Iterators in Python

What is an Iterator?

An iterator is an object in Python that enables you to traverse through all the elements of a collection, one element at a time. It implements two special methods: __iter__() and __next__(). The iter() function returns an iterator from an iterable, and next() retrieves the next item.

Creating Iterators

numbers = [1, 2, 3]
it = iter(numbers)
print(next(it))  # Output: 1
print(next(it))  # Output: 2

Examples

# 1. Using iter() and next()
fruits = ["apple", "banana", "cherry"]
fruit_iter = iter(fruits)
print(next(fruit_iter))  # Output: apple
print(next(fruit_iter))  # Output: banana
  

# 2. Iterating using a for loop (internally uses iter and next)
for fruit in fruits:
    print(fruit)
# Output:
# apple
# banana
# cherry
  

# 3. Custom iterator class
class CountDown:
    def __init__(self, start):
        self.current = start

    def __iter__(self):
        return self

    def __next__(self):
        if self.current <= 0:
            raise StopIteration
        num = self.current
        self.current -= 1
        return num

cd = CountDown(3)
for num in cd:
    print(num)
# Output: 3 2 1
  

# 4. Handling StopIteration
nums = [10, 20]
it = iter(nums)
print(next(it))  # Output: 10
print(next(it))  # Output: 20
# print(next(it))  # Raises StopIteration
  

# 5. Checking if object is an iterator
from collections.abc import Iterator

print(isinstance(it, Iterator))         # True
print(isinstance(nums, Iterator))       # False
print(isinstance(iter(nums), Iterator)) # True