A function groups statements under a name. When the same task appears in several places, define it once and call it whenever needed. This reduces duplication and makes updates easier.

Remember

Keep one source of truth: if the logic changes, update the function in one place.

Worked example

def show_welcome():
    print("Welcome to Python!")

show_welcome()
show_welcome()

🧠 Exam quick notes

Define a function using def, followed by a name and parentheses.
The function body must be consistently indented.
Call the function by writing its name followed by parentheses.
Test typical inputs and boundary cases.

✍️ Mini task

Why use a function when the same statements are needed several times?

🧩

TOPIC 02

Hiding Complexity

Give a meaningful name to a multi-step operation.

📘 Understand the idea

A function can hide implementation details behind a simple name. A caller needs to know what the function does and what inputs it expects, not every internal step.

Remember

Choose descriptive names and keep each function focused on one responsibility.

Worked example

def rectangle_area(length, width):
    return length * width

print(rectangle_area(5, 3))

🧠 Exam quick notes

Define a function using def, followed by a name and parentheses.
The function body must be consistently indented.
Call the function by writing its name followed by parentheses.
Test typical inputs and boundary cases.

✍️ Mini task

How does a function help hide complexity?

📥

TOPIC 03

Parameters

Parameters are names in a function definition.

📘 Understand the idea

Parameters act as local names that receive values when a function is called. They make a function reusable with different inputs.

Remember

Parameters appear in the function definition, inside the parentheses.

Worked example

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

greet("Asha")

🧠 Exam quick notes

Define a function using def, followed by a name and parentheses.
The function body must be consistently indented.
Call the function by writing its name followed by parentheses.
Test typical inputs and boundary cases.

✍️ Mini task

In def greet(name), what is name?

📨

TOPIC 04

Arguments

Arguments are the values passed to a function call.

📘 Understand the idea

When calling a function, supply arguments for its parameters. The values can be literals, variables, or expressions.

Remember

Remember: parameter = definition; argument = call.

Worked example

def square(number):
    return number * number

result = square(6)
print(result)

🧠 Exam quick notes

Define a function using def, followed by a name and parentheses.
The function body must be consistently indented.
Call the function by writing its name followed by parentheses.
Test typical inputs and boundary cases.

✍️ Mini task

In square(6), what is 6?

↩️

TOPIC 05

Return Values

Use return to send a result back to the caller.

📘 Understand the idea

A return statement ends the current function call and provides a result. The caller can store that result or use it in an expression. A function without an explicit return returns None.

Remember

print() displays a value; return gives a value back to the caller.

Worked example

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

total = add(8, 4)
print(total)

🧠 Exam quick notes

Define a function using def, followed by a name and parentheses.
The function body must be consistently indented.
Call the function by writing its name followed by parentheses.
Test typical inputs and boundary cases.

✍️ Mini task

What does return do?

🔁

TOPIC 06

Formal vs Actual Arguments

Distinguish parameter names from supplied values.

📘 Understand the idea

Formal parameters are the names written in a function definition. Actual arguments are the values or expressions supplied in a function call.

Remember

Use the exact terminology requested by your course: formal parameter in definition; actual argument in call.

Worked example

def multiply(x, y):  # x and y are formal parameters
    return x * y

print(multiply(3, 7))  # 3 and 7 are actual arguments

🧠 Exam quick notes

Define a function using def, followed by a name and parentheses.
The function body must be consistently indented.
Call the function by writing its name followed by parentheses.
Test typical inputs and boundary cases.

✍️ Mini task

Which are the formal parameters in multiply(x, y)?

🏷️

TOPIC 07

Named Arguments

Pass arguments using parameter names.

📘 Understand the idea

Keyword (named) arguments identify which parameter receives each value, making calls easier to read. In Python, positional arguments must come before keyword arguments in a call.

Remember

Example: describe(item="book", quantity=2). Names must match the function parameters.

Worked example

def describe(item, quantity):
    print(quantity, item)

describe(item="notebook", quantity=2)

🧠 Exam quick notes

Define a function using def, followed by a name and parentheses.
The function body must be consistently indented.
Call the function by writing its name followed by parentheses.
Test typical inputs and boundary cases.

✍️ Mini task

Why can named arguments improve readability?

🌀

TOPIC 08

Recursive Functions

A recursive function calls itself.

📘 Understand the idea

Recursion can express problems that break naturally into smaller versions of the same problem. Every correct recursive solution needs a base case to stop further calls.

Remember

Without a reachable base case, recursion may continue until Python raises RecursionError.

Worked example

def factorial(n):
    if n <= 1:  # base case
        return 1
    return n * factorial(n - 1)

print(factorial(5))

🧠 Exam quick notes

Define a function using def, followed by a name and parentheses.
The function body must be consistently indented.
Call the function by writing its name followed by parentheses.
Test typical inputs and boundary cases.

✍️ Mini task

What is the purpose of a base case?

⚡

TOPIC 09

Lambda Functions

Create a small anonymous function with one expression.

📘 Understand the idea

A lambda expression creates a function object, often for a short operation used briefly as an argument. A lambda body is a single expression, not a block of statements.

Remember

Use def for named, multi-step functions; use lambda for concise expressions.

Worked example

double = lambda n: n * 2
print(double(9))

🧠 Exam quick notes

Define a function using def, followed by a name and parentheses.
The function body must be consistently indented.
Call the function by writing its name followed by parentheses.
Test typical inputs and boundary cases.

✍️ Mini task

How many expressions can a lambda body contain?

HANDS-ON LEARNING

Practical Python Lab 🧪

Minimum three practicals are required; four runnable exercises are provided. Edit the code and run it directly in your browser.

PRACTICAL 01

Practical 1 · Calculator Functions

Define and call separate functions for addition, subtraction, multiplication, and safe division.

🐍 Python · Pyodide

Editable Python code

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 "Cannot divide by zero"
    return a / b

print("Add:", add(12, 4))
print("Subtract:", subtract(12, 4))
print("Multiply:", multiply(12, 4))
print("Divide:", divide(12, 4))

Ready to run

OUTPUT

Click “Run Python” to execute this example.

The Python runtime loads on first use and needs an internet connection. Modify the code to experiment.

PRACTICAL 02

Practical 2 · Student Marks and Grade

Use parameters, a return value, and if/elif logic to classify a mark.

🐍 Python · Pyodide

Editable Python code

def grade_for(mark):
    if mark < 0 or mark > 100:
        return "Invalid mark"
    if mark >= 90:
        return "A"
    elif mark >= 75:
        return "B"
    elif mark >= 50:
        return "C"
    return "Needs improvement"

for mark in [95, 82, 64, 42, 120]:
    print(mark, "->", grade_for(mark))

Ready to run

OUTPUT

Click “Run Python” to execute this example.

The Python runtime loads on first use and needs an internet connection. Modify the code to experiment.

PRACTICAL 03

Practical 3 · Recursive Factorial

Calculate factorial using a base case and a recursive call.

🐍 Python · Pyodide

Editable Python code

def factorial(n):
    if n < 0:
        raise ValueError("n must be non-negative")
    if n <= 1:
        return 1
    return n * factorial(n - 1)

for value in range(6):
    print(value, "! =", factorial(value))

Ready to run

OUTPUT

Click “Run Python” to execute this example.

The Python runtime loads on first use and needs an internet connection. Modify the code to experiment.

PRACTICAL 04

Practical 4 · Lambda and Sorting

Use a lambda expression as a key to sort records by score.

🐍 Python · Pyodide

Editable Python code

students = [("Maya", 82), ("Arun", 95), ("Leela", 76)]

by_score = sorted(students, key=lambda student: student[1], reverse=True)
for name, score in by_score:
    print(name, score)

double = lambda n: n * 2
print("Double 14:", double(14))

Ready to run

OUTPUT

Click “Run Python” to execute this example.

The Python runtime loads on first use and needs an internet connection. Modify the code to experiment.

YOUR LEARNING PROGRESS

Celebrate each step 🏆

Tick each topic after reading the notes and attempting its mini task.

Unit III progress0%

🎯 Learning outcomes

By the end of Unit III, you should be able to create reusable functions, distinguish parameters from arguments, return values, use named arguments, explain recursion, and write simple lambda expressions.

CONTINUE LEARNING

References & learning resources 📚

Python Tutorial — Defining Functions (official Python docs) ↗
Python language reference — Function definitions ↗
Python expressions — Lambda expressions ↗

Python #3 Recursive, Lambda Functions

Choose a topic 🌟

Hiding Redundancy

Hiding Complexity

Parameters

Arguments

Return Values

Formal vs Actual Arguments

Named Arguments

Recursive Functions

Lambda Functions

Hiding Redundancy

📘 Understand the idea

Remember

Worked example

🧠 Exam quick notes

✍️ Mini task

Hiding Complexity

📘 Understand the idea

Remember

Worked example

🧠 Exam quick notes

✍️ Mini task

Parameters

📘 Understand the idea

Remember

Worked example

🧠 Exam quick notes

✍️ Mini task

Arguments

📘 Understand the idea

Remember

Worked example

🧠 Exam quick notes

✍️ Mini task

Return Values

📘 Understand the idea

Remember

Worked example

🧠 Exam quick notes

✍️ Mini task

Formal vs Actual Arguments

📘 Understand the idea

Remember

Worked example

🧠 Exam quick notes

✍️ Mini task

Named Arguments

📘 Understand the idea

Remember

Worked example

🧠 Exam quick notes

✍️ Mini task

Recursive Functions

📘 Understand the idea

Remember

Worked example

🧠 Exam quick notes

✍️ Mini task

Lambda Functions

📘 Understand the idea

Remember

Worked example

🧠 Exam quick notes

✍️ Mini task

Practical Python Lab 🧪

Practical 1 · Calculator Functions

Practical 2 · Student Marks and Grade

Practical 3 · Recursive Factorial

Practical 4 · Lambda and Sorting

Unit 3 quiz 🧠

Celebrate each step 🏆

🎯 Learning outcomes

References & learning resources 📚

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