A mutable cell with the simplest API possible

cell.py

_SENTINEL = object()


class Cell:
    def __init__(self, value=_SENTINEL):
        if value is not _SENTINEL:
            self.value = value

    def __call__(self, value=_SENTINEL):
        if value is not _SENTINEL:
            self.value = value

        if not hasattr(self, "value"):
            raise ValueError("Cell is empty")
        
        return self.value


if __name__ == "__main__":
    cell = Cell()
    # cell()
    cell("x")
    print(cell())

    cel = Cell("y")
    print(cel())