Python Control Flow Statements: The else Clauses on Loops

In Python, the else clause can be used with loops (for and while). This may be surprising at first since most people associate else with if statements. However, in loops, the else clause has a unique behavior:

• The else block is executed only if the loop completes all its iterations without encountering a break statement.
• If the loop is exited early because of a break, the else block is skipped.

The else clause in loops (for and while) in Python is a bit unusual because most people associate else with if statements. In the context of loops, the else clause is executed only when the loop finishes normally, meaning it wasn’t interrupted by a break statement.

How It Works:

1. With a for loop:
   • The else block runs if the loop completes all its iterations without hitting a break.
   • If the loop is terminated by a break, the else block is skipped.
2. With a while loop:
   • The else block runs if the while loop condition becomes False naturally.
   • If the loop is terminated by a break, the else block is skipped.

Example with a for loop

Let’s say we’re searching for a specific number in a list.

# List of numbers
numbers = [1, 2, 3, 4, 5]

# Number we want to find
target = 6

# Iterate over the list
for num in numbers:
    if num == target:
        print("Found the target!")
        break
else:
    print("Target not found in the list.")

Explanation:

• The loop checks each number to see if it matches the target.
• If the number is found, the loop breaks, and the else clause is skipped.
• If the loop finishes without finding the target (i.e., without a break), the else block runs, printing “Target not found in the list.”

3. Combined Example

Use Case: Login system with limited attempts.

max_attempts = 3
correct_password = "secret123"

for attempt in range(1, max_attempts + 1):
    password = input(f"Attempt {attempt}: Enter password: ")
    if password != correct_password:
        print("Wrong password. Try again.")
        continue  # Skip to next attempt
    else:
        print("Login successful!")
        break  # Exit loop on success
else:
    print("Account locked. Too many failed attempts.")

Output (if user fails 3 times):

Attempt 1: Enter password: hello
Wrong password. Try again.
Attempt 2: Enter password: test
Wrong password. Try again.
Attempt 3: Enter password: 123
Wrong password. Try again.
Account locked. Too many failed attempts.

Example with a while loop

# Counter
i = 1

# Loop condition
while i <= 5:
    if i == 3:
        print("Breaking the loop")
        break
    print(i)
    i += 1
else:
    print("Loop finished without breaking.")

Explanation:

• The loop runs while i is less than or equal to 5.
• If i equals 3, the loop breaks.
• Since the loop is broken before it naturally ends, the else block is skipped.

Why Use the else Clause with Loops?

Using else with loops can be helpful when you’re performing a search or some operation where you want to know if the loop completed successfully or was interrupted by a break. It’s a clean way to handle scenarios where the loop might end early.

Example #: for..else

for x in range(3):
    print(x)
else:
    print('Final x = %d' % (x))