Python · Iterators

What for x in obj: really does:

1. it = iter(obj) asks the iterable for an iterator (obj.__iter__()).
2. It calls next(it) again and again (it.__next__()).
3. It stops when StopIteration is raised.

An iterable (list, string, dict, file) can hand out fresh iterators. An iterator remembers where it is and is used up after one pass. That explains why looping over the same zip or map object a second time gives nothing.

To write your own iterator, give the class __iter__ (which returns self) and __next__.

nums = [10, 20, 30]
it = iter(nums)
print(next(it), next(it), next(it))
try:
    next(it)
except StopIteration:
    print("StopIteration")

class Countdown:
    def __init__(self, start):
        self.n = start
    def __iter__(self):
        return self
    def __next__(self):
        if self.n <= 0:
            raise StopIteration
        self.n -= 1
        return self.n + 1

print(list(Countdown(5)))

z = zip("abc", [1, 2, 3])
print(list(z))
print(list(z))   # exhausted iterator

Output:

10 20 30
StopIteration
[5, 4, 3, 2, 1]
[('a', 1), ('b', 2), ('c', 3)]
[]