Recommended Free Tools
Use a for loop when you want to process items from an iterable, such as a list or string. Use a while loop when repetition should continue as long as a condition is true. For numeric sequences, range() supplies the values for a for loop.
How a Python for loop works
A for loop takes items from an iterable one at a time, in order, and runs its indented body for each item. It is useful for lists, strings, dictionary views, and other iterable objects; it is not limited to counting numbers. Python’s control-flow tutorial and language reference describe this behavior.
As an Amazon Associate I earn from qualifying purchases.
words = ["loop", "through", "items"]
for word in words:
print(word)Here, word receives each string in turn, and print() runs once for each one. The indented lines form the loop body.
The Tool Desk
Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Use range() for a numeric sequence
When a task needs a progression of integers, use range(). Its stop value is excluded: range(5) produces 0, 1, 2, 3, and 4, so it represents five values rather than including 5.
#1 Best Overall
for number in range(5):
print(number)You can also provide a start and a step. For example, range(2, 10, 2) produces 2, 4, 6, and 8. The start is included when it lies in the sequence; the stop is not.
If you only need each value in a sequence, iterate over the values directly. If you need both a sequence item and its index, enumerate() usually communicates that intent more clearly than combining range(len(items)) with indexing.
Choose between for and while
| Loop | Best suited to | Normal stopping point |
|---|---|---|
for |
Processing the items supplied by an iterable, or a numeric sequence from range(). |
The iterator has no more items. |
while |
Repeating while a condition remains true, when the repetition is condition-driven rather than defined by a collection of items. | The condition becomes false. |
Either kind of loop can also end early with break. A while loop needs a path to stop: its body should update the condition or otherwise exit. If neither happens, the condition can remain true indefinitely. The official Python tutorial introduction demonstrates condition-driven repetition.
What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
remaining = 3
while remaining > 0:
print(remaining)
remaining -= 1Each pass reduces remaining; the loop ends when it reaches zero.
Loop through indexes, dictionaries, or matching sequences
Python provides iterable-oriented patterns for common cases. The examples below follow the Python data-structures tutorial.
Get an index and its value with enumerate()
values = ["red", "green", "blue"]
for index, value in enumerate(values):
print(index, value)enumerate() yields an index and its corresponding item on each pass. By default, the index starts at zero.
Get dictionary keys and values with items()
settings = {"theme": "dark", "alerts": True}
for key, value in settings.items():
print(key, value)items() supplies each key-value pair, which can be unpacked into the loop variables.
Process corresponding items with zip()
questions = ["Name?", "Age?"]
answers = ["Mina", "28"]
for question, answer in zip(questions, answers):
print(question, answer)zip() pairs items by position. It stops when the shortest input iterable is exhausted.
Control a loop with break, continue, and else
breakexits the nearest enclosing loop immediately.continueskips the rest of the current pass and proceeds to the next one.- A loop’s optional
elsesuite runs when the loop completes normally, but not when it ends throughbreak.
The loop else is useful in a search: it can distinguish finding a match (and breaking) from checking every item without finding one.
numbers = [3, 7, 11]
for number in numbers:
if number % 2 == 0:
print("Found an even number")
break
else:
print("No even number found")The else here belongs to the for loop, not to the if. It runs only if the loop exhausts numbers without executing break.
Avoid changing a collection while iterating over it
Adding to or removing items from a collection during traversal can change what the loop visits and lead to skipped or unexpected work. For filtering, build a separate result instead of altering the collection being traversed.
users = {"Ari": "active", "Bo": "inactive", "Cam": "active"}
active_users = {}
for user, status in users.items():
if status == "active":
active_users[user] = statusThis leaves users unchanged and puts the selected entries in active_users. If you need to delete entries from a list, iterate over a copy or create a new filtered list, as the Python control-flow tutorial advises.
Quick Recap
Quick choice guide
- Use
for item in iterable:when the work is defined by the items to process. - Use
for number in range(...):for a numeric progression; remember that the stop value is excluded. - Use
enumerate(items)when you need both each item and its index. - Use
while condition:when repetition depends on a condition, and make sure there is a way for it to become false or for the loop to exit.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
