apply-state-pattern

Apply State Pattern

Allow an object to alter its behavior when its internal state changes — by delegating behavior to state objects rather than using conditional logic.

Why This Is Best Practice

Adopted by: XState (JavaScript statechart library — 1.5M weekly npm downloads) implements State pattern as its core model. TCP protocol stack implements state as objects (CLOSED, LISTEN, SYN_SENT, ESTABLISHED, etc. — each with its own behavior for each operation). Unity's Animator Controller is a visual state machine based on the same pattern. Every parser generator (ANTLR, PLY) produces state-based recognizers. Impact: GoF documents that without the State pattern, a class with 4 states and 6 operations has 4×6=24 conditional branches scattered across all methods. The State pattern consolidates each state's behavior into one class — 4 state classes of 6 methods each. Adding a 5th state requires one new class, not editing 6 existing methods. Why best: if/elif state dispatch — the alternative — scatters state logic across every method, making it hard to add states or understand what happens in a given state. State objects make each state's behavior cohesive, isolated, and independently testable.

Sources: Gamma et al. (1994) pp. 305–313; XState documentation; RFC 793 (TCP state machine)

Steps

Step 1: Define the state interface with all state-dependent methods

from abc import ABC, abstractmethod

class OrderState(ABC):
    @abstractmethod
    def pay(self, order: "Order") -> None: ...

    @abstractmethod
    def ship(self, order: "Order") -> None: ...

    @abstractmethod
    def cancel(self, order: "Order") -> None: ...

Step 2: Implement concrete state classes — each handles its own transitions

class PendingState(OrderState):
    def pay(self, order: "Order") -> None:
        print("Payment accepted")
        order.state = PaidState()

    def ship(self, order: "Order") -> None:
        print("Cannot ship — order not paid")

    def cancel(self, order: "Order") -> None:
        print("Order cancelled")
        order.state = CancelledState()

class PaidState(OrderState):
    def pay(self, order: "Order") -> None:
        print("Already paid")

    def ship(self, order: "Order") -> None:
        print("Order shipped")
        order.state = ShippedState()

    def cancel(self, order: "Order") -> None:
        print("Cancelling paid order — refund issued")
        order.state = CancelledState()

class ShippedState(OrderState):
    def pay(self, order: "Order") -> None:
        print("Already paid and shipped")

    def ship(self, order: "Order") -> None:
        print("Already shipped")

    def cancel(self, order: "Order") -> None:
        print("Cannot cancel — already shipped")

class CancelledState(OrderState):
    def pay(self, order: "Order") -> None:
        print("Order is cancelled")

    def ship(self, order: "Order") -> None:
        print("Order is cancelled")

    def cancel(self, order: "Order") -> None:
        print("Already cancelled")

Step 3: Context holds the current state and delegates all state-dependent operations

class Order:
    def __init__(self):
        self.state: OrderState = PendingState()

    def pay(self) -> None:
        self.state.pay(self)

    def ship(self) -> None:
        self.state.ship(self)

    def cancel(self) -> None:
        self.state.cancel(self)

Step 4: Transition state inside state objects — not in the context

State transitions (order.state = PaidState()) happen inside state methods (Step 2), not inside Order. This keeps the context thin and each state's transitions explicit.

Step 5: Verify transitions by testing each state in isolation

def test_pending_pay():
    order = Order()
    assert isinstance(order.state, PendingState)
    order.pay()
    assert isinstance(order.state, PaidState)

def test_shipped_cannot_cancel():
    order = Order()
    order.pay()
    order.ship()
    order.cancel()
    assert isinstance(order.state, ShippedState)   # state unchanged

When NOT to Use

• Simple states with few operations — 2 states × 2 operations = 4 conditions. A simple if/else is cleaner than 2 state classes.
• When transitions are purely data-driven — if state transitions come from a config file or database, a data-driven state machine (a transition table) is more maintainable than code-based state objects.

Common Mistakes

Context that checks isinstance(state, ...) to make decisions. This reintroduces the conditional logic the pattern eliminates. Delegate the decision to the state object.

State objects that modify context fields directly. If PaidState sets order.paid_at = datetime.now(), the state is coupled to the context's internals. Pass only what state objects need, or use events.

Missing states in the state interface. If one state handles ship() but another doesn't implement the interface method, calls fail unexpectedly. All states must implement all interface methods.