Iterator protocol in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of custom iterator class

What is the output of this Python code using a custom iterator?

Python

class CountDown:
    def __init__(self, start):
        self.current = start
    def __iter__(self):
        return self
    def __next__(self):
        if self.current <= 0:
            raise StopIteration
        self.current -= 1
        return self.current + 1

for number in CountDown(3):
    print(number)

A 1\n2\n3

B 2\n1\n0

C 4\n3\n2

D 3\n2\n1

💡 Hint

Look at how __next__ decreases self.current and what it returns.

The iterator starts at 3. Each call to __next__ returns the current value before decreasing it. It stops when current is 0.

❓ Predict Output

intermediate

Output of iter and next with list

What is the output of this code snippet?

Python

lst = [10, 20]
it = iter(lst)
print(next(it))
print(next(it))
print(next(it))

A 10\n20\nStopIteration

B 10\n20\nTraceback (most recent call last): StopIteration

C 10\n20\nNone

D 10\n20\n0

💡 Hint

What happens when next() is called but no items remain?

Calling next() on an exhausted iterator raises a StopIteration exception, causing a traceback if not caught.

🧠 Conceptual

advanced

Understanding iterator protocol methods

Which method must a Python object implement to be considered an iterator?

A __iter__ only

B __next__ only

C __iter__ and __next__

D __call__

💡 Hint

Think about what Python expects when using a for-loop on an iterator.

An iterator must have both __iter__ returning itself and __next__ to get the next item.

❓ Predict Output

advanced

Output of generator and iterator interaction

What is the output of this code?

Python

def gen():
    yield 1
    yield 2

g = gen()
print(next(g))
print(iter(g) is g)
print(next(g))

A 1\nTrue\n2

B 1\nTrue\nStopIteration

C 1\nFalse\n2

D 1\nFalse\nStopIteration

💡 Hint

Generators are their own iterators. What does iter(g) return?

Calling iter() on a generator returns the generator itself, so iter(g) is g is True.

❓ Predict Output

expert

Output of nested iterator with StopIteration handling

What is the output of this code?

Python

class Repeater:
    def __init__(self, value, times):
        self.value = value
        self.times = times
        self.count = 0
    def __iter__(self):
        return self
    def __next__(self):
        if self.count >= self.times:
            raise StopIteration
        self.count += 1
        return self.value

r = Repeater('Hi', 2)
it = iter(r)
try:
    while True:
        print(next(it))
except StopIteration:
    print('Done')

A Hi\nHi\nDone

B Hi\nDone

C Hi\nHi

D Done

💡 Hint

How many times does the iterator yield before stopping?

The iterator yields 'Hi' twice, then raises StopIteration, caught by the except block which prints 'Done'.