Interview question

What is the iterator protocol and how do iter and next work? Iterator protocol क्या है और iter और next कैसे काम करते हैं?

Answer

The iterator protocol requires an object to implement __iter__ (returns the iterator itself) and __next__ (returns the next value or raises StopIteration). This is what powers 'for' loops behind the scenes.

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

    def __iter__(self):
        return self  # the object IS its own iterator

    def __next__(self):
        if self.current >= self.limit:
            raise StopIteration  # signals the end of iteration
        self.current += 1
        return self.current

counter = CountUpTo(3)
print(next(counter))  # 1
print(next(counter))  # 2
print(next(counter))  # 3
# print(next(counter))  # StopIteration

# 'for' loops use this protocol automatically
for num in CountUpTo(5):
    print(num)  # 1, 2, 3, 4, 5

# What a for loop actually does under the hood:
iterator = iter(CountUpTo(3))  # calls __iter__
while True:
    try:
        value = next(iterator)  # calls __next__
        print(value)
    except StopIteration:
        break

# Iterable vs Iterator - an important distinction
# Iterable: has __iter__, can produce an iterator (e.g. a list)
# Iterator: has __iter__ AND __next__, tracks its own state
my_list = [1, 2, 3]  # iterable, but not an iterator itself
# next(my_list)  # TypeError: 'list' object is not an iterator
my_iterator = iter(my_list)  # NOW it's an iterator
print(next(my_iterator))  # 1

Iterator protocol के लिए object को __iter__ (खुद iterator return करे) और __next__ (अगली value return करे या StopIteration raise करे) implement करना ज़रूरी है। यही 'for' loops के पीछे का mechanism है।

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

    def __iter__(self):
        return self

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

counter = CountUpTo(3)
print(next(counter))  # 1
print(next(counter))  # 2

# 'for' loops यह protocol automatically use करते हैं
for num in CountUpTo(5):
    print(num)

# for loop असल में क्या करता है:
iterator = iter(CountUpTo(3))
while True:
    try:
        value = next(iterator)
        print(value)
    except StopIteration:
        break

# Iterable बनाम Iterator
my_list = [1, 2, 3]  # iterable, iterator नहीं
# next(my_list)  # TypeError
my_iterator = iter(my_list)
print(next(my_iterator))  # 1

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