Iterators in Python

Learn how Python iterators allow us to access elements one at a time.

What is an Iterator?

An iterator is an object that allows us to access elements of a collection one at a time.

Python uses the iter() function to create an iterator and the next() function to retrieve the next value.

Iterator Flow

Iterable and Iterator

An iterable is an object whose elements can be accessed one by one.

A list is an iterable. When we use iter() on the list, Python creates an iterator.

numbers = [10, 20, 30]

iterator = iter(numbers)

Here, numbers is an iterable and iterator is the iterator object.

Simple Iterator Example

numbers = [10, 20, 30]

iterator = iter(numbers)

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

Output:

10
20
30

Each call to next() returns the next element from the iterator.

Using next()

The next() function retrieves the next available value from an iterator.

colors = ["Red", "Green", "Blue"]

iterator = iter(colors)

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

Output:

Red
Green

The next value can be retrieved by calling next() again.

StopIteration

When an iterator has no more values, Python raises the StopIteration exception.

numbers = [10, 20]

iterator = iter(numbers)

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

The first two calls return values, but the third call raises StopIteration.

next() with Default Value

We can provide a default value to next() so that it does not raise StopIteration when the iterator is exhausted.

numbers = [10, 20]

iterator = iter(numbers)

print(next(iterator, "No value"))
print(next(iterator, "No value"))
print(next(iterator, "No value"))

Output:

10
20
No value

Iterators with for Loop

The for loop can automatically work with iterators.

numbers = [10, 20, 30]

for number in numbers:

    print(number)

Output:

10
20
30

The for loop internally gets values from the iterable one by one.

Iterator with String

Strings are also iterable objects.

text = "Python"

iterator = iter(text)

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

Output:

P
y
t

Iterator with Tuple

courses = ("Python", ".NET", "Java")

iterator = iter(courses)

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

Output:

Python
.NET
Java

Creating a Custom Iterator

We can create our own iterator by defining __iter__() and __next__() methods.

class Numbers:

    def __iter__(self):

        self.number = 1

        return self

    def __next__(self):

        if self.number <= 3:

            value = self.number

            self.number += 1

            return value

        else:

            raise StopIteration


numbers = Numbers()

for number in numbers:

    print(number)

Output:

1
2
3

Example 🤓👨‍🏫

The following example uses an iterator to process course names one at a time.

courses = [
    "Python",
    ".NET",
    "Java"
]

iterator = iter(courses)

print("Course:", next(iterator))
print("Course:", next(iterator))
print("Course:", next(iterator))

Output:

Course: Python
Course: .NET
Course: Java

Advantages of Iterators

Best Practices

Common Mistakes

• Calling next() after all values have been consumed.
• Forgetting that an iterator keeps its current position.
• Confusing an iterable with an iterator.
• Creating unnecessary custom iterators.

Practice Programs

1. Create an iterator from a list of numbers.
2. Use next() to print each element.
3. Create an iterator for a string.
4. Create an iterator for a tuple.
5. Create a custom iterator that generates numbers from 1 to 5.