__slots__
does not (significantly) speed up attribute access:
>>> class Foo(object):
... __slots__ = ('spam',)
... def __init__(self):
... self.spam = 'eggs'
...
>>> class Bar(object):
... def __init__(self):
... self.spam = 'eggs'
...
>>> import timeit
>>> timeit.timeit('t.spam', 'from __main__ import Foo; t=Foo()')
0.07030296325683594
>>> timeit.timeit('t.spam', 'from __main__ import Bar; t=Bar()')
0.07646608352661133
The goal of using __slots__
is to save memory; instead of using a .__dict__
mapping on the instance, the class has descriptors objects for each and every attribute named in __slots__
and instances have the attribute assigned wether or not they have an actual value:
>>> class Foo(object):
... __slots__ = ('spam',)
...
>>> dir(Foo())
['__class__', '__delattr__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__slots__', '__str__', '__subclasshook__', 'spam']
>>> Foo().spam
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: spam
>>> Foo.spam
<member 'spam' of 'Foo' objects>
>>> type(Foo.spam)
<type 'member_descriptor'>
So python still has to look at the class for each attribute access on an instance of Foo
(to find the descriptor). Any unknown attribute (say, Foo.ham
) will still result in Python looking through the class MRO to search for that attribute, and that includes dictionary searches. And you can still assign additional attributes to the class:
>>> Foo.ham = 'eggs'
>>> dir(Foo)
['__class__', '__delattr__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__slots__', '__str__', '__subclasshook__', 'ham', 'spam']
>>> Foo().ham
'eggs'
The slot descriptors are created when the class is created, and access memory assigned to each instance to store and retrieve a reference to the associated value (the same chunk of memory that tracks instance reference counts and a reference back to the class object). Without slots, a descriptor for __dict__
is used accessing a reference to a dict
object in the same manner.