Vending Machine

The Idea

Give every state its own small object: idle, paid, dispensing, sold out. Each answers the same actions differently, and a transition is a state replacing itself on the machine.

The machine holds no rules at all — no flags, no chain of checks that has to be repeated in every method.

Real-World Example

A ticket barrier. Tapping your card a second time does nothing, because the gate is already open; the identical tap a moment earlier is what opened it. The gate, not the card, decides.

The Code

class Idle:
    def coin(self, m): m.state = Paid(); return "paid"
    def select(self, m): return "pay first"

class Paid:
    def coin(self, m): return "already paid"
    def select(self, m): m.state = Idle(); return "dispensing"

class Machine:
    def __init__(self): self.state = Idle()
    def do(self, action): return getattr(self.state, action)(self)  # no flags

m = Machine()
print(m.do("select"), m.do("coin"), m.do("select"), m.do("select"))
# pay first paid dispensing pay first

The Tradeoff

Adding "sold out" is one new class plus one changed transition, in Paid, the only state that can sell the last item; the machine itself never changes. But four classes for four labels is a lot of ceremony, and the transition map is scattered instead of readable in one place. Use a table while states are only names.