Metaclasses and __slots__?

Question

So, I am reading a bit about metaclasses in Python, and how type()'s three-argument alter-ego is used to dynamically create classes. However, the third argument is usually a dict that initializes the to-be created class' __dict__ variable.

If I want to dynamically create classes based on a metaclass that uses __slots__ instead of __dict__, how might I do this? Is type() still used in some fashion along with overriding __new__()?

As an FYI, I am aware of the proper uses for __slots__, to save memory when creating large numbers of a class versus abusing it to enforce a form of type-safety.

Example of a normal (new-style) class that sets __metaclass__ and uses a __dict__:

class Meta(type):
    def __new__(cls, name, bases, dctn):
        # Do something unique ...
        return type.__new__(cls, name, bases, dctn)

class Foo(object):
    __metaclass__ = Meta

    def __init__(self):
        pass

In the above, type.__new__() is called and the fourth argument (which becomes the third when actually used) creates a __dict__ in Foo. But if I wanted to modify Meta to include __slots__, then I have no dictionary to pass on to type()'s __new__() function (as far as I know -- I haven't tested any of this yet, just pondering and trying to find some kind of a use-case scenario).

Edit: A quick, but untested guess, is to take a dict of the values to be put into the __slots__ variables and pass it to type.__new__(). Then add an __init__() to Meta that populates the __slots__ variables from the dict. Although, I am not certain how that dict would reach __init__(), because the declaration of __slots__ prevents __dict__ from being created unless __dict__ is defined in __slots__...

Answer 1

You can't create a type with a non-empty __slots__ attribute. What you can do is insert a __slots__ attribute into the new class's dict, like this:

class Meta(type): 
    def __new__(cls, name, bases, dctn):
         dctn['__slots__'] = ( 'x', )
         return type.__new__(cls, name, bases, dctn)

 class Foo(object):
    __metaclass__ = Meta

    def __init__(self):
        pass 

Now Foo has slotted attributes:

foo = Foo() 
foo.y = 1

throws

 AttributeError: 'Foo' object has no attribute 'y'