What is the difference between an iterable and an iterator?

Short answer

An iterable is an object you can get an iterator from through iter() (such as a list); an iterator remembers its position, returns the next item through next(), and is consumed only once.

Explanation

You can loop over a list many times because each loop creates a fresh iterator. A generator or a file object, however, is an iterator: after it is consumed, a second loop yields nothing. That is why passing a generator to two functions leaves the second one with empty data. If you need more than one pass, convert the result to a list or recreate the generator.

Code example

numbers = (n * n for n in range(3))  # generator = iterator
print(sum(numbers))   # 5
print(sum(numbers))   # 0 — Already consumed

items = [1, 2, 3]     # iterable
it = iter(items)
print(next(it), next(it))  # 1 2

Common mistakes

• Reusing a generator after it was consumed and expecting the same data.
• Converting every iterator to a list too early, which cancels the benefit of lazy processing.

Follow-up questions

• Which protocol makes an object usable in a for loop?
• How do generators and yield reduce memory usage?

Sources

• Python documentation — Iterator types