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    <p>One very important thing to be aware of -- if those attributes stay in the class, the <code>__slots__</code> generation will be useless... okay, maybe not <em>useless</em> -- it will make the class attributes read-only; probably not what you want.</p> <p>The easy way is to say, "Okay, I'll initialize them to None, then let them disappear." Excellent! Here's one way to do that:</p> <pre><code>class B(object): three = None four = None temp = vars() # get the local namespace as a dict() __slots__ = temp.keys() # put their names into __slots__ __slots__.remove('temp') # remove non-__slots__ names __slots__.remove('__module__') # now remove the names from the local for name in __slots__: # namespace so we don't get read-only del temp[name] # class attributes del temp # and get rid of temp </code></pre> <p>If you want to keep those initial values it takes a bit more work... here's one possible solution:</p> <pre><code>class B(object): three = 3 four = 4 def __init__(self): for key, value in self.__init__.defaults.items(): setattr(self, key, value) temp = vars() __slots__ = temp.keys() __slots__.remove('temp') __slots__.remove('__module__') __slots__.remove('__init__') __init__.defaults = dict() for name in __slots__: __init__.defaults[name] = temp[name] del temp[name] del temp </code></pre> <p>As you can see, it is possible to do this without a metaclass -- but who wants all that boilerplate? A metaclass could definitely help us clean this up:</p> <pre><code>class MakeSlots(type): def __new__(cls, name, bases, attrs): new_attrs = {} new_attrs['__slots__'] = slots = attrs.keys() slots.remove('__module__') slots.remove('__metaclass__') new_attrs['__weakref__'] = None new_attrs['__init__'] = init = new_init init.defaults = dict() for name in slots: init.defaults[name] = attrs[name] return super(MakeSlots, cls).__new__(cls, name, bases, new_attrs) def new_init(self): for key, value in self.__init__.defaults.items(): setattr(self, key, value) class A(object): __metaclass__ = MakeSlots one = 1 two = 2 class B(object): __metaclass__ = MakeSlots three = 3 four = 4 </code></pre> <p>Now all the tediousness is kept in the metaclass, and the actual class is easy to read and (hopefully!) understand.</p> <p>If you need to have anything else in these classes besides attributes I strongly suggest you put whatever it is in a mixin class -- having them directly in the final class would complicate the metaclass even more.</p>
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