Functions and Methods

Functions and Methods – Reusable Code Blocks and the DRY Principle

You have written a few lines of code. It works. But then you realize you need to do the same calculation in five different places. Copy-pasting the same code five times is messy, error-prone, and a nightmare to change later. Enter functions – reusable blocks of code that you can call by name.

1. What Is a Function?

A function is a named block of code that performs a specific task. You "call" the function by its name, and the computer executes the code inside it.

def greet():

    print("Hello, welcome to the program!")

 

# Call the function

greet()

2. Parameters and Arguments

Parameters are variables listed in the function definition. Arguments are the actual values you pass when calling.

def greet_user(name):

    print("Hello, " + name + "!")

 

greet_user("Alice")   # prints: Hello, Alice!

greet_user("Bob")     # prints: Hello, Bob!

 

# Multiple parameters

def rectangle_area(length, width):

    area = length * width

    print("Area is", area)

 

rectangle_area(5, 3)   # Area is 15

3. Return Values

def add(a, b):

    result = a + b

    return result

 

total = add(10, 20)

print(total)   # 30

 

# Multiple return values (using tuples)

def circle_stats(radius):

    area = 3.14159 * radius * radius

    circumference = 2 * 3.14159 * radius

    return area, circumference

 

a, c = circle_stats(5)

print(f"Area: {a}, Circumference: {c}")

4. Scope – Where Variables Live

Variables defined inside a function are local – they cannot be accessed outside. Variables defined outside any function are global.

def my_func():

    x = 10   # local variable

    print(x)

 

my_func()

# print(x)   # ERROR! x is not defined outside the function

Good practice: Avoid global variables. Pass everything as parameters and return results.

5. The DRY Principle – Don't Repeat Yourself

# Without functions (bad – repetitive)

area1 = 5 * 3

print(area1)

area2 = 7 * 2

print(area2)

area3 = 10 * 4

print(area3)

 

# With functions (good – DRY)

def area(length, width):

    return length * width

 

print(area(5, 3))

print(area(7, 2))

print(area(10, 4))

If you later need to change the formula, you change it in one place, not three.

6. Methods – Functions Attached to Objects

A method is a function that belongs to an object. You call it using dot notation: object.method().

name = "alice"

print(name.upper())      # "ALICE"

print(name.capitalize()) # "Alice"

 

numbers = [3, 1, 4, 1, 5]

numbers.sort()           # sorts the list in place

numbers.append(9)        # adds 9 at the end

7. Example – A Simple Calculator with Functions

def add(a, b):

    return a + b

 

def subtract(a, b):

    return a - b

 

def multiply(a, b):

    return a * b

 

def divide(a, b):

    if b == 0:

        return "Error: Division by zero"

    return a / b

 

# Main program

print("Simple Calculator")

while True:

    op = input("Enter operation (+, -, *, /) or 'q' to quit: ")

    if op == 'q':

        break

    if op not in ['+', '-', '*', '/']:

        print("Invalid operation")

        continue

 

    num1 = float(input("Enter first number: "))

    num2 = float(input("Enter second number: "))

 

    if op == '+':

        result = add(num1, num2)

    elif op == '-':

        result = subtract(num1, num2)

    elif op == '*':

        result = multiply(num1, num2)

    elif op == '/':

        result = divide(num1, num2)

 

    print(f"Result: {result}\n")

8. Common Mistakes

Mistake	Why it fails	Fix
Forgetting to return a value	The function returns None, causing unexpected results.	Add a return statement.
Using a variable inside a function without passing it	The function looks for a variable that does not exist in its scope.	Pass it as a parameter.
Defining a function but never calling it	No output; code inside never runs.	Add a call: function_name()

Summary

Term	Definition
Function	Reusable block of code with a name.
Parameter	Variable in the function definition that receives an argument.
Argument	Actual value passed to a function when called.
Return value	Value that the function sends back to the caller.
Local variable	Variable defined inside a function; not accessible outside.
DRY	Don't Repeat Yourself – principle of avoiding duplicate code.
Method	A function that belongs to an object (e.g., "hello".upper()).

Review Questions

1. Write a function called is_even that takes an integer and returns True if it is even, False otherwise.
2. What is the output of the following code?
   def test(x):
       x = x + 5
       return x
   y = 10
   result = test(y)
   print(y, result)
3. Explain why using functions makes a program easier to maintain than copy-pasting the same code multiple times.