Python Functions

What Is A Function?

A function is a reusable block of code. You give it a name, put related steps inside it, and call it whenever you need that work done.

def greet():
    print("Hello")

greet()

The def keyword starts a function definition. The indented block is the function body. The function runs only when it is called.

Parameters And Arguments

Parameters let a function receive values.

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

greet("Asha")
greet("Kabir")

In the definition, name is a parameter. In the calls, "Asha" and "Kabir" are arguments.

Returning Values

Use return when a function should send a value back to the caller.

def add(a, b):
    return a + b

total = add(3, 4)
print(total)

Output:

7

Printing and returning are different. print() displays a value. return gives a value back so the rest of the program can use it.

Default Parameter Values

A parameter can have a default value.

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

greet()
greet("Riya")

The first call uses "friend". The second call uses "Riya".

Function Scope

Variables created inside a function usually belong to that function.

def make_score():
    score = 10
    return score

print(make_score())

The variable score is local to the function. Code outside the function should use the returned value instead of depending on the local variable.

Why Functions Help

Functions make code easier to read and reuse. Instead of copying the same lines again and again, write the logic once and call it with different inputs.

def calculate_total(price, quantity):
    return price * quantity

print(calculate_total(50, 3))
print(calculate_total(120, 2))

Common Mistakes

A common mistake is forgetting to call the function.

def say_hi():
    print("Hi")

This defines the function but prints nothing. You still need:

say_hi()

Another mistake is expecting print() to return the value.

def add(a, b):
    print(a + b)

result = add(2, 3)

Here result becomes None because the function did not return anything.

Quick Summary

• Use def to define a function.
• A function body is indented.
• A function runs when it is called.
• Parameters receive values from arguments.
• return sends a value back to the caller.
• Variables inside a function are usually local.
• Functions reduce repetition and organize code.

Practice Quiz

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