Python Iterators: Understanding Iterables and Iterators in Python

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🔁 Python Iterators (Made Simple) 

Iterators are a way to go through all the elements in a collection (like a list or tuple), one item at a time. Python has built-in support for iterators, making it easy to loop through data.

If you've ever used a for loop in Python, you've already worked with iterators—even if you didn’t realize it!

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✅ What Is an Iterator?

An iterator is an object that can be looped through (iterated over). It must follow two main rules:

1. It must have a method called __iter__() → returns the iterator object itself.

2. It must have a method called __next__() → returns the next item from the sequence.

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📘 Example 1: Using an Iterator

my_list = [1, 2, 3]

# Get an iterator
my_iter = iter(my_list)

print(next(my_iter))  # Output: 1
print(next(my_iter))  # Output: 2
print(next(my_iter))  # Output: 3

🔍 Output:

1
2
3

If you call next() again after the list is finished, it will raise a StopIteration error.

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🛠 Behind the Scenes of a For Loop

When we use a for loop like this:

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

Python automatically:

• Calls iter() to get an iterator.

• Calls next() repeatedly until StopIteration is raised.

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🧱 Building a Custom Iterator

Let’s create our own iterator to return numbers from 1 to 5.

📘 Example 2: Custom Iterator

class MyNumbers:
    def __iter__(self):
        self.num = 1
        return self

    def __next__(self):
        if self.num <= 5:
            x = self.num
            self.num += 1
            return x
        else:
            raise StopIteration

# Create object
nums = MyNumbers()
my_iter = iter(nums)

for number in my_iter:
    print(number)

🔍 Output:

1
2
3
4
5

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💡 Important Notes

• iter() creates an iterator from an iterable (like a list or string).

• next() returns the next value.

• When there are no more items, StopIteration is raised to end the loop.

• You can create custom iterators by defining __iter__() and __next__().

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📘 Example 3: Iterating Over a String

Strings are also iterable!

my_str = "Hi"
my_iter = iter(my_str)

print(next(my_iter))  # Output: H
print(next(my_iter))  # Output: i

🔍 Output:

H
i

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🎯 Conclusion

Iterators are a powerful tool in Python that let you go through items one at a time. Whether you’re working with lists, strings, or building custom sequences, understanding iterators helps us write clean and efficient code.

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