Python Advanced
Python Iterators
An iterator is an object that lets you step through a collection of values one at a time. Understanding iterators helps you see what's really happening under the hood of every for loop.
Iterable vs Iterator
An iterable is anything you can loop over, like a list, tuple, string, or dictionary — it has an __iter__ method that produces an iterator. An iterator is the object that actually tracks your position and hands out one value at a time using __next__. Every iterator is iterable, but not every iterable is an iterator.
numbers = [10, 20, 30]
it = iter(numbers)
print(next(it))
print(next(it))
print(next(it))10
20
30StopIteration
Once an iterator runs out of values, calling next() again raises a StopIteration exception. A for loop catches this exception automatically, which is why loops end cleanly without you ever seeing an error.
it = iter([1, 2])
print(next(it))
print(next(it))
print(next(it)) # raises StopIterationWriting Your Own Iterator
You can build a custom class as an iterator by defining both __iter__ (which returns self) and __next__ (which returns the next value or raises StopIteration). This pattern is the foundation behind generators and many built-in tools like enumerate and zip.
class Countdown:
def __init__(self, start):
self.current = start
def __iter__(self):
return self
def __next__(self):
if self.current <= 0:
raise StopIteration
value = self.current
self.current -= 1
return value
for n in Countdown(3):
print(n)Every for loop in Python calls iter() on the object once, then next() repeatedly until StopIteration is raised — that is the entire mechanism behind looping.
| Concept | What it does |
|---|---|
| Iterable | Has __iter__(); can produce an iterator |
| Iterator | Has __iter__() and __next__(); tracks state |
| iter(x) | Returns an iterator for iterable x |
| next(it) | Returns the next value or raises StopIteration |
Generators (functions using yield) are the easiest way to write iterators in Python without manually managing __iter__ and __next__.
Frequently Asked Questions
What is the difference between an iterable and an iterator in Python?+
An iterable is any object you can loop over (like a list or string) because it defines __iter__. An iterator is the object returned by iter() that actually produces values one at a time via __next__ and remembers its position.
Why does a for loop never raise StopIteration in my code?+
The for loop catches StopIteration internally. When the iterator signals it has no more values, the loop simply stops instead of letting the exception propagate to your code.
Are lists iterators?+
No. Lists are iterables, not iterators — they don't have a __next__ method. Calling iter() on a list creates a separate list_iterator object that does the actual stepping through.