Python functions are declared with the def keyword:

def my_function():
    print("this is a function")

To run a function, use the function name, followed by parentheses, as follows:

>>> my_function()
this is a function

Parameters must be specified after the function name, inside the parentheses:

def my_function(x):
    print(x * 1234)

>>> my_function(7)
8638

Multiple parameters must be separated with a comma:

def my_function(x,y):
    print(x*5+ 2*y)

>>> my_function(7,9)
53

Use the equals sign to define a default parameter. If you call the function without the parameter, then the default value will be used:

def my_function(x,y=10):
    print(x*5+ 2*y)

>>> my_function(1)
25

>>> my_function(1,100)
205

The parameters of a function can be of any type of data (such as string, number, list, and dictionary). Here, the following list, lcities, is used as a parameter for my_function:

def my_function(cities):
    for x in cities:
        print(x)

>>> lcities=["Napoli","Mumbai","Amsterdam"]
>>> my_function(lcities)
Napoli
Mumbai
Amsterdam

Use the return statement to return a value from a function:

def my_function(x,y):
    return x*y

>>> my_function(6,29)
174 

Python supports an interesting syntax that allows you to define small, single-line functions on the fly. Derived from the Lisp programming language, these lambda functions can be used wherever a function is required.

An example of a lambda function, functionvar, is shown as follows:

# lambda definition equivalent to def f(x): return x + 1

functionvar = lambda x: x * 5
>>> print(functionvar(10))
50