Functions

Need for Functions

• Functions eliminate repetition. Instead of writing the same code multiple times, you define it once and reuse it.
• They help build modular, clean, and manageable programs.
• Functions can be imported and reused across different files.
• They group related logic into a named block, improving both readability and maintainability.

Example without a function:

a = 5
b = 4
c = a + b
print(c)

If you need to add numbers again, you must repeat the same lines → inefficient.

What is a Function?

A function is a named block of reusable code created using the def keyword.

Syntax:

def function_name():
    # indented block → function body

Defining a Function

def greet():
    print("Hello, welcome to Python!")

• def → keyword to define a function
• greet → function name
• () → parentheses (may contain parameters)
• : → indicates start of function block
• Indented block → body of the function

Importance of Indentation

• Python uses indentation instead of curly braces.
• All lines belonging to the function must be indented, otherwise Python cannot identify the function’s scope.

Defining and Calling a Function

To use the function, simply call it by name:

greet()

Example:

def add():
    a = 5
    b = 4
    c = a + b
    print(c)

add()      # Calling the function
add()      # Can be called multiple times

Output:

9
9

Function Parameters and Arguments

Functions can accept dynamic values using parameters, allowing the same function to work flexibly with different inputs each time it’s called.

Example:

def add(x, y):
    a = x
    b = y
    c = a + b
    print(c)

add(2, 3)      # 5

❌ Incorrect call: When the function was defined without parameters.

def add():

    a = x
    b = y
    c = a + b
    print(c)

add(2, 3)

Output:

TypeError: add() takes 0 positional arguments but 2 were given

Return Statement

• If a function does not explicitly return a value, it automatically returns None, indicating the absence of a result.

• Use return to send a specific output back to the caller, allowing the function's result to be reused or stored.

Example without return:

def add(x, y):
    c = x + y
    print(c)

result = add(2, 3)
print("Result is : ", result)

Output:

5
Result is : None

Example with return:

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

result = add(2, 3)
print("Result is : ", result)

Output:

Result is : 5

You can now reuse the returned value anywhere in your program.

Docstrings — Documenting Functions

• Docstrings are triple-quoted strings placed at the beginning of a function.
• They describe what the function does.
• IDEs display them when hovering over the function name.

Example:

def add(x, y):
    """Adds two numbers and returns the result."""
    return x + y

Summary

• Functions prevent repetition and improve modularity.
• Defined using def and identified by indentation.
• Functions can accept parameters and receive arguments.
• Without a return statement, functions return None.
• return allows the result to be reused.
• Docstrings describe the function’s purpose and act as built-in documentation.