Iterators & the Iterator Protocol

# Every iterable has an iterator
nums = [1, 2, 3]
iterator = iter(nums)       # get iterator
print(next(iterator))       # 1
print(next(iterator))       # 2
print(next(iterator))       # 3
# next(iterator)            # StopIteration!

# Custom iterator class
class Countdown:
    def __init__(self, start):
        self.start = start
    
    def __iter__(self):
        self.current = self.start
        return self
    
    def __next__(self):
        if self.current <= 0:
            raise StopIteration
        value = self.current
        self.current -= 1
        return value

for num in Countdown(5):
    print(num, end=" ")  # 5 4 3 2 1
print()

# Infinite iterator
class InfiniteCounter:
    def __init__(self, start=0):
        self.n = start
    def __iter__(self): return self
    def __next__(self):
        value = self.n
        self.n += 1
        return value

counter = InfiniteCounter()
for _ in range(5):
    print(next(counter), end=" ")  # 0 1 2 3 4
print()