Iterator (Python)

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Definition: In Python, an iterator is an object that allows you to traverse a sequence of elements one at a time. Each call to next() returns the next element, and when there are no more items, it raises a StopIteration exception.

Iterator vs Iterable

• Iterable: an object that can provide an iterator via iter() (e.g. list, tuple, set, dict).
• Iterator: the object returned by iter() and consumed by next().

fruits = ["apple", "banana", "cherry"]

# obtain an iterator
it = iter(fruits)

print(next(it))  # apple
print(next(it))  # banana
print(next(it))  # cherry
print(next(it))  # StopIteration (no more elements)

Main Characteristics

• Iterators remember their current position in the sequence.
• They can only be traversed once; once exhausted, a new one must be created.
• They are ideal for large datasets or infinite streams (because they generate elements on demand).

Creating a Custom Iterator

You can define a class implementing the special methods __iter__() and __next__():

class Counter:
    def __init__(self, limit):
        self.limit = limit
        self.count = 0

    def __iter__(self):
        return self

    def __next__(self):
        if self.count < self.limit:
            self.count += 1
            return self.count
        else:
            raise StopIteration

for num in Counter(3):
    print(num)
# 1, 2, 3

Relation with Generators

Generators are a convenient way to create iterators using the yield keyword:

def count(limit):
    for i in range(1, limit + 1):
        yield i

for n in count(3):
    print(n)

Common Errors and Best Practices

• Iterator exhaustion: once an iterator is consumed, it cannot be reset; use iter() again to restart.
• StopIteration: signals the end of iteration, not a runtime error.
• Safe iteration: for loops automatically handle StopIteration.

See Also

• Iterable (Python): an object capable of providing an iterator via iter().
• Generator (Python): a simpler way to create iterators using yield.
• Set (set): an iterable collection that can be turned into an iterator.

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