What are iterators and the iteration protocol?

An iterator is an object that returns elements one by one using the __next__() method and signals completion by raising a StopIteration exception.

The iteration protocol:

• __iter__() — returns the iterator object itself.
• __next__() — returns the next element or raises a StopIteration exception.

How a for loop works:

# What 'for' does under the hood
nums = [1, 2, 3]

# for num in nums:
#     print(num)

# Equivalent:
iterator = iter(nums)       # Calls nums.__iter__()
while True:
    try:
        num = next(iterator)  # Calls iterator.__next__()
        print(num)
    except StopIteration:
        break

Creating a custom iterator:

class Countdown:
    def __init__(self, start):
        self.current = start

    def __iter__(self):
        return self

    def __next__(self):
        if self.current <= 0:
            raise StopIteration
        value = self.current
        self.current -= 1
        return value

for num in Countdown(5):
    print(num)  # 5, 4, 3, 2, 1

Built-in iterables:

• list, tuple, str, dict, set are iterable (they have an __iter__ method).
• range(), map(), filter(), zip() return iterators.
• An iterator can only be traversed once, while an iterable object can be looped over multiple times.

In practice:

The iteration protocol underpins for loops, generators, and many built-in functions. Custom iterators are useful when you need to process data in chunks — for example, reading a large file line by line instead of loading it all into memory.