Function

• In this lecture, we learn how to define and use functions in Python.
• A function summarizes several instructions (or commands) into one.
• First you define a function, then you call it to use.

• Let's try to calculate the average score of Math, English, and Science for students A and B.

  stu_A_score = {"Math": 89, "English": 30, "Science": 86}
  stu_B_score = {"Math": 59, "English": 70, "Science": 65}
  
  aveA = (stu_A_score["Math"] + stu_A_score["English"] + stu_A_score["Science"])/3
  aveB = (stu_B_score["Math"] + stu_B_score["English"] + stu_B_score["Science"])/3
  
  print(f"Average of A is {aveA:.2f}, B is {aveB:.2f}")
  

• Clearly, taking average is common procedure so you do not want to write it twice.

  def average(scores):
      sum = 0
      for key, val in scores.items():
          sum += val
      ave = sum/len(scores)
      return ave
  
  stu_A_score = {"Math": 89, "English": 30, "Science": 86}
  stu_B_score = {"Math": 59, "English": 70, "Science": 65}
  
  aveA = average(stu_A_score)
  aveB = average(stu_B_score)
  
  print(f"Average of A is {aveA:.2f}, B is {aveB:.2f}")
  

Basic

• You can define a function like

  def say_hello():
      return "hello"
  

• A function will return the result when it is called, and returned value will be stored to other variable.

  def say_hello():
      return "hello"
  
  s = say_hello()
  print(s)
  

• The return statement is not mandatory, so you can omit it like

  def say_hello2():
      print("hello")
  
  say_hello2()
  

• But I recommend writing return for safety. If no need to return, you can put None.

  def say_hello3():
      print("hello")
      return None
  
  say_hello3()
  

Exercise

• Define and call the function that repeats the string you inputted ("abc" -> "abcabc")

answer

Argument

• argument is the variable that one passes to the function. The function usually does some procedure using that argument. You can pass an argument to the function, like

  def say_something(s):
      print(s)
      return None
  
  say_something("hello")
  

• You can set the default value for arguments like

  def say_something(s="hello"):
      print(s)
      return s
  
  say_something("good morning")
  say_something()
  

• You can use multiple arguments

  def combine_string(s1, s2):
      new_string = s1 + " " + s2
      return new_string
  
  s = combine_string("Good", "Morning")
  print(s)
  

• You can also use multiple return-value, as

  def say_strong(s1, s2):
      s1new = s1 + "!"
      s2new = s2 + "!"
      return s1new, s2new
  
  s1new, s2new = say_strong("Good", "Morning")
  print(s1new, s2new)
  

Scope of variables

• When you define the function, you have to know about the scope of variables.

• The variables defined inside the function cannot be used outside the function.

  def hello():
    i = 10
    print(i)
    return
  
  hello()   # => 10
  print(i)  # => not defined
  

• This is because i in the above funcion is local variable.

• Instead, variables defined outside the function is global variable so they can be accessed even inside the function.

  i = 10
  def hello():
      print(i)
      return
  
  hello()
  

• Note that the same variable name is used for both local and global variables, the global variable is used.

  a = 10
  
  def hello():
      a = 20
      return
  
  hello()
  print(a)
  

• It is safer to use global inside the function to access the global variable.

  a = 10
  
  def hello():
      global a
      a = 20
      return
  
  hello()
  print(a)
  

Exercise

1. Create a function that takes two numbers as arguments and returns their sum.
2. Create a function that recieves a list (e.g. [10, 20, 30]), and then returns the new list in which each elements are doubled. answer