Python How-Tos, Examples, Exercises, Challenges, Syllabus, Study Plan & Interview Prep

fruits = ["apple", "banana", "apple", "cherry", "banana"]

# Step 1: Start with a list that has duplicates
my_list = [1, 2, 3, 2, 1, 4, 5, 3]

# Step 2: Convert it to a set (duplicates disappear automatically)
unique_set = set(my_list)

# Step 3: Convert it back to a list if you need a list
unique_list = list(unique_set)

print(unique_list)

[1, 2, 3, 4, 5]

# Original list with duplicates
my_list = ["banana", "apple", "banana", "cherry", "apple"]

# New empty list to collect unique items
unique_list = []

# Go through each item in the original list
for item in my_list:
    # Only add the item if it's NOT already in unique_list
    if item not in unique_list:
        unique_list.append(item)

print(unique_list)

['banana', 'apple', 'cherry']

my_list = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]

unique_list = list(dict.fromkeys(my_list))

print(unique_list)

[3, 1, 4, 5, 9, 2, 6]

# Original string
text = "Hello, World!"

# Reverse using slicing
reversed_text = text[::-1]

print(reversed_text)

!dlroW ,olleH

H  e  l  l  o
0  1  2  3  4

text = "Python"

# reversed() returns an iterator, join() combines the characters
reversed_text = "".join(reversed(text))

print(reversed_text)

nohtyP

text = "science"
reversed_text = ""

for char in text:
    reversed_text = char + reversed_text  # each new char goes to the FRONT

print(reversed_text)

ecneics

word = "racecar"

if word == word[::-1]:
    print(word, "is a palindrome!")
else:
    print(word, "is NOT a palindrome.")

racecar is a palindrome!

x = 5
y = 3
result = x + y
print(result)

8

# Ask the user to type two numbers
num1 = input("Enter first number: ")
num2 = input("Enter second number: ")

# IMPORTANT: input() always returns a STRING, not a number
# We must convert to int (whole number) or float (decimal)
sum_result = int(num1) + int(num2)

print("The sum is:", sum_result)

Enter first number: 12
Enter second number: 7
The sum is: 19

print("5" + "3")   # Output: 53  ← wrong! This joins strings
print(5 + 3)       # Output: 8   ← correct! This adds numbers

a = 4.5
b = 2.3

total = a + b
print("Total:", total)

Total: 6.8

num1 = float(input("Enter first decimal: "))
num2 = float(input("Enter second decimal: "))
print("Sum:", num1 + num2)

numbers = [10, 20, 30, 40, 50]
total = sum(numbers)
print("Total:", total)

Total: 150

print("Hello, World!")

Hello, World!

name = "Alice"
age = 25
city = "London"

print(name, "is", age, "years old and lives in", city)

Alice is 25 years old and lives in London

text = "Python"          # String
whole_number = 42        # Integer
decimal = 3.14           # Float
is_fun = True            # Boolean

print(type(text))
print(type(whole_number))
print(type(decimal))
print(type(is_fun))

<class 'str'>
<class 'int'>
<class 'float'>
<class 'bool'>

sentence = "Hello, Python!"
print(len(sentence))

14

word = "Python"
print(word.upper())
print(word.lower())

PYTHON
python

text = "Programming"
print(text[0:4])    # First 4 characters
print(text[-3:])    # Last 3 characters

Prog
ing

sentence = "I love Java"
new_sentence = sentence.replace("Java", "Python")
print(new_sentence)

I love Python

data = "Alice,Bob,Charlie,Diana"
names = data.split(",")
print(names)

['Alice', 'Bob', 'Charlie', 'Diana']

fruits = ["apple", "banana", "cherry"]
print(fruits[0])    # First item
print(fruits[-1])   # Last item
print(fruits[1:3])  # Slice: items at index 1 and 2

apple
cherry
['banana', 'cherry']

numbers = [3, 1, 4, 1, 5, 9, 2, 6]

numbers.append(7)        # Add to end
numbers.remove(1)        # Remove first occurrence of 1
numbers.sort()           # Sort ascending

print(numbers)

[1, 2, 3, 4, 5, 6, 7, 9]

colours = ["red", "green", "blue"]

for colour in colours:
    print("Colour:", colour)

Colour: red
Colour: green
Colour: blue

student = {
    "name": "Bob",
    "age": 20,
    "grade": "A"
}

print(student["name"])
print(student["grade"])

Bob
A

person = {"name": "Carol", "city": "Paris", "job": "Engineer"}

for key, value in person.items():
    print(key, "→", value)

name → Carol
city → Paris
job → Engineer

temperature = 35

if temperature > 30:
    print("It's hot outside!")
elif temperature > 20:
    print("It's warm.")
else:
    print("It's cool.")

It's hot outside!

score = 78

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print("Grade:", grade)

Grade: C

for i in range(1, 6):
    print("Step", i)

Step 1
Step 2
Step 3
Step 4
Step 5

count = 0

while count < 5:
    print("Count:", count)
    count += 1  # count = count + 1

Count: 0
Count: 1
Count: 2
Count: 3
Count: 4

for num in range(1, 11):
    if num == 5:
        continue       # Skip number 5
    if num == 8:
        break          # Stop at number 8
    print(num)

1
2
3
4
6
7

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

greet("Alice")
greet("Bob")

Hello, Alice!
Hello, Bob!

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

result = add(10, 25)
print("Sum:", result)

Sum: 35

def greet(name, message="Welcome!"):
    print(name + ":", message)

greet("Alice")
greet("Bob", "Nice to meet you!")

Alice: Welcome!
Bob: Nice to meet you!

# Regular function
def square(x):
    return x * x

# Equivalent lambda
square_lambda = lambda x: x * x

print(square(5))         # Output: 25
print(square_lambda(5))  # Output: 25

class Dog:
    def __init__(self, name, breed):
        self.name = name
        self.breed = breed

    def bark(self):
        print(self.name, "says: Woof!")

# Create objects from the class
dog1 = Dog("Rex", "Labrador")
dog2 = Dog("Bella", "Poodle")

dog1.bark()
dog2.bark()
print(dog1.breed)

Rex says: Woof!
Bella says: Woof!
Labrador

with open("notes.txt", "w") as file:
    file.write("Hello, this is a note.\n")
    file.write("Python file handling is easy!")

with open("notes.txt", "r") as file:
    content = file.read()
    print(content)

Hello, this is a note.
Python file handling is easy!

My name is Sam, I am 22 years old, and my favourite colour is blue.

name = ___
age = ___
colour = ___

print("My name is", ___, "I am", ___, "years old, and my favourite colour is", ___ + ".")

name = "Sam"
age = 22
colour = "blue"

print("My name is", name + ", I am", age, "years old, and my favourite colour is", colour + ".")

current_year = 2025
age = int(input("How old are you? "))
birth_year = current_year - age
print("You were probably born in", birth_year)

How old are you? 20
You were probably born in 2005

word = input("Enter a word: ").lower()

if word == word[::-1]:
    print(word, "is a palindrome.")
else:
    print(word, "is not a palindrome.")

Enter a word: level
level is a palindrome.

Enter a word: hello
hello is not a palindrome.

text = input("Enter a sentence: ")
vowels = "aeiouAEIOU"
count = 0

for char in text:
    if char in vowels:
        count += 1

print("Number of vowels:", count)

Enter a sentence: Hello World
Number of vowels: 3

numbers = [45, 12, 78, 34, 90, 23]

largest = numbers[0]

for num in numbers:
    if num > largest:
        largest = num

print("Largest number:", largest)

Largest number: 90

data = [5, 3, 8, 3, 1, 5, 9, 1, 7]

unique_data = list(set(data))
unique_data.sort()

print("Cleaned and sorted:", unique_data)

Cleaned and sorted: [1, 3, 5, 7, 8, 9]

scores = [85, 92, 76, 88, 91, 67, 73]

total = sum(scores)
average = total / len(scores)

print("Total:", total)
print("Average:", round(average, 2))

Total: 572
Average: 81.71

number = int(input("Enter a number for its multiplication table: "))

for i in range(1, 11):
    print(number, "x", i, "=", number * i)

7 x 1 = 7
7 x 2 = 14
...
7 x 10 = 70

for i in range(1, 31):
    if i % 3 == 0 and i % 5 == 0:
        print("FizzBuzz")
    elif i % 3 == 0:
        print("Fizz")
    elif i % 5 == 0:
        print("Buzz")
    else:
        print(i)

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz

def celsius_to_fahrenheit(celsius):
    return (celsius * 9/5) + 32

def fahrenheit_to_celsius(fahrenheit):
    return (fahrenheit - 32) * 5/9

print(celsius_to_fahrenheit(100))   # Expected: 212.0
print(fahrenheit_to_celsius(32))    # Expected: 0.0

def calculate(a, b, operation):
    if operation == "add":
        return a + b
    elif operation == "subtract":
        return a - b
    elif operation == "multiply":
        return a * b
    elif operation == "divide":
        if b != 0:
            return a / b
        else:
            return "Error: Cannot divide by zero"
    else:
        return "Unknown operation"

print(calculate(10, 5, "add"))       # 15
print(calculate(10, 5, "subtract"))  # 5
print(calculate(10, 5, "multiply"))  # 50
print(calculate(10, 5, "divide"))    # 2.0
print(calculate(10, 0, "divide"))    # Error: Cannot divide by zero

x = 10
y = 3
print(x % y)

text = "Hello"
print(text[1])

numbers = [1, 2, 3, 4, 5]
print(numbers[-1])

for i in range(0, 10, 2):
    print(i)

0
2
4
6
8

mixed = [1, "hello", 3.14, True]

def mystery(n):
    return n * 2 + 1

print(mystery(4))

my_dict = {"a": 1, "b": 2, "c": 3}
print(my_dict["b"])

A) function greet():
B) def greet():
C) define greet():
D) func greet():

sentence = "the cat sat on the mat the cat"

words = sentence.split()
frequency = {}

for word in words:
    if word in frequency:
        frequency[word] += 1
    else:
        frequency[word] = 1

for word, count in frequency.items():
    print(word, ":", count)

the : 3
cat : 2
sat : 1
on : 1
mat : 1

def fibonacci(n):
    a, b = 0, 1
    result = []
    for _ in range(n):
        result.append(a)
        a, b = b, a + b
    return result

print(fibonacci(10))

[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

def is_prime(n):
    if n < 2:
        return False
    for i in range(2, int(n**0.5) + 1):
        if n % i == 0:
            return False
    return True

# Test it
for num in range(2, 20):
    if is_prime(num):
        print(num, "is prime")

2 is prime
3 is prime
5 is prime
7 is prime
11 is prime
13 is prime
17 is prime
19 is prime

nested = [[1, 2, 3], [4, 5], [6, 7, 8, 9]]

flat = []
for sublist in nested:
    for item in sublist:
        flat.append(item)

print(flat)

[1, 2, 3, 4, 5, 6, 7, 8, 9]

flat = [item for sublist in nested for item in sublist]
print(flat)

def are_anagrams(word1, word2):
    return sorted(word1.lower()) == sorted(word2.lower())

print(are_anagrams("listen", "silent"))   # True
print(are_anagrams("hello", "world"))     # False
print(are_anagrams("Triangle", "Integral"))  # True

True
False
True

def find_missing(numbers, n):
    expected_sum = n * (n + 1) // 2
    actual_sum = sum(numbers)
    return expected_sum - actual_sum

nums = [1, 2, 4, 5, 6, 7, 8, 9, 10]
print("Missing number:", find_missing(nums, 10))

Missing number: 3

students = {}

# Add students and their scores
students["Alice"] = [85, 92, 78, 90]
students["Bob"] = [70, 65, 80, 75]
students["Carol"] = [95, 98, 92, 97]

# Calculate and display averages
for name, scores in students.items():
    average = sum(scores) / len(scores)
    print(name + ":", round(average, 1), "average")

Alice: 86.2 average
Bob: 72.5 average
Carol: 95.5 average

cart = {
    "Milk": 1.50,
    "Bread": 2.00,
    "Eggs": 3.25,
    "Butter": 4.75,
    "Coffee": 6.99
}

total = 0
print("=== Shopping Cart ===")

for item, price in cart.items():
    print(f"  {item}: £{price:.2f}")
    total += price

tax = total * 0.20  # 20% VAT
grand_total = total + tax

print("-------------------")
print(f"Subtotal: £{total:.2f}")
print(f"Tax (20%): £{tax:.2f}")
print(f"Total: £{grand_total:.2f}")

=== Shopping Cart ===
  Milk: £1.50
  Bread: £2.00
  Eggs: £3.25
  Butter: £4.75
  Coffee: £6.99
-------------------
Subtotal: £18.49
Tax (20%): £3.70
Total: £22.19

import random

secret = random.randint(1, 100)
attempts = 0

print("Guess the number between 1 and 100!")

while True:
    guess = int(input("Your guess: "))
    attempts += 1

    if guess < secret:
        print("Too low! Try higher.")
    elif guess > secret:
        print("Too high! Try lower.")
    else:
        print(f"Correct! You got it in {attempts} attempts.")
        break

name = input("Enter your name: ")
age = int(input("Enter your age: "))
city = input("Enter your city: ")
hobby = input("Enter your hobby: ")

print("\n=== Your Profile ===")
print("Name:", name)
print("Age:", age)
print("City:", city)
print("Hobby:", hobby)
print("In 10 years you'll be:", age + 10)

my_list = [1, 2, 3]
my_list[0] = 99    # Works fine

my_tuple = (1, 2, 3)
my_tuple[0] = 99   # TypeError! Tuples are immutable

a = [1, 2, 3]
b = [1, 2, 3]

print(a == b)   # True  (same values)
print(a is b)   # False (different objects in memory)

c = a
print(a is c)   # True  (same object)

result = None

if result is None:
    print("No result yet")

person = {
    "name": "Alice",
    "age": 30,
    "job": "Engineer"
}

print(person["name"])     # Alice
print(person.get("age"))  # 30
person["city"] = "London" # Add a new key

list1 = [1, 2, 3]
list1.append([4, 5])
print(list1)   # [1, 2, 3, [4, 5]] ← nested list added

list2 = [1, 2, 3]
list2.extend([4, 5])
print(list2)   # [1, 2, 3, 4, 5] ← items added individually

squares = []
for x in range(1, 6):
    squares.append(x ** 2)

squares = [x ** 2 for x in range(1, 6)]
print(squares)   # [1, 4, 9, 16, 25]

# break: exits the loop entirely
for i in range(10):
    if i == 5:
        break
    print(i)        # Prints 0 1 2 3 4

# continue: skips the current iteration, continues loop
for i in range(10):
    if i == 5:
        continue
    print(i)        # Prints 0 1 2 3 4 6 7 8 9

# pass: does nothing, placeholder
for i in range(3):
    pass            # No output, but no error either

double = lambda x: x * 2
print(double(5))    # 10

# Common use: sort a list of tuples by second element
pairs = [(1, 'b'), (3, 'a'), (2, 'c')]
pairs.sort(key=lambda pair: pair[1])
print(pairs)   # [(3, 'a'), (1, 'b'), (2, 'c')]

def show_args(*args):
    for item in args:
        print(item)

show_args(1, 2, 3)   # Prints 1, 2, 3 each on a new line

def show_kwargs(**kwargs):
    for key, value in kwargs.items():
        print(key, "=", value)

show_kwargs(name="Alice", age=25)
# name = Alice
# age = 25

def my_decorator(func):
    def wrapper():
        print("Before the function runs")
        func()
        print("After the function runs")
    return wrapper

@my_decorator
def say_hello():
    print("Hello!")

say_hello()

Before the function runs
Hello!
After the function runs

import copy

original = [[1, 2], [3, 4]]

shallow = copy.copy(original)
deep = copy.deepcopy(original)

original[0][0] = 99

print(original)   # [[99, 2], [3, 4]]
print(shallow)    # [[99, 2], [3, 4]] ← affected!
print(deep)       # [[1, 2], [3, 4]]  ← NOT affected

def count_up(n):
    for i in range(n):
        yield i

gen = count_up(5)

for value in gen:
    print(value)

0
1
2
3
4

class Animal:
    def __init__(self, name):
        self.name = name

    def speak(self):
        print(self.name, "makes a sound")

class Dog(Animal):    # Dog inherits from Animal
    def speak(self):  # Override parent method
        print(self.name, "says: Woof!")

class Cat(Animal):
    def speak(self):
        print(self.name, "says: Meow!")

dog = Dog("Rex")
cat = Cat("Whiskers")

dog.speak()   # Rex says: Woof!
cat.speak()   # Whiskers says: Meow!

r = range(1_000_000)  # Does NOT create 1 million numbers in memory immediately
print(r[0])   # 0 — only computed when accessed

class Car:
    def __init__(self, brand, model, year):
        self.brand = brand
        self.model = model
        self.year = year

    def describe(self):
        print(f"{self.year} {self.brand} {self.model}")

car1 = Car("Toyota", "Corolla", 2022)
car1.describe()   # 2022 Toyota Corolla

# Stage 1: Create student data
students_raw = [
    {"name": "Alice", "scores": [85, 92, 78]},
    {"name": "Bob", "scores": [70, 65, 80]},
    {"name": "Carol", "scores": [95, 98, 91]},
    {"name": "Alice", "scores": [85, 92, 78]},   # duplicate
    {"name": "Dave", "scores": [60, 55, 70]},
]

print("Raw student count:", len(students_raw))

Raw student count: 5

# Stage 2: Remove duplicate students by name
seen_names = []
students = []

for student in students_raw:
    if student["name"] not in seen_names:
        seen_names.append(student["name"])
        students.append(student)

print("After removing duplicates:", len(students), "students")

After removing duplicates: 4 students

# Stage 3: Add average and grade to each student
def get_grade(average):
    if average >= 90:
        return "A"
    elif average >= 80:
        return "B"
    elif average >= 70:
        return "C"
    elif average >= 60:
        return "D"
    else:
        return "F"

for student in students:
    avg = sum(student["scores"]) / len(student["scores"])
    student["average"] = round(avg, 1)
    student["grade"] = get_grade(avg)

# Stage 4: Print formatted report
print("\n" + "="*40)
print("      STUDENT PERFORMANCE REPORT")
print("="*40)

for student in students:
    print(f"Name:    {student['name']}")
    print(f"Scores:  {student['scores']}")
    print(f"Average: {student['average']}")
    print(f"Grade:   {student['grade']}")
    print("-"*40)

# Stage 5: Find top performer
top_student = max(students, key=lambda s: s["average"])

print(f"\n🏆 Top Student: {top_student['name']}")
print(f"   Average: {top_student['average']} | Grade: {top_student['grade']}")

Raw student count: 5
After removing duplicates: 4 students

========================================
      STUDENT PERFORMANCE REPORT
========================================
Name:    Alice
Scores:  [85, 92, 78]
Average: 85.0
Grade:   B
----------------------------------------
Name:    Bob
Scores:  [70, 65, 80]
Average: 71.7
Grade:   C
----------------------------------------
Name:    Carol
Scores:  [95, 98, 91]
Average: 94.7
Grade:   A
----------------------------------------
Name:    Dave
Scores:  [60, 55, 70]
Average: 61.7
Grade:   D
----------------------------------------

🏆 Top Student: Carol
   Average: 94.7 | Grade: A

# ❌ Wrong
age = input("Enter age: ")
next_year = age + 1        # TypeError: can only concatenate str (not "int") to str

# ✅ Correct
age = int(input("Enter age: "))
next_year = age + 1
print(next_year)

# ❌ Wrong — wants to print 1 to 10 but...
for i in range(10):
    print(i)    # Prints 0 to 9! Not 1 to 10.

# ✅ Correct
for i in range(1, 11):
    print(i)    # Prints 1 to 10

# ❌ Wrong — skips items
nums = [1, 2, 3, 4, 5]
for n in nums:
    if n % 2 == 0:
        nums.remove(n)   # Unexpected behaviour

# ✅ Correct — iterate over a copy
nums = [1, 2, 3, 4, 5]
for n in nums[:]:
    if n % 2 == 0:
        nums.remove(n)

print(nums)   # [1, 3, 5]

# ❌ Wrong — assignment, not comparison
if x = 5:     # SyntaxError

# ✅ Correct — comparison
if x == 5:
    print("x is 5")

# ❌ Wrong
def greet():
print("Hello")   # IndentationError

# ✅ Correct
def greet():
    print("Hello")

# ❌ If you want to add items individually
my_list = [1, 2, 3]
my_list.append([4, 5])     # Adds a nested list: [1, 2, 3, [4, 5]]

# ✅ Use extend to add items individually
my_list = [1, 2, 3]
my_list.extend([4, 5])     # Adds items: [1, 2, 3, 4, 5]

# ❌ Wrong — the list persists between calls!
def add_item(item, lst=[]):
    lst.append(item)
    return lst

print(add_item("a"))   # ['a']
print(add_item("b"))   # ['a', 'b'] ← Bug! Expected ['b']

# ✅ Correct — use None as default
def add_item(item, lst=None):
    if lst is None:
        lst = []
    lst.append(item)
    return lst