How Loops Work¶

Try This First: Before reading, open Python and type for i in range(5): print(i). Watch what happens. How many numbers print? Does it start at 0 or 1?

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A loop repeats code. Instead of writing the same thing 100 times, you write it once and let the loop handle repetition.

Visualize It¶

Watch how a for loop steps through a list, one item at a time: Open in Python Tutor

Python has two kinds of loops:

For loops — "do this for each item"¶

colors = ["red", "blue", "green"]
for color in colors:
    print(color)

Output:

red
blue
green

The variable color takes a different value each time through the loop. First it is "red", then "blue", then "green".

Looping over numbers with range()¶

for i in range(5):       # 0, 1, 2, 3, 4
    print(i)

for i in range(1, 6):    # 1, 2, 3, 4, 5
    print(i)

for i in range(0, 10, 2): # 0, 2, 4, 6, 8 (step by 2)
    print(i)

While loops — "keep doing this until something changes"¶

count = 1
while count <= 5:
    print(count)
    count = count + 1

The loop checks the condition (count <= 5) before each repetition. When it becomes False, the loop stops.

When to use which¶

Use a for loop when...	Use a while loop when...
You know how many times to repeat	You do not know when to stop
You are going through a list	You are waiting for a condition
`for item in my_list:`	`while not done:`

The walrus operator `:=` in loops (Python 3.8+)¶

The walrus operator lets you assign a value and use it in the same expression. This is especially useful in while loops:

# Without walrus — must call input() in two places:
line = input("Enter text (or 'quit'): ")
while line != "quit":
    print(f"You said: {line}")
    line = input("Enter text (or 'quit'): ")

# With walrus — cleaner:
while (line := input("Enter text (or 'quit'): ")) != "quit":
    print(f"You said: {line}")

Reading a file in chunks:

# Read a large file 8KB at a time:
with open("big_file.txt") as f:
    while chunk := f.read(8192):
        process(chunk)

The walrus operator assigns the result of the right side to the variable and returns it, so the while condition can test the value in the same line.

Common mistakes¶

Forgetting to update the while condition (infinite loop):

count = 1
while count <= 5:
    print(count)
    # Missing: count = count + 1
    # This loop NEVER ends! Press Ctrl+C to stop it.

Off-by-one errors with range():

range(5)     # Gives 0,1,2,3,4 — NOT 1,2,3,4,5
range(1, 5)  # Gives 1,2,3,4   — NOT 1,2,3,4,5
range(1, 6)  # Gives 1,2,3,4,5 — this is what you want

Modifying a list while looping over it:

# WRONG — unpredictable behavior
for item in my_list:
    if item == "bad":
        my_list.remove(item)

# RIGHT — loop over a copy or build a new list
good_items = [item for item in my_list if item != "bad"]

Practice¶

Quick check: Take the quiz

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