Python Iterators

1. What is an Iterator

An iterator is an object that allows traversal through all elements of a collection, one element at a time.

numbers = [1, 2, 3, 4]
iterator = iter(numbers)

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

An iterator follows two core methods: __iter__() and __next__().

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2. Iterable vs Iterator

data = [10, 20, 30]

print(iter(data))     # Iterable converted to iterator
print(isinstance(data, list))  # Iterable

• Iterable → Can be looped over

• Iterator → Produces values one-by-one

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3. Using next() with Iterators

items = ["A", "B", "C"]
it = iter(items)

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

next() retrieves the next value from the iterator.

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4. StopIteration Exception

items = [1, 2]
it = iter(items)

print(next(it))
print(next(it))
# print(next(it))  # Raises StopIteration

When no items remain, StopIteration is raised.

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5. Iterators in for Loops

colors = ["red", "green", "blue"]

for color in colors:
    print(color)

The for loop automatically handles iterator creation and termination.

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6. Creating Custom Iterator Class

class CountUp:
    def __init__(self, limit):
        self.limit = limit
        self.current = 0

    def __iter__(self):
        return self

    def __next__(self):
        if self.current < self.limit:
            self.current += 1
            return self.current
        raise StopIteration

counter = CountUp(3)

for number in counter:
    print(number)

Defines user-controlled iteration logic.

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7. Infinite Iterator Example

class InfiniteNumbers:
    def __iter__(self):
        self.num = 0
        return self

    def __next__(self):
        self.num += 1
        return self.num

Produces an endless sequence unless manually stopped.

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8. Iterator from Built-in Functions

data = "Python"

it = iter(data)
print(list(it))  # ['P', 'y', 't', 'h', 'o', 'n']

Many built-in data types support iteration natively.

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9. Checking if an Object is an Iterator

from collections.abc import Iterator

numbers = iter([1, 2, 3])
print(isinstance(numbers, Iterator))  # True

Ensures an object conforms to iterator protocol.

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10. Real-World Iterator Example

class FileLineReader:
    def __init__(self, filename):
        self.file = open(filename, "r")

    def __iter__(self):
        return self

    def __next__(self):
        line = self.file.readline()
        if line:
            return line.strip()
        self.file.close()
        raise StopIteration

for line in FileLineReader("data.txt"):
    print(line)

Efficient for processing large datasets line-by-line.