Python Iterators

Introduction

Iterators are objects that implement the iterator protocol with iter and next methods. They provide a way to traverse collections lazily.

Basic Iterator

class Counter:
    def __init__(self, limit):
        self.current = 0
        self.limit = limit
    
    def __iter__(self):
        return self
    
    def __next__(self):
        if self.current >= self.limit:
            raise StopIteration
        result = self.current
        self.current += 1
        return result

for i in Counter(5):
    print(i)  # 0, 1, 2, 3, 4

Iterator from Iterable

class FileLines:
    def __init__(self, filename):
        self.filename = filename
    
    def __iter__(self):
        with open(self.filename, "r") as f:
            for line in f:
                yield line.rstrip()

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

Infinite Iterator

class Fibonacci:
    def __init__(self):
        self.a, self.b = 0, 1
    
    def __iter__(self):
        return self
    
    def __next__(self):
        result = self.a
        self.a, self.b = self.b, self.a + self.b
        return result

fib = Fibonacci()
for i in range(10):
    print(next(fib))  # 0, 1, 1, 2, 3, 5, 8, 13, 21, 34

Iterator with StopIteration

class RangeIterator:
    def __init__(self, start, stop):
        self.current = start
        self.stop = stop
    
    def __iter__(self):
        return self
    
    def __next__(self):
        if self.current >= self.stop:
            raise StopIteration
        value = self.current
        self.current += 1
        return value

Built-in Iterator Functions

items = [1, 2, 3]
iterator = iter(items)
print(next(iterator))  # 1
print(next(iterator, "default"))  # 2
print(next(iterator, "default"))  # 3
print(next(iterator, "default"))  # default

Practice Problems

1. Create iterator for even numbers
2. Implement iterator from list
3. Build infinite generator
4. Use iter() and next() functions
5. Create iterator with state