Function: a block of code which is executed (called, launched) from another part of the program.

  • It might accept information (arguments) at the time of the call.
  • It might produce (return) a result.

Calling a function

Calling a function means launching it. Simple function call:


Functions can be called inside of other calls, since function calls are expressions.



The pieces of data passed to a function call are called arguments. Here two arguments are passed: 3 and 2.


Here the result of calling pow becomes the argument for the print call:


Creating a function

Here circle_area function is created (defined). It:

  • accepts one parameter r
  • calculates the area of a circle of radius r
  • returns the answer.
def circle_area(r):
  area = 3.14 * r * r
  return area

Here a call to circle_area is made, one argument is passed and the result is saved into a variable:

answer = circle_area(12)


Parameters specify the number and the order of possible arguments. They are a part of the definition

The number of parameters in the definition should match the number of arguments in the call, unless there are default paramters. There could be zero parameters.


return keyword specifies the result to be produced by a function. The usefulness of functions comes from the results they return.

Default values for parameters

A function definition might include the predefined default value for a parameter. It is used only if the corresponding argument is not provided in the call.

def greeting(name = "friend"):
  return "Hello " + name


Hello John Hello friend Hello Mike

Examples of function definitions and calls

Example 1

def calculate_full_cost(price, tax):
  full_cost = price + (price * tax / 100)
  return full_cost

print("The total cost is:")
print(calculate_full_cost(100, 13))

The total cost is: 113.0

Example 2

def generate_welcome(name = "friend"):
  return "Welcome home, " + name

welcome_1 = generate_welcome()
welcome_2 = generate_welcome("Paul")


Welcome home, friend Welcome home, Paul