What is Iterator in Python: Simple Explanation and Example

An iterator in Python is an object that lets you go through all the items in a collection one by one, like reading pages in a book. It uses the __iter__() and __next__() methods to keep track of where you are and to get the next item until there are no more.

How It Works

Think of an iterator like a bookmark in a book. When you read a book, you don't read all pages at once; you read one page at a time and use a bookmark to remember where you stopped. Similarly, an iterator remembers the current position in a collection (like a list or a set) and gives you the next item when you ask for it.

In Python, an iterator is any object that has two special methods: __iter__(), which returns the iterator object itself, and __next__(), which returns the next item. When there are no more items, __next__() raises a StopIteration exception to signal the end.

This mechanism allows Python to loop over collections efficiently without loading everything at once, which is helpful for large data or streams.

Example

This example shows how to create an iterator from a list and use it to get items one by one.

python

numbers = [10, 20, 30]
iterator = iter(numbers)  # Get iterator from the list

print(next(iterator))  # Output: 10
print(next(iterator))  # Output: 20
print(next(iterator))  # Output: 30

# next(iterator) now would raise StopIteration

Output

10
20
30

When to Use

Use iterators when you want to process items one at a time without loading everything into memory. This is useful for reading large files, streaming data, or looping over custom collections.

For example, when reading a big text file, an iterator can read one line at a time instead of loading the whole file. Also, iterators let you create your own objects that can be looped over with for loops, making your code cleaner and more flexible.

Key Points

• An iterator remembers its current position in a collection.
• It uses __iter__() and __next__() methods.
• When no items are left, StopIteration is raised.
• Iterators help handle large data efficiently.
• You can create custom iterators for your own objects.

Key Takeaways

An iterator lets you access items one by one from a collection.

It uses special methods __iter__() and __next__() to work.

When items run out, it raises StopIteration to stop.

Iterators are useful for handling large or streaming data efficiently.

You can make your own iterators to customize looping behavior.