Python Top Exercises

Python program to swap two variables

Description: Write a Python program to swap the values of two variables.

Rules and hints:

• Do it first with a third (temporary) variable.
• Then do it the Python way, in one line.

Python program to swap two variables

a = 10
b = 20
print("Before swap: a =", a, "b =", b)

a, b = b, a

print("After swap: a =", a, "b =", b)

Output:

How it works

• The line a, b = b, a builds a tuple (b, a) and unpacks it into a and b.
• In many other languages you need a third variable: temp = a, a = b, b = temp. That also works in Python.
• Swapping works with any type, not only numbers.

Python program to check even or odd number

Description: Write a Python program to check whether a number is even or odd.

Rules and hints:

• A number is even if it is divisible by 2.
• Use the modulus operator %, which gives the remainder.

Python program to check even or odd

for number in (4, 7):
    if number % 2 == 0:
        print(number, "is even")
    else:
        print(number, "is odd")

Output:

How it works

• 4 % 2 is 0, so 4 is even.
• 7 % 2 is 1, so the else part runs and 7 is odd.
• Negative numbers also work, because -3 % 2 is 1 in Python.

Learn the basics in Python Conditional Statements.

Python program to check leap year

Description: Write a Python program to check whether a year is a leap year.

Rules and hints:

• A year is a leap year if it is divisible by 4 but not by 100.
• A year divisible by 400 is also a leap year.

Python program to check a leap year

def is_leap(year):
    return (year % 4 == 0 and year % 100 != 0) or year % 400 == 0

for year in (2024, 1900, 2000, 2023):
    if is_leap(year):
        print(year, "is a leap year")
    else:
        print(year, "is not a leap year")

Output:

How it works

• 2024 is divisible by 4 and not by 100, so it is a leap year.
• 1900 is divisible by 100 but not by 400, so it is not a leap year.
• 2000 is divisible by 400, so it is a leap year.
• 2023 is not divisible by 4.

Python program to find factorial of a number

Description: Write a Python program to find the factorial of a number using a loop.

Rules and hints:

• Factorial of 5 is 5 x 4 x 3 x 2 x 1 = 120.
• Start with 1 and multiply by each number.

Python program for factorial using a loop

num = 5
factorial = 1

for i in range(1, num + 1):
    factorial = factorial * i

print("Factorial of", num, "is", factorial)

Output:

How it works

• factorial starts with 1.
• The loop multiplies it with 1, 2, 3, 4 and 5 one by one: 1, 2, 6, 24, 120.
• For 0, the loop does not run, and the answer stays 1, which is correct.

You can also solve it with a function that calls itself. See Python Functions (recursion).

Python program to print Fibonacci series

Description: Write a Python program to print the first 10 numbers of the Fibonacci series.

Rules and hints:

• Each number is the sum of the two numbers before it.
• The series starts with 0 and 1.

Python program for Fibonacci series

a, b = 0, 1

for _ in range(10):
    print(a, end=" ")
    a, b = b, a + b

Output:

How it works

• a and b hold two neighbour numbers of the series.
• print(a, end=" ") prints on the same line with a space.
• a, b = b, a + b moves to the next pair at once.
• The loop variable _ means we do not use its value.

Python program to check prime number

Description: Write a Python program to check whether a number is prime.

Rules and hints:

• A prime number is greater than 1 and has no divisor other than 1 and itself.
• You only need to test divisors up to the square root of the number.

Python program to check a prime number

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

for n in (2, 15, 17, 1):
    if is_prime(n):
        print(n, "is prime")
    else:
        print(n, "is not prime")

Output:

How it works

• Numbers below 2 are not prime.
• For 15 the loop tests 2 and 3. Since 15 % 3 == 0, it returns False.
• For 17 the loop tests 2, 3 and 4. None divides it, so it is prime.
• As soon as one divisor is found, return False ends the function early.

Python program to check palindrome string

Description: Write a Python program to check whether a string is a palindrome.

Rules and hints:

• A palindrome reads the same from both sides, like madam.
• Ignore upper and lower case.

Python program to check a palindrome

def is_palindrome(text):
    text = text.lower()
    return text == text[::-1]

for word in ("madam", "Level", "python"):
    if is_palindrome(word):
        print(word, "is a palindrome")
    else:
        print(word, "is not a palindrome")

Output:

How it works

• lower() changes the text to lowercase, so “Level” becomes “level”.
• text[::-1] is the reversed string.
• If the text equals its reverse, it is a palindrome.

Read about slicing in Python Strings.

Python program to reverse a string

Description: Write a Python program to reverse a string in three different ways.

Rules and hints:

• Strings are immutable, so you create a new string.

Python program to reverse a string

text = "Python"

# 1. slicing
print(text[::-1])

# 2. reversed() and join()
print("".join(reversed(text)))

# 3. loop
result = ""
for ch in text:
    result = ch + result
print(result)

Output:

How it works

• Slicing with step -1 goes through the string from the end.
• reversed() gives the characters in reverse order, and join() makes a string again.
• In the loop, every new character is added to the front of result.

Related: join() function.

Python program to remove duplicates from a list

Description: Write a Python program to remove the duplicate items from a list.

Rules and hints:

• A set keeps only unique values, but it does not keep the order.
• dict.fromkeys() keeps the order of the first appearance.

Python program to remove duplicates from a list

items = [1, 2, 2, 3, 4, 4, 5]

unique_sorted = sorted(set(items))
unique_ordered = list(dict.fromkeys(items))

print(unique_sorted)
print(unique_ordered)

Output:

How it works

• set(items) removes the duplicates, and sorted() makes a sorted list of it.
• A dictionary cannot have duplicate keys, and in Python 3.7+ it keeps the insertion order. So dict.fromkeys(items) followed by list() keeps the first order.

Related: Python Sets and Python Dictionary.

Python program to find the sum of natural numbers using recursion

Description: Write a Python program to find the sum of the first n natural numbers using recursion.

Rules and hints:

• A recursive function calls itself.
• Every recursive function needs a stopping (base) case.

Python program for the sum of natural numbers using recursion

def sum_to(n):
    if n == 0:
        return 0          # base case
    return n + sum_to(n - 1)

print(sum_to(5))
print(sum_to(10))

Output:

How it works

• sum_to(5) returns 5 + sum_to(4), and so on, down to sum_to(0) which returns 0.
• The results come back as 0, 1, 3, 6, 10 and 15.
• Without the base case the function would never stop and Python would raise a RecursionError.

See the Recursion section of Python Functions.