Loops in Python

Overview

Loops let you run a block of code repeatedly, which saves time and avoids duplicate statements.

Learning Objectives

• Iterate over sequences of items using for loops.
• Repeat code blocks based on conditions using while loops.
• Use the range() helper to iterate a specific number of times.

Concept Explanation

A for loop goes through items in a collection (like a list or string) one-by-one. A while loop keeps running as long as a specified condition is True. We must change the condition inside while loops, or they will run forever.

Code Examples

Example 1 — Basics

This example introduces the fundamental syntax and concepts.

# Loop three times
for i in range(3):
    print(f'Loop index: {i}')

Example 2 — Everyday Usage

This example demonstrates a realistic scenario handling business parameters.

# Print values of scores list
scores = [88, 92, 79]
total = 0
for score in scores:
    total += score
print(f'Total score: {total}')

Example 3 — Advanced Example

This example shows clean, production-grade code structure following senior development standards.

# Process task list items sequentially
pending_tasks = ['clean', 'verify', 'upload']
while pending_tasks:
    current = pending_tasks.pop(0) # Take the first task
    print(f'Completed task: {current}')

Visual Flow

The following execution flow represents the step-by-step evaluation inside the interpreter:

Evaluate Loop Condition → True? Run Block → Update iteration status → Repeat check

Common Mistakes

Review these common pitfalls when working with this topic:

• Forgetting to update counter variables inside while loops, causing infinite loops.
• Modifying the size of a list directly while iterating over it.
• Using range(len(items)) to loop list elements instead of looping items directly.
• Indenting code statements incorrectly inside loop bodies.

Best Practices

Enforce these Pythonic best practices in your codebase:

for i in range(len(fruits)):
    print(fruits[i]) # Unnecessary index lookups

for fruit in fruits:
    print(fruit) # Clear direct iteration

Quick Revision

Use these key summaries for last-minute revision:

• for loops iterate over collections and ranges.
• while loops repeat statements while checks stay True.
• range(n) generates numbers from 0 up to n-1.
• Change loop variables inside while loops to prevent infinite loops.
• Looping directly over list items is cleaner than indexing.