By the end of this lesson
Use for, range and a bounded while loop.
Before you start
- Python 3.12 or a compatible newer Python 3 interpreter; no external packages.
- Complete lesson 7 in this route, or be comfortable with its examples.
Three words to know
- Iteration
- One pass through a loop.
- Accumulator
- A value updated as items are processed.
- Loop condition
- The rule deciding whether a while loop continues.
1. Iterate over values
A for loop visits each item. The body runs once per item; statements after it run after the loop finishes. An empty list causes zero iterations, so initialise totals before the loop.
sizes = [2, 3, 1]
total = 0
for size in sizes:
total += size
print(total)
What you should see · example output
6
2. Keep positions readable
range stops before its end value. enumerate pairs each value with an index; start=1 makes human-facing numbering convenient. It does not change the list or its actual zero-based indices.
for step in range(1, 4):
print(step)
for number, item in enumerate(["read", "test"], start=1):
print(f"{number}. {item}")
What you should see · example output
1 2 3 1. read 2. test
3. Make the stopping rule visible
A while loop checks its condition before every pass. This countdown changes the same value used by the condition, so it terminates. If a practice loop runs endlessly, press Ctrl+C in the terminal and check which value should have changed. Avoid editing a list while iterating over that same list; build a separate result instead.
remaining = 3
while remaining > 0:
print(remaining)
remaining -= 1
print("done")
What you should see · example output
3 2 1 done
YOUR TURN
A small check before you continue
Sum the numbers from 1 through 5 using a loop. Which range includes 5?
Show a suggested answer
Start total at 0 and add each value from range(1, 6). The sum is 15. range(1, 5) stops at 4.
Sources and scope
Documentation and examples reviewed 2026-10-18. Provider screens, package versions and output wording can differ. Examples show expected outcomes; they are not a claim that OpenViST connected to your server or verified your setup.