metaclass

meta.py

from inspect import getmembers, isroutine


class Descriptor:

    def __init__(self, value):
        self.value = value

    def __get__(self, instance, owner):
        print("get")
        if self.value in instance.__slots__.keys():
            return instance.__slots__[self.value]
        else:
            print('Not exists')
            return None

    def __set__(self, instance, value):
        print("set")
        instance.__slots__[self.value] = value

    def __delete__(self, instance):
        instance.__slots__[self.value] = None


class MetaA(type):
    def __new__(mcs, *args, **kwargs):
        variables = [a for a in args[2] if not (a.startswith('__') and a.endswith('__')) and not isroutine(args[2][a])]

        args[2]['__slots__'] = {}
        for var_name in variables:
            args[2]['__slots__'][var_name] = args[2][var_name]
            args[2].pop(var_name)

        res = super().__new__(mcs, *args, **kwargs)

        for key in args[2]['__slots__']:
            flag = False
            for name, value in getmembers(args[2]['__slots__'][key]):
                if name == '__get__' or name == '__set__' or name == '__delete__' or name == '__set_name__':
                    flag = True
                    break
            if flag:
                setattr(res, key, args[2]['__slots__'][key])
                args[2]['__slots__'][key] = None

        def _setattr(self, _name, _value):
            if _name not in self.__slots__:
                print(f"Not allowed to assign '{_name}' with value '{_value}'")
                return None
            else:
                return object.__setattr__(self, _name, _value)

        res.__setattr__ = _setattr
        return res

    def __init__(cls, *args, **kwargs):
        type.__init__(cls, *args, **kwargs)

    def __call__(cls, *args, **kwargs):
        print(cls.__slots__)
        prev_values = cls.__slots__.copy()
        new_obj = super().__call__(*args, **kwargs)
        print(new_obj.__slots__)
        for key in new_obj.__slots__:
            if prev_values[key] is not None:
                try:
                    exec(f'v = new_obj.{key}')
                except:
                    exec(f"new_obj.{key} = {new_obj.__slots__[key]}")
        return new_obj


class A(metaclass=MetaA):
    foo = Descriptor("foo")
    bar = 2

    def __init__(self, foo):
        self.foo = foo

    def f(self):
        print(self.foo)


print('hello there')
a = A(10)
print(a.bar)
print(a.foo)
a.bar = -1
a.foo = 12
del a.foo
a.foo = 10
a.zoo = 9