Iterators Explained: The Magic Behind Your For Loops 🪄

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The for loop

Ever wondered what really happens when you write a for loop? Or why some objects work with loops while others don't? Let me share a story. I was pair programming with a colleague when they asked, “Why can we loop through a list but not an integer?” That’s when I realized: many Python developers use iterators daily without truly understanding them. Let’s change that!

Iterables vs Iterators: The Simple Truth 🎭

Think of it this way:

• An iterable is like a book 📚
• An iterator is like your bookmark 🔖

The book (iterable) contains all the content, but the bookmark (iterator) keeps track of where you are.

numbers = [1, 2, 3]      # This is an iterable ✨
iterator = iter(numbers) # This creates an iterator 🔄

What’s Really Happening in a For Loop? 🔄

Here’s the mind-blowing part: every for loop you've ever written is secretly using iterators! This:

for number in [1, 2, 3]:
    print(number)

Is actually doing this behind the scenes:

# What Python does under the hood 🎩✨
iterator = iter([1, 2, 3])
while True:
    try:
        number = next(iterator)
        print(number)
    except StopIteration:
        break

Cool, right? 😎

When we use a for loop in Python, we're actually using an iterator to traverse over a sequence (like a list, tuple, or string) or any other iterable object. The behavior of the for loop is more than just a simple syntactic structure—under the hood, Python is working with iterators and the iterator protocol.

In technical terms, an iterable is an object that implements the __iter__() method. This method is responsible for returning an iterator—an object that keeps track of the current position and knows how to retrieve the next value.

my_list = [1, 2, 3]

In this case, my_list is an iterable.

What is an Iterator?