Functions

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
    ...

The code below defines a function named say_hello and calls it twice.

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.

[Click here to visualize the execution]

Note how the statements inside the function are not executed unless the method is called.

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

Good (this works)

print('starting ...')

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

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

say_hello()

Bad (this will not work!)

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

# call appears before definition
say_hello()

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

Defining your own functions

Exercise: Hip Hip Hooray!

Exercise : Hip Hip Hooray

Define the missing functions in the code below so that it produces the given output:

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 code inside the function can use the parameters like variables. 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, ...):
    ...

Parameters vs arguments: When you define a function, you can specify parameters that it can accept. When you call the function, you can assign arguments (i.e. values) to each of those parameters.

The say_hello function below takes one parameter. The first time we call it we pass the argument Gina to that parameter, and the next time we pass a different argument John to the same parameter.

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 ?

[Click here to visualize the execution]

The code below has a say_hello function that takes one parameter and a repeat_hello function that takes two parameters. Furthermore, note how one function calls the other.

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')

Knock knock Penny
Knock knock Penny
Knock knock Penny
Knock knock Sheldon

Parameter values are forgotten after the function returns.

The code below produces an error because variable v1 is not available after the function has returned.

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)

Function parameters

Exercise: Grader print_score function

Exercise : Grader - `print_score` Function

The code below prints the grade based on exam and project score, somewhat similar to a previous exercise you did.

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)

Read the above code to understand how it works. Do you remember the meaning of // operator used in the code above?

Add a print_score function to the code and replace the following three lines,

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

with these three lines,

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

without changing the external behavior of the program.

Return Value

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

The play() function below returns one of the strings Rock, Paper, Scissors randomly.

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, even if the function does not return anything.

The print_all_products function below uses an empty return statement to return from the function early if one of the arguments is 0:

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)

Returning values from functions

Exercise: Grader - calculate_grade function

Exercise : Grader - `calculate_grade` Function

In a previous exercise you wrote a function to print scores (an example solution is given below).

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)

Add a calculate_grade function to the code and replace the following lines,

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'

with the line,

grade = calculate_grade(project_score, exam_score)

without changing the external behavior of the program.

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). 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.

The breed_cows function below can increase the global variable total_cows to 10 from the local scope because it has a global total_cows statement. If you remove that statement, the last print statement will print 5 instead of 10

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

without the global total_cows statement

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

Global and local scope

Exercise: Grader - Add main Function

Exercise : Grader - Add a `main` Function

Take the a grader you wrote in the previous exercise. Here is an example of how the code is likely to look like:

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

def calculate_grade(project, exam):
  if total_score >= 60 and project >= 25 and exam >= 25:
    return 'A'
  elif (total_score >= 50) and (project >= 25 or exam >= 25):
    return 'B'
  elif total_score >= 40:
    return 'C'
  else:
    return 'D'

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

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

grade = calculate_grade(project_score, exam_score)

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

Move all the statements that are not inside a function (i.e., the last 10 statements in the code above) into a new function called main. Given below is the expected structure of the code. The external behavior of the code should remain as before.

def print_score(name, value):
  # ...

def calculate_grade(project, exam):
  # ...

def main():
  # MOVE CODE HERE

main()

You might have to do minor changes to the existing function calculate_grade

Exercise: Grader - Restructure the main Function

Exercise : Grader - Restructure the `main` Function

Take the a grader you wrote in the previous exercise. Restructure the main function so that its code looks as follows:

def print_score(name, value):
  # ...

def calculate_grade(project, exam):
  # ...

# ADD MISSING FUNCTIONS

def main():
  project_score = read_number('Enter project score:')
  exam_score = read_number('Enter exam score:')
  grade = calculate_grade(project_score, exam_score)
  print_report(project_score, exam_score, grade)

main()

The external behavior of the code should remain as before.

You need to add functions read_number and print_report.

Exercise: Grader - Analyze Grades in a Loop

Exercise : Grader - Analyze Grades in a Loop

Take the program you wrote in the previous exercise. Enhance it to analyze grades in a loop until the user chooses to stop. A sample output is given below:

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