如何在没有__dict__的情况下创建类对象

Question

class B(type):
    __slots__ = ()

class A(metaclass=B):
    pass

A.test = "test"

"A" is instance of metaclass "B" and "B" have __slots__ defined - why I have __dict__ at class A ? How I can create class object without __dict__ ?

Answer 1

You cannot do this; classes always have a __dict__ .

You can only use __slots__ on classes to produce instances without a __dict__ , not on meta types. You'd normally only produce a few classes, so there is not much point in supporting __slots__ on metaclasses.

Don't use __slots__ to prevent attributes being set. Use __setattr__ for that instead:

class NoAttributesClassMeta(type):
    def __setattr__(cls, name, value):
        if name not in cls.__dict__:
            raise AttributeError("Cannot set attributes")
        type.__setattr__(cls, name, value)

Answer 2

__slots__ won't stop you from setting an attribute to a class, you need to override __setattr__ for that. Something like this should do it:

class B(type):
    def __new__(cls, clsname, bases, dct):
        dct['__slots__']  = ('x',)
        return type.__new__(cls, clsname, bases, dct)
    def __setattr__(cls, attr, val):
        if attr not in cls.__slots__:
            raise AttributeError('Can\'t set {!r}'.format(attr))
        else:
            type.__setattr__(cls, attr, val)

class A(metaclass=B):
    pass

Demo:

>>> A.x = 1
>>> A.y = 2
Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    A.y = 2
  File "C:\Python27\so.py", line 7, in __setattr__
    raise AttributeError('Can\'t set {!r}'.format(attr))
AttributeError: Can't set 'y'