[python] What exactly are iterator, iterable, and iteration?

Here's another view using collections.abc. This view may be useful the second time around or later.

From collections.abc we can see the following hierarchy:

builtins.object
    Iterable
        Iterator
            Generator

i.e. Generator is derived from Iterator is derived from Iterable is derived from the base object.

Hence,

  • Every iterator is an iterable, but not every iterable is an iterator. For example, [1, 2, 3] and range(10) are iterables, but not iterators. x = iter([1, 2, 3]) is an iterator and an iterable.
  • A similar relationship exists between Iterator and Generator.
  • Calling iter() on an iterator or a generator returns itself. Thus, if it is an iterator, then iter(it) is it is True.
  • Under the hood, a list comprehension like [2 * x for x in nums] or a for loop like for x in nums:, acts as though iter() is called on the iterable (nums) and then iterates over nums using that iterator. Hence, all of the following are functionally equivalent (with, say, nums=[1, 2, 3]):
    • for x in nums:
    • for x in iter(nums):
    • for x in iter(iter(nums)):
    • for x in iter(iter(iter(iter(iter(nums))))):

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