Slots

A slot is nothing more than a memory management nicety: when you define __slots__ on a class, you’re telling the Python interpreter that the list of attributes described within are the only attributes this class will ever need, and a dynamic dictionary is not needed to manage the references to other objects within the class:

class X(object):
    __slots__ = ["m", "n"]

>>> x = X()
>>> x.m = 10
>>> x.n = 10
>>> x.k = 3
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
AttributeError: 'X' object has no attribute 'k'

• __slots__ reserves space for the listed variables directly in the instance.

• Classes that define slots don’t have an instance dictionary (__dict__).

• If you try to assign to an attribute that’s not in __slots__, you receive an error. This may be quite useful for struct-like classes, because it prevents problems with misspelled attribute names.

• Just be warned that a slot in a derived class hides a slot of the same name in the base class.