Chapter 6: Functions in Python

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“The only way to do great work is to love what you do.”
– Steve Jobs

6.1 What is a Function?

A function is a block of reusable code that performs a specific task. It’s reusable, which means you can call it multiple times in your program. This helps to organize your code, make it more readable, and avoid repetition.

**Why Do We Use Functions? **

We use functions in Python for several reasons:

Code Reusability: You can call a function multiple times instead of repeating code. This saves time and effort.
Modularity: Breaking down a large program into smaller, manageable chunks (functions) makes it easier to understand and maintain.
Avoiding Repetition: Functions prevent you from writing the same code over and over, reducing errors and improving efficiency.”

6.2 How to Write a Function

To define a function, you use the def keyword followed by the function name, parentheses for parameters, and a colon. The code block that defines the function is indented.

Syntax:

def function_name(parameters):
  # Function body
  # Code to be executed

Example 6.1: Defining and Calling a Function

def greet(name):
  print("Hello,", name + "!")

# Calling the function
greet("Ahmad")  # Output: Hello, Ahmad!

Explanation:

def greet(name): defines a function named greet that takes one parameter, name.
print("Hello,", name + "!") is the function body, which prints a greeting message using the provided name.
greet("Ahmad") calls the function with the argument “Ahmad”.

Key Points:

Parameters: These are variables passed to the function when it’s called. For more details, See Appendix B: Parameters and Arguments
Return Value: A function can optionally return a value using the return statement.
Docstrings: It’s good practice to include a docstring (a string that explains the function’s purpose) after the function definition.

6.3 Return Statement

Functions can return values using the return keyword.

Example 6.2: Function with a Return Value

def add(x, y):
  return x + y

result = add(3, 5)
print(result)  # Output: 8

Task 1: Create a Function to Calculate the Area of a Rectangle

Function Requirements:

Define a function named calculate_area that takes two parameters: length and width.
The function should calculate the area of the rectangle (Area = Length × Width) and return the result.

Input:

Length (a positive float or integer)
Width (a positive float or integer)

Output:

The area of the rectangle (a float)

Expected Output

The area of the rectangle with length 5 and width 3 is: 15

Additional Test Cases

Input: length = 7, width = 2
- Output: The area of the rectangle with length 7 and width 2 is: 14
Input: length = 10.5, width = 4.2
- Output: The area of the rectangle with length 10.5 and width 4.2 is: 44.1

Task 2: Create a Function to Check if a Number is Even or Odd

Function Requirements:

Define a function named is_even that takes one parameter: number.
The function should determine if the number is even or odd.
It should return the string "Even" if the number is even, and "Odd" if the number is odd.

Input:

A single integer (positive or negative)

Output:

A string: either "Even" or "Odd"

Expected Output

The number 4 is: Even

Additional Test Cases

Encourage beginners to test the function with various numbers:

Input: num = 7
- Output: The number 7 is: Odd
Input: num = -2
- Output: The number -2 is: Even
Input: num = 0
- Output: The number 0 is: Even

6.4 Default Arguments

You can assign default values to parameters, which makes them optional when calling the function.
Video: Learn How to Use Default Parameters in Function Definition

Example 6.3:

   def greet(name, message="Hello"):
       print(f"{message}, {name}!")

   greet("Alice")           # Uses default message "Hello"
   greet("Alice", "Hi")      # Overrides default with "Hi"

6.5 Keyword Arguments

Python allows you to specify arguments by name, making your code more readable.
Example:

Example 6.4:

   def multiply(a, b):
       return a * b

   result = multiply(b=3, a=5)  # You can specify arguments in any order

Task 3: Basic Default Argument Task

Task: Write a function greet that takes a name as an argument and a greeting message with a default value of “Hello”. If no greeting is provided, the function should use “Hello.”
Example: greet("Alice") should output "Hello, Alice!" and greet("Alice", "Good morning") should output "Good morning, Alice!"

Task 4: Create a Function with Multiple Defaults and Modify One

Task: Write a function calc_price that accepts price, tax=0.05, and discount=0. Calculate the final price after applying tax and discount. Test with various keyword arguments to see how changes affect the result.
Example: calc_price(100), calc_price(100, discount=0.1), calc_price(100, tax=0.07, discount=0.1)

video: Guard Statements in Python: Explained Simply

Python Quiz - Functions

Fix the Errors

Fixing Errors in Function Calls and Assignments

def greet():
    print("Hello World!")

greeting = greet

True/False (Mark T for True and F for False)

Multiple Choice (Select the best answer)

What is the output of the following code?

def myfunction(val):
	return val % 4 == 0
print(myfunction (13) or myfunction (8))

A) 0
B) 13
C) False
D) True
E) 3.5
Watch the Video Tutorial for the Answer: https://youtu.be/laKpsLlq60I

What is the output of the following code? Python Quiz #88

  def greet(name="User"):
    return "Hello, " + name
  print(greet("Ahmad"))

A) `Hello, User`  
B) `Hello, Ahmad`  
C) `Hello`  
D) `Error`  

What is the output of the following code? Python Quiz #89

def my_function():
  pass
print(my_function())

- A) `None`  
- B) `0`  
- C) `True`  
- D) `Error`  

What is the output of the following code? Python Quiz #90

def my_func(a, b=2, c=3):
    return a + b + c
print(my_func(5, c=4))

A) 11
B) 12
C) 10
D) Error

Which of the following function calls is invalid for this function definition? [Python Quiz #93]

def my_func(a, b, c=3):
    return a + b + c

A) my_func(1, 2)
B) my_func(1, 2, 4)
C) my_func(a=1, b=2, c=5)
D) my_func(1, c=4, b=2, 5)

What is the output of the following code? [Python Quiz #91]

def change_value(x):
    x = 10

num = 5
change_value(num)
print(num)

A) 5
B) 10
C) Error
D) None

What is the output of the following code? [Python Quiz #96]

def greet(name: str) -> str:
    return "Hello, " + name + "!"

result = greet(5)
print(result)

 - A) Hello, 5!
 - B) TypeError
 - C) None
 - D) Hello, !

What will be the output of this code? [Python Quiz #87]
```
 def func(x, y=2):
     return x * y
 print(func(3))
```
- A) 2
- B) 6
- C) 3
- D) Error
What is the main purpose of a function in Python?

A) To group a set of related code into a single unit
B) To create a new type of data
C) To write a program in a single line
D) To change the value of global variables

What is the purpose of the return statement in a function in Python?

A) To print the output of the function
B) To exit the function and return a value
C) To execute the function without returning anything
D) To stop the function and start a new one

What is the correct way to define a function in Python?

A) function my_function():
B) def my_function():
C) define my_function():
D) my_function() {

What happens if you don’t include a return statement in a function?
- A) The function will return None.
- B) The function will cause an error.
- C) The function will return 0.
- D) The function will return the last variable used.

Exercises

Beginner

Write a Python program that takes two numbers as input and prints their sum.
- [Watch the Solution Now ✨](https://www.youtube.com/watch?v=CQHXsGnUns0&list=PLKYRx0Ibk7Vi-CC7ik98qT0VKK0F7ikja&index=24
Write a function sum3(num1,num3,num3) that takes three numbers as input and returns the sum.
Write a function SumNum(num1) that takes a number as input and returns the sum of numbers from 1 to that number (num1).
Write a function sumSquares(x) that takes a list of numbers as input and returns the sum of their squares.
Write a function order_food that accepts a main_dish, an optional side_dish with a default value of "fries", and an optional drink with a default of "water". Call this function using both positional and keyword arguments.
- Example: order_food("burger"), order_food("pizza", drink="soda"), and order_food("salad", side_dish="breadsticks", drink="juice")
Create a function introduce that takes three parameters: name, age, and city. Set default values for age to 18 and city to “Unknown”. Test calling the function with different combinations of arguments.
- Example: introduce("John"), introduce("John", 25), introduce("John", 25, "New York")
Define a function student_profile that accepts name, grade, and subject with a default value of "Math". Use keyword arguments to call this function in different orders.
- Example: student_profile(grade="A", name="Emma") and student_profile("Sophia", "B", subject="History")
Write a function add_numbers that takes two numbers and returns their sum.
- Example: add_numbers(3, 5) should return 8
Write a function circle_area that calculates the area of a circle given its radius. Use the formula: area = π * radius² (you can use 3.14 for π).
- Example: circle_area(3) should return approximately 28.26
Write a function celsius_to_fahrenheit that takes a temperature in Celsius and converts it to Fahrenheit using the formula: Fahrenheit = Celsius * (9/5) + 32.

Example: celsius_to_fahrenheit(25) should return 77.0

Write a function is_even that takes a number and returns True if the number is even, and False otherwise.

Example: is_even(4) should return True and is_even(7) should return False

Write a function max_of_three that takes three numbers and returns the largest one.

Example: max_of_three(3, 7, 5) should return 7

Write a function simple_interest that calculates simple interest given principal, rate, and time using the formula: interest = (principal * rate * time) / 100.

Example: simple_interest(1000, 5, 2) should return 100.0

Intermediate

Write a function is_prime that checks if a number is prime. A prime number has only two divisors: 1 and itself.

Example: is_prime(7) should return True and is_prime(8) should return False

Write a function factorial that takes a number and returns its factorial. (Factorial of 5 is 5 * 4 * 3 * 2 * 1 = 120)

Example: factorial(5) should return 120

Write a program with three functions:
isEven(n): This function takes an integer n as input and returns True if n is even and False otherwise. You can use the modulo operator (%) to check for evenness.
printTable(n): This function takes an integer n as input and prints its multiplication table. The table should show the product of n with each number from 1 to 10, formatted like n * i = n * i, where i is the current number in the loop.
main: The main program should:
- Prompt the user to enter an integer.
- Use the isEven(n) function to check if the entered number is even.
- If the number is even, call the printTable(n) function to print its multiplication table.
- If the number is odd, print a message indicating the number is odd and not eligible for printing a table.

Example output:

Enter an integer: 4
is even! Here's its multiplication table:
* 1 = 4
* 2 = 8
* 3 = 12
...
* 10 = 40

Write a function fibonacci that takes a number n and returns the nth number in the Fibonacci sequence.

Example: fibonacci(5) should return 5 (sequence: 0, 1, 1, 2, 3, 5, …)

Advanced

Projects

Create a Number Guessing Game

Function Requirements:

Define a function named guess_number that takes no parameters.
The function should randomly select a number between 1 and 100.
Prompt the user to guess the number, providing feedback on whether their guess is too high, too low, or correct.
The game should continue until the user guesses the correct number.
Once the user guesses correctly, the function should print a congratulatory message and the number of attempts it took.

Input:

User input (guesses) from the console

Output:

Feedback on each guess and a congratulatory message upon a correct guess

Expected Output

When the user plays the game, the interaction might look like this:

Welcome to the Number Guessing Game!
Guess a number between 1 and 100.
Enter your guess: 50
Too low! Try again.
Enter your guess: 75
Too high! Try again.
Enter your guess: 60
Congratulations! You've guessed the number 60 in 3 attempts.

Notes for Beginners

Random Number Generation: You can use the random module to select a random number.
Input Handling: Use input() to get the user’s guess and convert it to an integer.
Loops and Conditionals: This task will help practice loops for continuous guessing and conditionals for feedback.

Review Questions

References and Bibliography

Which of the following will cause a syntax error due to incorrect indentation in Python?

print("Hello World!")

def my_function():
print("Hello World!")

if x == 10:
    print("x is 10")

x = 10

Answer: B

Appendices

Appendix A: Parameters and Arguments

Parameters are defined by the names that appear in a function definition, whereas arguments are the values actually passed to a function when calling it. Parameters define what kind of arguments a function can accept.

Parameters

Definition: Variables declared in a function’s definition.
Purpose: Act as placeholders for values that will be passed to the function when it’s called.
Location: Inside the function’s parentheses.

Arguments

Definition: Actual values passed to a function when it’s called.
Purpose: Provide data for the function to work with.
Location: Inside the function call parentheses.

See also the FAQ question of Python Documentation on the difference between arguments and parameters.

Example 6.3: Defining a Function with Parameters and Passing Arguments

def greet(name):  # 'name' is a parameter
    print("Hello,", name + "!")

greet("Alice")  # "Alice" is an argument

In this example:

name is a parameter in the function greet.
"Alice" is an argument passed to the function when it’s called.

To summarize:

Parameters are defined before the function is called.
Arguments are provided when the function is called.

Think of it like this:

A parameter is like an empty box that expects a value.
An argument is the value you put into the box.

Sure! Here’s a simple task for beginners to practice writing functions in Python, along with input and output examples.