Python Iterators

What Is an Iterator in Python?

An iterator is an object that implements two methods: __iter__ and __next__. The __iter__ method returns the iterator object itself. The __next__ method returns the next value in the sequence. When there are no more values, it raises a StopIteration exception.

Think of an iterator like a bookmark in a book. It remembers where we are. Each time we ask for the next item, it gives us the next one and moves the bookmark forward.

Why Do We Need Iterators in Python?

Iterators are everywhere in Python. The for loop uses them behind the scenes. Lists, tuples, dictionaries, and strings are all iterable. They can be looped over because they have an iterator.

Iterators let us work with large data sets without loading everything into memory at once. They produce values one at a time, on demand.

How to Use an Iterator in Python

We can get an iterator from any iterable using the iter() function. Then we use next() to get values one by one.

numbers = [1, 2, 3]
iterator = iter(numbers)

print(next(iterator))
print(next(iterator))
print(next(iterator))

1
2
3

If we call next() again, we get a StopIteration exception.

print(next(iterator))

StopIteration

That is how a for loop works internally. It calls iter() to get an iterator, then calls next() repeatedly until StopIteration is raised.

How to Create a Custom Iterator in Python

We can create our own iterator by defining a class with __iter__ and __next__ methods.

Let us create an iterator that counts from 1 to a given limit.

class Counter:
    def __init__(self, limit):
        self.limit = limit
        self.current = 1

    def __iter__(self):
        return self

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

counter = Counter(3)

for num in counter:
    print(num)

1
2
3

The __iter__ method returns self. The __next__ method returns the next value and raises StopIteration when done.

A Real-Life Example – Fibonacci Iterator

Let us create an iterator that generates Fibonacci numbers.

class Fibonacci:
    def __init__(self, limit):
        self.limit = limit
        self.a = 0
        self.b = 1
        self.count = 0

    def __iter__(self):
        return self

    def __next__(self):
        if self.count >= self.limit:
            raise StopIteration
        value = self.a
        self.a, self.b = self.b, self.a + self.b
        self.count += 1
        return value

fib = Fibonacci(5)

for num in fib:
    print(num)

0
1
1
2
3

That is a useful iterator for generating sequences.

Common Mistakes with Iterators in Python

class Counter:
    def __init__(self, limit):
        self.limit = limit
        self.current = 1

    def __iter__(self):
        return self

    def __next__(self):
        value = self.current
        self.current += 1
        return value

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

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

iterator = iter([1, 2, 3])
for x in iterator:
    print(x)
for x in iterator:
    print(x)