Defining Python Functions

Overview

• Prerequisite Topics
• What is a Function?
• What is an Input?
• What is an output_value?
• return statement
• contrasting return and print

What is a Function?

• A function is a block of reusable instruction(s).
• A function relates an input to an output_value.
• The intent of a function is to re-execute the same lines of in different areas of a program.
• Functions provide better modularity for an application

Function defintions

• The function signature is the first line of a function definition.
• A function signature is denoted by the keyword def, followed by the name of the function, followed by parenthesis (), followed by a :.
• A code block begins with a colon : and is followed by a new line and an indentation.
• A function definition is a function signature and its respective code block.

# this is outside the scope of the of the function code-block
def my_function_name(): # this is the function signature
    output_value = "Hello world" # this is the beginning of the function code-block
    print(output_value) # this is the ending of the function code-block
# this is outside the scope of the of the function code-block

Function Parameters

• Input parameters are denoted by the parenthesis ()
• In simple examples, an input is typically thought to be a numeric datatype or textual datatype
  • though, any type is valid as an input.

# this is outside the scope of the of the function code-block
def my_function_name(input_value): # this is the function signature
    output_value = "Hello " + input_value # this is the beginning of the function code-block
    print(output_value) # this is the ending of the function code-block
    return None # explicit return
# this is outside the scope of the of the function code-block

return

• The return statement exits a function, optionally passing back an expression to the caller.
  • The expression passed back to the caller is named
• Omitting a return statement is the same as explicitly returning None

# this is outside the scope of the of the function code-block
def my_function_name(input_value): # this is the function signature
    output_value = "Hello " + input_value # this is the beginning of the function code-block
    return output_value # this is the ending of the function code-block
# this is outside the scope of the of the function code-block

return vs print

• In contrast to print statements, return statements allow a system’s state change to be observable in a programmatic way
• In contrast to return statements, print statements allow a system’s state change to be observable in a human-readable