Loops — Python Course · MasterCoding

What you'll learn

Computers excel at repetition. Loops run a block of code again and again — over a list, a range of numbers, or until a condition changes. Python's loops are especially elegant, and this module makes them second nature.

By the end you'll be able to:

Loop over any collection with for
Generate number sequences with range()
Repeat until a condition changes with while
Control loops with break and continue
Loop with an index using enumerate(), and pair lists with zip()
Nest loops for grids and patterns

The for loop

Python's for loop is a "for each" loop — it walks directly through the items of any collection, handing you one at a time. There's no manual index or counter (unless you want one), which makes it clean and hard to get wrong:

for = 'for each item'

A collection

["a", "b", "c"]

──▶

for item in ...

item = "a", then "b", then "c"

for_loop.py

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

for fruit in ["apple", "banana", "cherry"]:   # iterate directly!
    print(fruit)

Tip

Notice you can loop straight over a list of strings — no list[i] indexing needed. This is a big difference from C-style languages, and it's the Pythonic way to iterate.

range()

When you do want to loop a certain number of times, range() generates a sequence of numbers. It takes up to three arguments — range(start, stop, step) — and the stop value is excluded:

ranges.py

print(list(range(5)))         # [0, 1, 2, 3, 4]
print(list(range(2, 8)))      # [2, 3, 4, 5, 6, 7]
print(list(range(0, 10, 2)))  # [0, 2, 4, 6, 8]  (step of 2)

Note

range() is lazy — it doesn't build the whole list in memory, it produces numbers on demand. That's why range(1_000_000) is instant and memory-light. We wrapped it in list() above just to see the values.

The while loop

A while loop repeats as long as its condition stays true. Use it when you don't know the number of repetitions in advance. The golden rule: something inside must eventually make the condition false, or it loops forever:

while_loop.py

count = 3

while count > 0:
    print(count)
    count -= 1        # MUST change, or the loop runs forever

print("Liftoff!")

break & continue

Two keywords give you fine control. break stops the loop completely; continue skips just the current pass and moves to the next:

break vs continue

break

Exits the loop entirely and moves on.

continue

Skips the rest of this pass and starts the next.

break.py

for n in range(2, 10):
    if n == 4:
        continue      # skip only 4
    if n == 7:
        break         # stop the whole loop at 7
    print(n, end=" ")

Note

Python has a rare feature: a loop can have an else clause that runs only if the loop finished without hitting a break — handy for search loops ("did we find it?").

enumerate() & zip()

Two built-ins make loops even nicer. enumerate() gives you the index and the value on each pass — no manual counter. zip() walks several lists together, pairing up their items:

enumerate_zip.py

names = ["Sara", "Omar", "Aisha"]

for i, name in enumerate(names):        # index + value together
    print(i, name)

scores = [90, 85, 100]
for name, score in zip(names, scores):  # walk two lists in step
    print(f"{name}: {score}")

Tip

If you ever find yourself writing for i in range(len(items)): just to index a list, reach for enumerate() instead — it's cleaner and more Pythonic.

Nested loops

A loop can live inside another. For every single pass of the outer loop, the inner loop runs completely — exactly what you need for grids, tables, and patterns:

nested.py

for row in range(1, 4):
    for col in range(1, 4):
        print(row * col, end=" ")
    print()               # newline after each row

Recap & quick check

Key takeaways

Python's for loop is a 'for each' — iterate directly over any collection, no manual index.
range(start, stop, step) generates numbers lazily; stop is excluded.
while repeats while a condition is true — make sure something changes it.
break exits the loop; continue skips to the next pass.
Use enumerate() for index + value, and zip() to walk lists together.

Quick check

1. How does Python's for loop work?

2. What does range(2, 8) produce?

3. What does continue do?

4. Which built-in gives you both the index and the value while looping?

5. What does zip() do?

Fantastic — you can now automate repetitive work. Next up: Module 7 — Functions, where you'll package code into reusable building blocks.