21783b57_ython_in_Action__Magic_Methods__Method
[Description] When __slots__ is set as a class variable (a list of strings), it reserves space in the class for the named members. Instances of classes with __slots__ will not have a __dict__ dictionary, and attempting to set an attribute not listed in __slots__ will raise an AttributeError. It is intended as a memory optimization, saving the space required for a dictionary per instance. In IronPython classes that define __slots__ are heavily optimised; attribute lookup is approximately 4x faster. __slots__ has several caveats about its use. See the Python documentation for more details. (a) [Method] 
__slots__
IronPython in Action: Magic Methods
Method