Independent booleans can represent combinations the domain never intended. Named states and checked transitions make the legal combinations and actions explicit.

Use an enum for LOCKED and UNLOCKED. Each operation checks the current state and performs exactly the allowed transition. payment returns False if already unlocked; it is a simulated authorization signal, not an actual money-taking operation. push returns True only when it consumes one unlocked passage. A full State pattern with per-state classes is an optional refactoring when behavior grows; two clear branches do not require that machinery.

What the function promises: A Turnstile starts locked. payment signals one accepted authorization if locked. push attempts one passage. Rejected actions leave state unchanged. Real payment/refund processing is outside this simulation.

from enum import Enum, auto

class GateState(Enum):
    LOCKED = auto()
    UNLOCKED = auto()

class Turnstile:
    def __init__(self):
        self._state = GateState.LOCKED

    @property
    def state(self):
        return self._state

    def payment(self):
        if self._state is GateState.UNLOCKED:
            return False
        self._state = GateState.UNLOCKED
        return True

    def push(self):
        if self._state is GateState.LOCKED:
            return False
        self._state = GateState.LOCKED
        return True

gate = Turnstile()
assert not gate.push()
assert gate.payment()
assert not gate.payment()
assert gate.push()
assert gate.state is GateState.LOCKED
assert not gate.push()

Work and storage: O(1) time and storage for each transition. Clarity comes from the explicit behavior table, not runtime optimization.

When to recognize it: The same action means different things depending on the object’s current state.

Check before applying it: A state diagram does not enforce itself. Route mutations through checked transitions; do not scatter independent boolean flags.

Write the legal action/state transitions and failure outcomes first. Then choose whether a transition table or separate State objects makes those rules easier to maintain and test.