Understanding __iter__() and return self in Python

[Rajeev Bagra]

Python provides a powerful mechanism called the iterator protocol that allows objects to be looped over using constructs like:

for item in obj:
    ...

Many beginners see code like this and wonder what exactly it means:

def __iter__(self):
    self.n = 0
    return self

This post explains the idea from scratch in plain English.

---

What is __iter__()?

The __iter__() method is part of Python’s iterator protocol.

Whenever Python encounters:

for x in obj:
    print(x)

Python internally does something similar to:

iterator = iter(obj)

The iter() function then calls:

obj.__iter__()

So the purpose of __iter__() is:

• To prepare the object for iteration
• To return an iterator object

---

Breaking Down the Code

def __iter__(self):
    self.n = 0
    return self

Line 1: Resetting State

self.n = 0

This initializes or resets a counter variable.

If the iterator is supposed to generate values one by one, the object needs some way to remember its current position.

For example:

0, 1, 2, 3, 4 ...

The variable self.n stores the current position during iteration.

Resetting it to 0 means:

“Start iteration from the beginning.”

---

Line 2: Returning self

return self

This is the most important part.

It means:

“The current object itself will behave as the iterator.”

Instead of creating a separate iterator object, the same object handles iteration.

---

Why Can the Object Return Itself?

An iterator object must implement two methods:

• __iter__()
• __next__()

Example:

class Counter:

    def __iter__(self):
        return self

    def __next__(self):
        ...

Since the same class already contains both methods, the object can safely return itself.

---

Complete Working Example

class Numbers:

    def __iter__(self):
        self.n = 1
        return self

    def __next__(self):

        if self.n <= 5:
            value = self.n
            self.n += 1
            return value

        raise StopIteration

Usage:

nums = Numbers()

for x in nums:
    print(x)

Output:

1
2
3
4
5

---

What Happens Internally?

Python roughly performs these steps:

nums = Numbers()

iterator = nums.__iter__()

Since __iter__() returns self, this becomes:

iterator = nums

Python then repeatedly calls:

iterator.__next__()

until:

StopIteration

is raised.

---

Iterable vs Iterator

These two terms are related but different.

Concept	Meaning
Iterable	An object that can be looped over
Iterator	An object that produces values one at a time

Sometimes they are separate objects.

Sometimes the same object performs both roles.

Using return self means the object itself acts as the iterator.

---

Important Detail

Returning self alone is not enough.

The class must also implement:

def __next__(self):

Without __next__(), Python cannot retrieve the next value during iteration.

---

Real-World Example: File Objects

Python file objects work similarly.

f = open("data.txt")

iter(f) is f

The file object itself behaves as its own iterator.

---

Key Takeaway

The statement:

return self

inside __iter__() means:

“Use this same object as the iterator.”

This design is common in Python when the iterable object itself already contains the iteration logic through __next__().

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