What Happens If a Class Has __iter__() But Not __next__() — And Vice Versa?

[Rajeev Bagra]

This is one of the most important conceptual questions in understanding Python iteration.

To understand it properly, we must clearly separate:

• iterable
• iterator

---

First Important Clarification

A Python list is mainly an:

Iterable

not an iterator itself.

Example:

numbers = [1, 2, 3]

This object has:

__iter__()

but the actual iteration is usually handled by a separate iterator object internally.

---

What Happens Internally for a List?

When you do:

numbers = [1, 2, 3]

iterator = iter(numbers)

Python internally creates something conceptually like:

<list_iterator object>

That iterator object contains:

__next__()

So:

Object	Main Role
list	Iterable
list_iterator	Iterator

---

Case 1 — Class Has __iter__() But No __next__()

Example:

class MyClass:

    def __iter__(self):
        return self

Now:

obj = MyClass()

iter(obj)

works.

BUT:

next(obj)

fails.

Why?

Because Python expects the returned iterator object to implement:

__next__()

Without it, Python does not know:

“How to fetch the next value.”

---

Error Example

class MyClass:

    def __iter__(self):
        return self


obj = MyClass()

next(obj)

Output:

TypeError:
'MyClass' object is not an iterator

---

What Does This Mean Conceptually?

If a class has only:

__iter__()

it means:

“This object knows how iteration should begin, but does not know how to produce values.”

So the iteration mechanism is incomplete.

---

Real Meaning of __iter__() Alone

Having only:

__iter__()

usually means:

“This object can create or return an iterator.”

That returned iterator might be:

• another object
• a generator
• a built-in iterator

---

Example of Proper Iterable with Only __iter__()

class MyNumbers:

    def __iter__(self):
        return iter([1, 2, 3])

Usage:

obj = MyNumbers()

for x in obj:
    print(x)

Output:

1
2
3

Here:

• MyNumbers is iterable
• the returned list iterator handles __next__()

Very important distinction.

---

Case 2 — Class Has __next__() But No __iter__()

Example:

class MyClass:

    def __next__(self):
        return 1

Now:

next(obj)

works.

BUT:

iter(obj)

fails.

---

Error Example

obj = MyClass()

iter(obj)

Output:

TypeError:
'MyClass' object is not iterable

---

What Does This Mean Conceptually?

If a class has only:

__next__()

it means:

“This object knows how to produce next values, but Python does not know how iteration should start.”

So Python cannot use it in a normal loop.

---

Proper Iterator Structure

A true iterator usually implements BOTH:

__iter__()
__next__()

Example:

class Counter:

    def __init__(self):
        self.n = 1

    def __iter__(self):
        return self

    def __next__(self):

        if self.n <= 3:

            value = self.n
            self.n += 1

            return value

        raise StopIteration

Now this works fully:

c = Counter()

for x in c:
    print(x)

Output:

1
2
3

---

Very Important Insight

If Object Has	Meaning
__iter__() only	Can provide an iterator
__next__() only	Can produce values but not iterable properly
Both	Fully functional iterator

---

The Most Common Design in Python

Usually:

• collections are iterables
• separate iterator objects handle traversal

Example:

Iterable	Iterator Produced
list	list_iterator
tuple	tuple_iterator
dict	dict_keyiterator
file	file iterator

---

Key Takeaway

__iter__()

Means:

“I can provide an iterator.”

---

__next__()

Means:

“I can provide the next value.”

---

Both Together

Means:

“I am a complete iterator object.”

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