simple Python observable implementation

observable.py

T = TypeVar('T')

class Observable(Generic[T]):
    def __init__(self, value: T) -> None:
        self.value: T = value
        # list of callable items
        self.listeners: List[Callable[[T], Any]] = []
    
    def update(self, new_value: T) -> None:
        self.value = new_value
        for listener in self.listeners:
            listener(self.value)

a = Observable(0)
b = Observable(1)

a.listeners.append(lambda new_a: b + new_a)
b.listeners.append(lambda new_b: a - new_a)

# attention à pas créer des boucles infinies !
a.listeners.append(lambda new_a: b.update(b + new_a))
b.listeners.append(lambda new_b: a.update(a - new_b)) # does not terminate!