`iter()` and `next()`

Introduction

Every time you write a for loop in Python, it uses iter() and next() under the hood.

While you rarely need to call them manually in standard algorithmic questions, they are critical for object-oriented design questions, specifically when asked to "Design an Iterator" (e.g., LeetCode 284: Peeking Iterator or LeetCode 173: Binary Search Tree Iterator).

How Iteration Works

When you run for item in sequence:, Python does two things: 1. It calls iter(sequence) to get an iterator object. 2. It repeatedly calls next(iterator) to get elements until a StopIteration exception is raised.

Manual Iteration

You can do this manually:

nums = [1, 2, 3]

# Get the iterator
iterator = iter(nums)

# Advance manually
print(next(iterator)) # 1
print(next(iterator)) # 2
print(next(iterator)) # 3

# print(next(iterator)) # Raises StopIteration

Providing a Default to `next()`

If you call next() and the iterator is empty, it raises StopIteration.

To prevent the exception and instead return a default value, pass a second argument to next().

iterator = iter([])

# Avoids exception, returns None
val = next(iterator, None)
print(val) # None

Interview Application: Designing an Iterator

If an interviewer asks you to build a custom iterator, you must implement the __iter__ and __next__ magic methods.

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

    def __iter__(self):
        # Must return the iterator object itself
        return self

    def __next__(self):
        if self.current > self.limit:
            raise StopIteration

        result = self.current
        self.current += 2
        return result

# Now we can use our custom class in a for loop!
evens = EvenNumbers(6)
for num in evens:
    print(num)
# 0, 2, 4, 6

Summary

iter(obj) gets an iterator from an iterable.
next(iterator, default) fetches the next item, raising StopIteration (or returning the default) when exhausted.
To design a custom iterable object, implement __iter__ and __next__.

iter() and next()