))

print((lambda a, b: a+b)(1, 2))
            

Functional Programming in Python: When and How to Use It