Python supports functional programming, a programming paradigm in which functions can be values to be passed between functions.

Having functions as values create new patterns in programming.

When a function is declared, the function name is actually a variable that refers to the function object.

# Define a function
def hello():
    print("Hello world.")

# The function can be treated as a data value
x = hello
y = x

# Variables assigned to functions can
# be used as functions themselves
x() # -> Hello world.
y() # -> Hello world.

Functions can be the return values of functions.

# This main function returns a function.  
# The returned function is constructed based
# on the argument(s) of the main function.
def make_adder_fn(increment):
    def f(x):
        return x + increment
    return f

Functions can be parameters of functions.

# This function expects a function as its first
# argument, and a number as a second argument.
# The main function applies the argument function
# twice to the second argument.
def apply_twice(f, x):
    return f(f(x))

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Definition:

A higher order function is a function that accepts another function as an input.

Anonymous functions.

add = lambda x,y: x + y