An iterator in Python is an object that allows sequential traversal (iteration) of elements from a collection such as a list, tuple, set, or dictionary — one item at a time.

Iterators provide a way to access elements without using indexing, and are fundamental to Python’s looping constructs like for loops.

🔹 Key Concepts

🔹 Iterator Protocol

An object is an iterator if it implements the following two methods:

1. __iter__() → returns the iterator object itself.
2. __next__() → returns the next value from the data source.

When there are no more items, __next__() raises a StopIteration exception.

🧩 Example 1: Using iter() and next()

numbers = [1, 2, 3, 4]
iterator = iter(numbers)   # Create an iterator object

print(next(iterator))  # 1
print(next(iterator))  # 2
print(next(iterator))  # 3
print(next(iterator))  # 4

numbers = [1, 2, 3, 4]
iterator = iter(numbers)   # Create an iterator object

print(next(iterator))  # 1
print(next(iterator))  # 2
print(next(iterator))  # 3
print(next(iterator))  # 4

Output:

1
2
3
4

1
2
3
4

(If you call next(iterator) again → it raises StopIteration.)

🧩 Example 2: Iterating Using a for Loop

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:  # Internally uses iter() and next()
    print(fruit)

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:  # Internally uses iter() and next()
    print(fruit)

Output:

apple
banana
cherry

apple
banana
cherry

Explanation:
The for loop automatically:

1. Calls iter(fruits) to get an iterator.
2. Repeatedly calls next() until StopIteration is raised.

🧩 Example 3: Creating a Custom Iterator Class

class Counter:
    def __init__(self, low, high):
        self.current = low
        self.high = high

    def __iter__(self):
        return self  # Returns itself as iterator

    def __next__(self):
        if self.current > self.high:
            raise StopIteration
        else:
            num = self.current
            self.current += 1
            return num

for i in Counter(1, 5):
    print(i)

class Counter:
    def __init__(self, low, high):
        self.current = low
        self.high = high

    def __iter__(self):
        return self  # Returns itself as iterator

    def __next__(self):
        if self.current > self.high:
            raise StopIteration
        else:
            num = self.current
            self.current += 1
            return num

for i in Counter(1, 5):
    print(i)

Output:

1
2
3
4
5

1
2
3
4
5

🧩 Example 4: Using Iterators with Strings

word = "Dheeraj"
it = iter(word)

print(next(it))
print(next(it))
print(next(it))

word = "Dheeraj"
it = iter(word)

print(next(it))
print(next(it))
print(next(it))

Output:

D
h
e

D
h
e

🧩 Example 5: Handling StopIteration Exception

nums = [10, 20]
it = iter(nums)

try:
    print(next(it))
    print(next(it))
    print(next(it))  # No more elements
except StopIteration:
    print("End of iteration reached")

nums = [10, 20]
it = iter(nums)

try:
    print(next(it))
    print(next(it))
    print(next(it))  # No more elements
except StopIteration:
    print("End of iteration reached")

Output:

10
20
End of iteration reached

10
20
End of iteration reached

🔹 Difference Between Iterable and Iterator

🔹 In Summary

• An iterator is an object that represents a stream of data.
• It uses the iter() and next() functions to traverse items one by one.
• for loops internally use the iterator protocol.
• Custom iterators can be created by defining __iter__() and __next__() methods.
• When no more data is available, a StopIteration exception is raised.

✅ In short:

Iterator = object that remembers where it is during iteration and fetches next data only when requested.