"""
State Design Pattern
Intent: Lets an object alter its behavior when its internal state changes. It
appears as if the object changed its class.
"""
from __future__ import annotations
from abc import ABC, abstractmethod
class Context:
"""
The Context defines the interface of interest to clients. It also maintains
a reference to an instance of a State subclass, which represents the current
state of the Context.
"""
_state = None
"""
A reference to the current state of the Context.
"""
def __init__(self, state: State) -> None:
self.transition_to(state)
def transition_to(self, state: State):
"""
The Context allows changing the State object at runtime.
"""
print(f"Context: Transition to {type(state).__name__}")
self._state = state
self._state.context = self
"""
The Context delegates part of its behavior to the current State object.
"""
def request1(self):
self._state.handle1()
def request2(self):
self._state.handle2()
class State(ABC):
"""
The base State class declares methods that all Concrete State should
implement and also provides a backreference to the Context object,
associated with the State. This backreference can be used by States to
transition the Context to another State.
"""
@property
def context(self) -> Context:
return self._context
@context.setter
def context(self, context: Context) -> None:
self._context = context
@abstractmethod
def handle1(self) -> None:
pass
@abstractmethod
def handle2(self) -> None:
pass
"""
Concrete States implement various behaviors, associated with a state of the
Context.
"""
class ConcreteStateA(State):
def handle1(self) -> None:
print("ConcreteStateA handles request1.")
print("ConcreteStateA wants to change the state of the context.")
self.context.transition_to(ConcreteStateB())
def handle2(self) -> None:
print("ConcreteStateA handles request2.")
class ConcreteStateB(State):
def handle1(self) -> None:
print("ConcreteStateB handles request1.")
def handle2(self) -> None:
print("ConcreteStateB handles request2.")
print("ConcreteStateB wants to change the state of the context.")
self.context.transition_to(ConcreteStateA())
if __name__ == "__main__":
context = Context(ConcreteStateA())
context.request1()
context.request2()