"The presence of slots does several things. First, it restricts the valid set of attribute names on an object to exactly those names listed. Second, since the attributes are now fixed, it is no longer necessary to store attributes in an instance dictionary, so the dict attribute is removed (unless a base class already has it; it can also be added back by a subclass that doesn't use slots). Instead, the attributes can be stored in predetermined locations within an array. Thus, every slot attribute is actually a descriptor object that knows how to set/get each attribute using an array index. Underneath the covers, the implementation of this feature is done entirely in C and is highly efficient."