Although they are interchangeable in many programming languages, Python uses the term function for non-object related functional units of code and the term method for object related functional units of code. We will concentrate on functions for the time being.

A function is defined with the def keyword.

• The signature of a function consists of
  • a function name
  • an opening parenthesis("(")
  • any number of input arguments
  • a closing parenthesis(")")
  • a colon(":").
• Function names are case sensitive, i.e. upper case and lower case letters are not interchangeable when we are referring to a function in other parts of our code.
• By the older convention, function names start with a lower case letter and if it consists of multiple words, camel case (or mixed case) is used to differentiate the individual words where initial letter of each word is typed in upper case, e.g. thisIsAMixedCaseFunctionName.
• By the newer convention, function names consist of lower case letters with underscore("_") characters separating the individual words, if there are more than one, e.g. this_is_a_function_name_in_python.
• No space characters are allowed in function names.
• Function names cannot start with a number, however they can contain numbers.
• The functions name should give a clue about the main purpose of that function.
• Functions can be defined to receive zero or more arguments. If no arguments is necessary, we simply do not write in between the pair of parentheses. If more than one argument is necessary, we need to add a comma in between the consecutive arguments.
• Avoid functions that are too long to keep track of and maintain.
• Writing a function is necessary if there is no standard built-in way of doing that work.
• Indentation defines the boundaries and the end of the function code.
• The order of arguments is very important, that when calling the function you must specify the argument values as specified in the function signature.

The following code defines a function with an argument that is used inside the function to drawing squares of variable sizes:

import turtle

def draw_a_square_for_me(square_size):
    turtle.forward(square_size)
    turtle.right(90)
    turtle.forward(square_size)
    turtle.right(90)
    turtle.forward(square_size)
    turtle.right(90)
    turtle.forward(square_size)
    turtle.right(90)


turtle.penup()
turtle.goto(-50,50)
turtle.pendown()
turtle.pensize(5)

turtle.pencolor("Red")
draw_a_square_for_me(100)

turtle.penup()
turtle.backward(10)
turtle.left(90)
turtle.forward(10)
turtle.right(90)
turtle.pendown()
turtle.pencolor("Blue")
draw_a_square_for_me(120)

turtle.penup()
turtle.backward(10)
turtle.left(90)
turtle.forward(10)
turtle.right(90)
turtle.pendown()

turtle.pencolor("Green")
draw_a_square_for_me(140)

turtle.done()