Usage of 'else' in Python

The else keyword in Python has a few different applications, but it primarily functions within conditional statements:

Conditions

  • if...else statements: This is the most common use of else.
  • It allows you to define an alternate block of code to execute if the condition in the if statement is False.
age = 12
if age >= 18:
    print("You are eligible to vote.")
else:
    print("You are not eligible to vote.")
  • else with ternary operator
age = 12
print("You are eligible to vote.") if age >= 18 else print("You are not eligible to vote.")

# You are not eligible to vote.
  • else with comprehension
# Even/Odd Marker
numbers_list = ["even" if num %2 == 0 else "odd" for num in range(10)]

# ['even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd', 'even', 'odd']

Loops

  • for...else and while...else loops: In Python, you can optionally use an else block with for and while loops.
  • This code block will only execute if the loop terminates normally (i.e., it completes all iterations without a break statement being called).

for else loop

  • Imagine you have a list of groceries and want to check if a specific item is present. Traditionally, you might use a flag variable to track if the item is found:
found = False
grocery_list = ["Milk", "Eggs", "Bread", "Apples"]
item_to_find = "Cheese"

for item in grocery_list:
    if item == item_to_find:
        found = True
        break  # Exit the loop after finding the item

if found:
    print(f"{item_to_find} is in your grocery list.")
else:
    print(f"{item_to_find} is not on your grocery list.")

With for...else, you can achieve the same functionality without needing a flag:

grocery_list = ["Milk", "Eggs", "Bread", "Apples"]
item_to_find = "Cheese"

for item in grocery_list:
    if item == item_to_find:
        print(f"{item_to_find} is in your grocery list.")
        break  # Exit the loop after finding the item
else:
    print(f"{item_to_find} is not on your grocery list.")
  • The else block here executes only if the loop finishes iterating through the entire list without finding a match.
  • `break` statement needs to be added once the condition inside the loop is satisfied to come out of the loop. Otherwise the else statement will execute.
  • This keeps the code cleaner and avoids the need for an extra variable.

while else loop

  • Suppose you want a valid integer input from the user within a specific range. Here’s how you might do it with a while loop:
lower_limit = 5
upper_limit = 10

while True:
    user_input = int(input("Enter an integer between 5 and 10: "))
    if lower_limit <= user_input <= upper_limit:
        print("Valid input!")
        break  # Exit the loop on valid input

print(f"Your input: {user_input}")

With while...else, you can express the validation process more clearly:

lower_limit = 5
upper_limit = 10

while True:
    user_input = int(input("Enter an integer between 5 and 10: "))
    if lower_limit <= user_input <= upper_limit:
        print("Valid input!")
        break  # Exit the loop on valid input
else:
    print("Invalid input. Please enter a number between 5 and 10.")

print(f"Your input (if valid): {user_input}")  # Print input only if valid
  • The else block here executes only if the while loop completes all iterations without finding a valid input.
  • This provides a clear message to the user if their input doesn’t meet the criteria.

Exception Handling

  • try...except...else blocks: The else block in a try...except statement handles exceptions.
  • If no exceptions occur while executing the code in the try block, the code within the else block will be executed.
    try:
    result = 10 / 0  # This will cause a ZeroDivisionError
except ZeroDivisionError:
    print("Division by zero error!")
else:
    print("The result is:", result)  # This won't execute because of the exception
  • Remember that the else block is always optional and only executes under specific conditions depending on the context (i.e., if the if condition is False, the loop completes normally, or no exceptions occur in the try block).

Summary

We have gone through various ways we can use `else` keyword in Python programming language.

Further Read: