Modularity

• So far, we've created only small programs
  • When programs get large, they become more complex to write, debug, modify, and understand
  • Modularity can make it easier to comprehend large programs
  • Modularity can also make it possible to reuse part of our program
• Types of modularity:
  • Functions
  • Objects

Functions

• A function is a module of program code
  • A function takes input (arguments)
    • The arguments allow us to customize the operation performed by the function
  • A function produces output (return value)
    • The return value is often the result of executing the code
  • Calling a function causes the code in a function to execute
    • If we call the function again, the code executes again

Defining Functions

• Functions without arguments or return value:

def sayHello():
	print('Hello!')
                            

• To call this function:

sayHello()
                            

Defining Functions

• Functions with a return value:

def getAnswer():
   return 42
                            

• To call this function:

answer = getAnswer()
                            

Defining Functions

• Functions with arguments:

def getDogAge(hAge):
   return hAge * 7
                            

• To call this function:

answer1 = getDogAge(5)

humanAge = 7
answer2 = getDogAge(humanAge)
                            

Documenting Functions

• To document a function, use a multi-line comment immediately after the def line:
  • Don't forget to indent the comment

def getAgeInDogYears(humanAge):
   """
   This function, given an age in human
   years, returns the age in dog years.
   """
   return humanAge * 7
                        

Local Variables

• If you create any variables inside functions, they are local variables
  • A local variable is accessible/usable within that function only
  • The word local refers to the variable's scope
    • The scope is local to the function

def getAgeInDogYears(humanAge):
   dogYearsFactor = 7  # local variable
   return humanAge * dogYearsFactor
                        

Global Variables

• Variables created outside of a function are called global variables
  • Global variables' scope includes both inside and outside of functions
  • We explicitly declare when we use global variables

dogYearsFactor = 7  # global variable
def getAgeInDogYears(humanAge):
   global dogYearsFactor
   return humanAge * dogYearsFactor
                        

Arguments

• Arguments are values passed into (and sometimes out of) functions
  • They are used to customize the behaviour of the function
  • Compare this Python function:

  
  def eightTimesTwo():
     return 8 * 2
                          

  • ... with this Python function:

  
  def timesTwo(num):
     return num * 2
                          

  • Which one can be used in more situations?

Arguments

• At first, arguments can be confusing
  • Consider the following situation:

  
  def timesTwo(num):
     return num * 2
  
  value = 8
  print(timesTwo(value))
                          

  • The number 8 seems to have two different names: num and value

Arguments: Passing by Value

• In Python, arguments are passed by value
  • By value means that the value passed to the function when it is called is copied, and the copy is put into the argument variable
  • An argument variable has the same scope as a local variable
• Advantage:
  • When calling a function by passing its arguments from variables, you don't have to worry about those variables' values being modified

Arguments: Passing by Value

Arguments: Passing by Value

Arguments: Passing by Value

Arguments: Passing by Reference

• Another way that arguments are passed into a function is by reference
  • By reference means that the value passed to the function is linked to the argument variable
  • Any change to the argument also changes the original value
  • Advantage:
    • When calling a function by passing its arguments by reference, you can have side effects elsewhere in the program
• Note: In Python, passing by reference is not supported