Functions

Writing Functions

You can write your own functions in Python. Function are useful when you want to execute a bunch of statements multiple times at different points of a program.

Format:

def function_name():
    statement_1
    statement_2
    ...

def say_hello():
  print('Hello!')
  print('How are you?')
  print('Nice to meet you.')
  
# call the function twice
print('Going to call the function ...')
say_hello()
print('Going to repeat ...')
say_hello()

Going to call the function ...
Hello!
How are you?
Nice to meet you.
Going to repeat ...
Hello!
How are you?
Nice to meet you.

The function definition should appear in the code before it is called.

print('starting ...')

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

print('Going to say hello ...')

say_hello()

print('starting ...')
print('Going to say hello ...')

# call appears before definition
say_hello() 

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

for i in range(3):
  say_hip()
  say_hip()
  say_horray()
  print() # print empty line
  
  
  
  
  

Hip
Hip
Hooray!

Hip
Hip
Hooray!

Hip
Hip
Hooray!

Function Parameters

You can configure a function to have parameters. The parameters can be used by the code inside the function like a variable. That means we can pass arguments (i.e., values for those parameters) to affect the behavior of a function so that the function behaves differently each time it is executed.

Format:

def function_name(parameter1_name, parameter2_name, ...):
    ...

A note about terminology: a function can have parameters and when we call the function we can assign arguments (i.e. values) to each of those parameters.

def say_hello(name):
  print('Hello ' + name)
  print('How are you', name, '?')
  
say_hello('Gina')
say_hello('John')

Hello Gina
How are you Gina ?
Hello John
How are you John ?

def say_hello(name):
  print('Knock knock ' + name)

def repeat_hello(name, times):
  print('Greeting ', name, times, 'times')
  for i in range(times):
    say_hello(name)
  
repeat_hello('Penny', 3)
say_hello('Sheldon')

Parameter values are are forgotten after the function returns.

def print_uniqueness(v1, v2, v3):
  print(v1 != v2 and v2 != v3 and v3 != v1)
  
print_uniqueness(1,2,4) # True
print_uniqueness(1,1,2) # False

# Error. v1 is not available after function returns
print(v1) 

project_score = int(input('Enter project score:'))
exam_score = int(input('Enter exam score:'))
total_score = project_score + exam_score
print('Total:', total_score)

if total_score >= 60 and project_score >= 25 and exam_score >= 25:
    grade = 'A'
elif (total_score >= 50) and (project_score >= 25 or exam_score >= 25):
    grade = 'B'
elif total_score >= 40:
    grade = 'C'
else:
    grade = 'D'

print('Project :', '=' * (project_score//5))
print('Exam    :', '=' * (exam_score//5))
print('Total   :', '=' * (total_score//5))
print('Grade   :', grade)

print('Project :', '=' * (project_score//5))
print('Exam    :', '=' * (exam_score//5))
print('Total   :', '=' * (total_score//5))

print_score('Project :', project_score)
print_score('Exam    :', exam_score)
print_score('Total   :', total_score)

Enter project score:
30

Enter exam score:
40

Total: 70

Project : ======
Exam    : ========
Total   : ==============
Grade   : A

Return Value

You can use a return statement to make a function return a value.

import random

def play():
  value = random.randint(1,3)
  if value == 1:
    return 'Rock'
  elif value == 2:
    return 'Paper'
  else:
    return 'Scissors'
  
print('Player1 response : ' + play())
print('Player2 response : ' + play())

You can use an empty return statement to return from the function without executing the remainder of the function.

def print_product(a, b):
  print(a, 'x', b, '=', a*b)

def print_all_products(n1, n2, n3):
  # return early if any value is 0
  if n1 == 0 or n2 == 0 or n3 == 0:
    print('Values cannot be zeros')
    return
  
  # print all possible products
  print_product(n1, n2)
  print_product(n2, n3)
  print_product(n3, n1)
  
  
print_all_products(2,3,4)
print_all_products(0,2,3)

def print_score(name, value):
  print(name, '=' * (value//5))

project_score = int(input('Enter project score:'))
exam_score = int(input('Enter exam score:'))

total_score = project_score + exam_score
print('Total:', total_score)

if total_score >= 60 and project_score >= 25 and exam_score >= 25:
    grade = 'A'
elif (total_score >= 50) and (project_score >= 25 or exam_score >= 25):
    grade = 'B'
elif total_score >= 40:
    grade = 'C'
else:
    grade = 'D'

print_score('Project :', project_score)
print_score('Exam    :', exam_score)
print_score('Total   :', total_score)
print('Grade   :', grade)

if total >= 60 and project_score >= 25 and exam_score >= 25:
    grade = 'A'
elif (total >= 50) and (project_score >= 25 or exam_score >= 25):
    grade = 'B'
elif total >= 40:
    grade = 'C'
else:
    grade = 'D'

grade = calculate_grade(project_score, exam_score)

Local and Global Scope

The scope of a variable is which part of the code it can be read/modified. Think of a scope as a container for variables. When a scope is destroyed, all the values stored in the scope’s variables are forgotten.

The global scope is the scope that is applicable to the entire program. There is only one global scope, and it is created when your program begins. When your program terminates, the global scope is destroyed, and all its variables are forgotten. Otherwise, the next time you ran your program, the variables would remember their values from the last time you ran it. A variable that exists in the global scope is called a global variable.

A local scope is a scope that applies only during the execution of a function. A local scope is created whenever a function is called. Parameters and variables that are assigned in the called function are said to exist in that function’s local scope. A variable that exists in a local scope is called a local variable. When the function returns, the local scope is destroyed, and these variables are forgotten. The next time you call this function, the local variables will not remember the values stored in them from the last time the function was called.

A variable must be in the global scope or the local scope; it cannot be in both.

_{[Some parts of the above explanation were adapted from Automate the Boring Stuff]}

Consider the code given below, apparently from a program related to a farm, with the global scope and the local scope of each function indicated by shaded areas.

Note the following rules about scope:

• Rule 1: Local scopes can read global variables e.g., raise_chicken function can access the global variable total_chickens (see line 6).
• Rule 2: Global scope cannot read/write local variables e.g., line 11 will be rejected because it reads the local variable eggs.
• Rule 3: Local scopes cannot read/write variables of other local scopes e.g., line 15 will be rejected because it reads the local variable eggs.
• Rule 4: If a variable is assigned in a local scope, it becomes a local variable even if a global variable has the same name. That is, there can be separate global and local variables with the same name. e.g., line 16 (total_cows = 10) creates a local variable total_cows although there is also a global variable total_cows; the line 19 prints 5 because the total_cows global variable remains 5 and the local variable total_cows which has the value 10 is destroyed after the function milk_cows is executed.

To modify a global variable within a local scope, use the global statement on that variable.

total_cows = 5
print('total cows before breeding:', total_cows)

def breed_cows():
    global total_cows
    print('breeding cows')
    total_cows = 10
    print('total cows at the end of breeding:', total_cows)

breed_cows()
print('total cows after breeding:', total_cows) # prints 10

total cows before breeding: 5
breeding cows
total cows at the end of breeding: 10
total cows after breeding: 10

total cows before breeding: 5
breeding cows
total cows at the end of breeding: 10
total cows after breeding: 5

Enter project score: 10
Enter exam score: 50
Total: 60
Project : ==
Exam    : ==========
Total   : ============
Grade   : B
Continue y/n? y
Enter project score: 50
Enter exam score: 49
Total: 99
Project : ==========
Exam    : =========
Total   : ===================
Grade   : A
Continue y/n? n

Enter project score:
30

Enter exam score:
40

Total: 70

Project : ======
Exam    : ========
Total   : ==============
Grade   : A