Python Loops

A loop is simply telling Python: “Repeat this block of code multiple times for me.”

Basic Idea

Without a loop, Python reads your code line by line, once.
With a loop, you take a block of code and say: “Do it again, and again… until I tell you to stop.”

Two Types You Need to Know

Think of two everyday situations:

• You know how many times you’ll repeat → “I jump 5 times on the trampoline” → That’s a for loop (you know the number of iterations).

• You don’t know in advance how many times → “I jump until I’m tired” → That’s a while loop (as long as the condition is true).

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For Loop (You Know the Number of Iterations)

Magic phrase to remember:
“For each element in the list, execute the indented block.”

Examples to think about:

• “For each student in the class, I say hello.”
• “For each number in the list, I print it.”

Mental model:

1. Python takes the 1st element from the list.
2. It executes the indented block.
3. It takes the 2nd element, repeats, and so on.

Quick Reference

# Loop through a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)
 
# Loop through a range of numbers
for i in range(5):  # 0, 1, 2, 3, 4
    print(i)
 
# Loop with enumerate (get index + value)
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")
 
# Loop through a dictionary
person = {"name": "Alice", "age": 30, "city": "Paris"}
for key, value in person.items():
    print(f"{key}: {value}")

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While Loop (As Long As…)

Magic phrase:
“While the condition is true, keep looping.”

Real-life examples:

• “While there’s pizza, I eat.”
• “While the player has lives, the game continues.”

Warning: If the condition never becomes false, the loop never stops (infinite loop).

Quick Reference

# Simple while loop
count = 0
while count < 5:
    print(count)
    count += 1  # Don't forget to update the condition!
 
# Loop until user types "quit"
while True:
    user_input = input("Enter something (or 'quit'): ")
    if user_input == "quit":
        break  # Exit the loop
    print(f"You said: {user_input}")
 
# Loop with multiple conditions
player_lives = 3
enemies_defeated = 0
while player_lives > 0 and enemies_defeated < 10:
    # Game logic...
    player_lives -= 1
    enemies_defeated += 1

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Memory Aid

• for → You can count on your fingers before you start (3 times, 10 times, for each element in a list).
• while → You watch a condition that can change (score, lives, user input, counter…).

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Common Loop Control Statements

Statement	Purpose	Example
break	Exit the loop immediately	if score < 0: break
continue	Skip to next iteration	if x < 0: continue
else (with loop)	Runs after loop completes normally	for x in range(5): ... else: print("Done")
range(start, stop, step)	Generate sequence of numbers	range(0, 10, 2) → 0, 2, 4, 6, 8

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Tips & Gotchas

• range() is exclusive of the stop value. range(5) gives 0–4, not 0–5.

• Forgetting to update the while condition is the #1 cause of infinite loops. Always update a counter or change state.

• for loops are usually better than while when iterating over data. for is safer (harder to create infinite loops).

• List comprehensions are faster than loops. [x*2 for x in items] is cleaner and ~30% faster than a for loop with .append().

• Indentation matters! Python uses indentation to define the loop body. Wrong indent = code outside the loop.

• You can nest loops, but keep it simple. More than 2 levels deep gets hard to read. Consider a function instead.

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Related

• Python-Modules-Functions-Lists — Lists are the most common data structure you loop through.
• Python-List-Comprehensions — The Pythonic alternative to for loops with transformations.
• Python-Control-Flow — Often paired with loops for conditional logic.
• Official Python Loop Documentation

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Key Takeaway:
Use for when you have a known collection (list, range, dict). Use while when you have a condition that changes over time. Prefer loops to repeated code—never copy-paste the same block three times!