For Loop vs While Loop in Python

The for loop vs while loop in Python choice comes down to one thing: a for loop walks through a sequence or a fixed count, and a while loop keeps repeating until a condition turns false, however many passes that takes.

for item in items:        # you know what you're looping over
    ...

while not done:           # you loop until something changes
    ...

My test machine had Python 3.12.5; each output on this page is pasted from a real Command Prompt session.

For loop vs while loop: the same task written both ways

Here’s one job, adding up weekly sales, done with each loop. Watch how much bookkeeping the while version needs:

weekly_sales = [1250.00, 980.50, 1430.25, 1105.75]

total = 0.0
for amount in weekly_sales:
    total += amount
print(f"for loop total:   ${total:,.2f}")

total = 0.0
i = 0
while i < len(weekly_sales):
    total += weekly_sales[i]
    i += 1
print(f"while loop total: ${total:,.2f}")

Output:

for loop total:   $4,766.50
while loop total: $4,766.50

The while loop creates, tests and bumps i by hand; forget i += 1 and it never ends. If you’re new to lists, see looping through a list in Python.

When should you use a for loop?

Reach for for when there’s something to walk through: a list, file, dict or range(). You don’t manage an index, and enumerate() hands you a position when you want one:

cities = ["Austin", "Denver", "Seattle"]
for rank, city in enumerate(cities, start=1):
    print(rank, city)

for day in range(1, 4):
    print(f"2026-10-{day:02d}: nightly backup done")

Output:

1 Austin
2 Denver
3 Seattle
2026-10-01: nightly backup done
2026-10-02: nightly backup done
2026-10-03: nightly backup done

If you want counting, you use range(), explained in for i in range() in Python. The full syntax lives in my Python for loop guide.

When should you use a while loop?

Use while when you can’t predict the number of passes. This plan deposits $400.00 a month and stops once the balance hits $10,000.00:

balance = 2500.00
deposit = 400.00
month = 0

while balance < 10_000:
    balance = balance * 1.004 + deposit   # 0.4% monthly interest
    month += 1

print(f"Goal reached after {month} months: ${balance:,.2f}")

Output:

Goal reached after 18 months: $10,136.35

The Python while loop tutorial covers the basics, and while loops with multiple conditions shows how to combine tests with and and or.

Which is faster in Python, a for loop or a while loop?

For counting, the for loop wins in CPython: range() counts in C, while the while loop runs its test and += as bytecode on every pass. I timed both:

import timeit

def with_for(n=1_000_000):
    total = 0
    for i in range(n):
        total += i
    return total

def with_while(n=1_000_000):
    total = 0
    i = 0
    while i < n:
        total += i
        i += 1
    return total

def with_sum(n=1_000_000):
    return sum(range(n))

print("Same result:", with_for() == with_while() == with_sum())
for f in (with_for, with_while, with_sum):
    best = min(timeit.repeat(f, number=5, repeat=3))
    print(f"{f.__name__:<11} {best:.3f} s for 5 runs")

Output:

Same result: True
with_for    0.162 s for 5 runs
with_while  0.204 s for 5 runs
with_sum    0.049 s for 5 runs

Your times will differ, but the order holds. A built-in beats either loop, as shown in summing list elements in Python.

Avoid infinite loops in a Python while loop

A while loop stops only when its condition turns false or a break runs. while True plus break is the cleanest retry pattern; I used iter() instead of input() so the run repeats exactly:

answers = iter(["abc", "-5", "42"])
attempts = 0

while True:
    attempts += 1
    text = next(answers)
    if text.isdigit() and int(text) > 0:
        print(f"Accepted {text} after {attempts} tries")
        break
    print(f"{text!r} is not a positive number, try again")

Output:

'abc' is not a positive number, try again
'-5' is not a positive number, try again
Accepted 42 after 3 tries

Python has no do-while keyword; this is how you write a do-while loop in Python. To skip a pass instead, see continue in a Python while loop.

Keep going with these loop tutorials:

• Iterate through a dictionary in Python
• Write a while loop condition in Python
• Iterate through tuples in Python
• Use break and continue in Python

Frequently asked questions