Explain the Python Iterators

ICSM Computer 420 02 Oct 2025

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Updated 02 Oct 2025

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ICSM Computer 02 Oct 2025

What is an Iterator?

An iterator in Python is an object that lets you loop through a sequence (like a list, tuple, or string) one element at a time, without needing to store the whole sequence in memory.

It follows the iterator protocol, which means it must implement two methods:

__iter__() → returns the iterator object itself.
__next__() → returns the next item in the sequence. If no items are left, it raises a StopIteration exception.

Example 1: Basic Iterator

numbers = [1, 2, 3]
it = iter(numbers)  # create an iterator

print(next(it))  # 1
print(next(it))  # 2
print(next(it))  # 3
# print(next(it))  # raises StopIteration

Here, iter() turns the list into an iterator, and next() fetches items one by one.

Example 2: Custom Iterator

You can build your own iterator class by defining __iter__() and __next__():

class CountUpTo:
    def __init__(self, limit):
        self.limit = limit
        self.current = 0

    def __iter__(self):
        return self  # an iterator must return itself

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

counter = CountUpTo(3)
for num in counter:
    print(num)

Output:

1
2
3

Why Iterators Matter

Memory efficiency: They don’t load everything at once (useful for large datasets).
Lazy evaluation: Items are produced only when needed.
Works with loops: for loops, comprehensions, and many built-ins use iterators under the hood.

Iterators vs Iterables

Iterable: Any object you can loop over (like list, tuple, dict, string). It has __iter__().
Iterator: The actual object that produces values one by one. It has both __iter__() and __next__().

Example:

nums = [1, 2, 3]   # iterable
it = iter(nums)    # iterator

An iterator is like a TV remote. The __next__() button gets you the next channel (item). Once you reach the end, pressing it again gives you a "no signal" (StopIteration).

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