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How to Check If a Loop Has Completed in Python

In Python, detecting whether a loop has finished normally (iterated through all items) versus being interrupted (by a break statement) is a common requirement. While flag variables are the traditional way to handle this in many languages, Python offers a unique and elegant else clause specifically for loops.

This guide explains how to track loop completion using boolean flags and the Pythonic for...else construct.

Method 1: Using a Flag Variable (Traditional)​

The most universal way to check if a loop completed is to initialize a boolean variable (flag) before the loop, update it if a specific condition is met (like finding an item), and check it after the loop.

Example: Searching for a Value​

numbers = [1, 3, 5, 7, 9]
target = 4
found = False  # The flag

for num in numbers:
    if num == target:
        found = True
        print(f"Found {target}!")
        break  # Exit loop early

# Check flag after loop
if not found:
    print(f"{target} was not found in the list.")

Output:

4 was not found in the list.

This method is explicit and understood by programmers coming from languages like C++ or Java.

Method 2: Using the else Clause (Pythonic)​

Python allows an else block to be attached directly to a for or while loop.

  • Executed: If the loop finishes naturally (exhausts the iterable).
  • Skipped: If the loop is terminated by a break statement.

This structure eliminates the need for the found flag variable in search logic.

Example: The "Search and Fallback" Pattern​

numbers = [1, 3, 5, 7, 9]
target = 4

for num in numbers:
    if num == target:
        print(f"Found {target}!")
        break # Skips the else block
else:
    # This runs ONLY if the loop completed without breaking
    print(f"{target} was not found in the list.")

Output:

4 was not found in the list.

Example: Successful Completion (Validating All Items)​

You can use this to verify a condition holds for all items.

# Check if all numbers are odd
numbers = [1, 3, 5, 7]

for num in numbers:
    if num % 2 == 0:
        print(f"Found an even number: {num}")
        break
else:
    print("Loop completed: All numbers are odd!")

Output:

Loop completed: All numbers are odd!

Comparison and Best Practices​

FeatureFlag Variablefor...else
ReadabilityHigh (Explicit logic)Medium (Can be confusing to beginners)
Code LengthVerboseConcise
Use CaseComplex logic, multiple break conditionsSimple search/validation logic

Best Practice: Use for...else when the logic is a simple "search and find" or "validate all" scenario. Use flags if the loop body is complex or if you need the status variable for logic later in the script.

Conclusion​

To check if a loop has completed in Python:

  1. Use for...else for a concise, flag-free way to execute code only if the loop was not interrupted by break.
  2. Use a Boolean Flag if you prefer explicit logic or need to track the state beyond the immediate scope of the loop.

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