To use this website fully, you first need to accept the use of cookies. By agreeing to the use of cookies you consent to the use of functional cookies. For more information read this page.

Part 3.1Procedures in Python


Python, like many scripting syntaxes, has just one procedure declaration format.

Often called a procedure, a function, a subroutine or a subprogram, it is a division of code that is separated from the rest. A procedure is a way of organising code to perform one particular function. For instance, (in an abstract example), a procedure could be used to turn on a light bulb or to turn off a plug socket. When it is needed, it is run (called). The part that gives the procedure a name and parameters is called the procedure signature. Below is a simple diagram that explains the procedure structure:


Defining a procedure

Below is an example procedure:

def main():
  print ("Hello world")

Procedures can however, return a value. If a procedure returns a value, the value that it gives back can be used elsewhere.

Returning a value

A procedure that returns, or gives back, a value can be known as a function, since it's actions mimic that of a mathematical function:

A ⇒ B

In this example, A is an input the arrow is the process and B is the output. For every input there is exactly one output.

A function returning a value looks like the following sample:

def main():
  return "Hello world"

With this, assigning the function call to a variable will give the variable the value "Hello world".

Calling a procedure

A procedure can be called through a very simple syntax. Arguments are also given to procedure calls within the brackets - these line up with parameters specified in the procedure signature.

def main():
  print ("Hello world")


The previous sample will call the procedure named main. It will also provide no arguments to the call.

A procedure with parameters can be called by inserting values in the positions of those parameters with arguments:

def add(n1, n2):
  print (n1 + n2)

add(4, 7)


Recursion is a big part of learning to program efficently and it is basically a procedure calling itself over and over again until a termination condition (base case) is met. Recursion can be used to multiply two numbers by simply using addition.

def multiply(n1, n2, total):
if n2 == 0:
return n1 else: total = multiply(n1, n2 - 1, total) return total + n1 print(multiply(4, 3, 0))

Here is what happens in this program:

  • multiply(4, 3, 0) calls multiply (4, 2, 0)
  • multiply(4, 2, 0) calls multiply (4, 1, 0)
  • multiply(4, 1, 0) calls multiply (4, 0, 0)
  • multiply (4, 0, 0) then returns 0 + 4 (4) to multiply(4, 1, 0)
  • multiply (4, 1, 0) then returns 4 + 4 (8) to multiply(4, 2, 0)
  • multiply (4, 2, 0) then returns 8 + 4 (12) to multiply(4, 3, 0)
Recursion is something that to understand, one must first understand recursion. For more information on recursion, look at recursion.
Code previewClose
Feedback 👍
Comments are sent via email to me.