Why Can Python Sequences Be Iterated? Uncover the Magic of iter and __getitem__

We all know that sequences are iterable, but why? This article explores the underlying mechanism.

The built‑in iter() function is used by the interpreter: it first checks whether the object implements an __iter__ method; if so, it calls it to obtain an iterator. If not, it looks for a __getitem__ method with integer indexes starting at zero and creates an iterator that accesses elements sequentially. If both checks fail, a TypeError is raised.

An object is therefore iterable when it either provides an __iter__ method that returns an iterator, or a __getitem__ method that supports zero‑based indexing.

Example using __getitem__

class Eg1:
    def __init__(self, text):
        self.text = text
        self.sub_text = text.split(' ')
    def __getitem__(self, index):
        return self.sub_text[index]

o1 = Eg1('Hello, the wonderful new world!')
for i in o1:
    print(i)

Output:

Hello,
the
wonderful
new
world!

We created Eg1 with a __getitem__ method, making its instance o1 iterable.

Example using __iter__

class Eg2:
    def __init__(self, text):
        self.text = text
        self.sub_text = text.split(' ')
    def __iter__(self):
        return Eg2Iterator(self.sub_text)

class Eg2Iterator:
    def __init__(self, sub_text):
        self.sub_text = sub_text
        self.index = 0
    def __next__(self):
        try:
            subtext = self.sub_text[self.index]
        except IndexError:
            raise StopIteration()
        self.index += 1
        return subtext
    def __iter__(self):
        return self

o2 = Eg2('Hello, the wonderful new world!')
for i in o2:
    print(i)

The output is identical to the previous example.

Using a generator with yield

class Eg3:
    def __init__(self, text):
        self.text = text
        self.sub_text = text.split(' ')
    def __iter__(self):
        for item in self.sub_text:
            yield item

This concise implementation leverages the yield keyword, turning the method into a generator function.

Further simplification with yield from

class Eg4:
    def __init__(self, text):
        self.text = text
        self.sub_text = text.split(' ')
    def __iter__(self):
        yield from self.sub_text

Generator expression

class Eg5:
    def __init__(self, text):
        self.text = text
        self.sub_text = text.split(' ')
    def __iter__(self):
        return (item for item in self.sub_text)

In Python, all generators are iterators.

Summary

Iterable objects either implement __iter__ returning an iterator or implement __getitem__ with zero‑based indexing.

Iterators implement a no‑argument __next__ method that raises StopIteration when exhausted and an __iter__ method that returns themselves.

Generator functions contain the yield keyword and produce generator objects.

Generator expressions provide a compact syntax for creating generators without defining a separate function.