flowchart TB
    Iterable["📦 Iterable
has __iter__()
list, tuple, str, dict"] -->|"iter(iterable)"| Iterator["🔄 Iterator
has __iter__() + __next__()
remembers its position"]
    Iterator -->|"next(iterator)"| Value["📤 Returns next value
1, 2, 3, ..."]
    Value -->|"More values?"| Iterator
    Value -->|"No more values!"| Stop["⛔ StopIteration
Exception raised"]

    style Iterable fill:#7c3aed,color:#fff
    style Iterator fill:#4f46e5,color:#fff
    style Value fill:#059669,color:#fff
    style Stop fill:#dc2626,color:#fff

# An object is iterable if it has __iter__() that returns an iterator
# An object is an iterator if it has __next__() that returns values

numbers = [1, 2, 3]
it = iter(numbers)  # Same as numbers.__iter__()

print(next(it))  # 1
print(next(it))  # 2
print(next(it))  # 3
# print(next(it))  # StopIteration!

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)  # 5, 4, 3, 2, 1

# enumerate — indexed iteration
for i, char in enumerate("hello"):
    print(i, char)

# zip — parallel iteration
for a, b in zip([1, 2, 3], ['a', 'b', 'c']):
    print(a, b)

# reversed — reverse iteration
for char in reversed("hello"):
    print(char)

# iter with sentinel
def read_until_empty():
    while True:
        line = input("> ")
        if line == "":
            break
        yield line

# itertools module
from itertools import cycle, chain, islice, count

# Infinite cycle
colors = cycle(["red", "green", "blue"])
for _ in range(6):
    print(next(colors))  # red, green, blue, red, green, blue

# Chain iterables
combined = chain([1, 2, 3], ['a', 'b', 'c'])
print(list(combined))  # [1, 2, 3, 'a', 'b', 'c']

# Infinite count
for i in islice(count(10, 2), 5):
    print(i)  # 10, 12, 14, 16, 18