标签归档:slots

__slots__的用法?

问题:__slots__的用法?

__slots__Python 的目的是什么-尤其是关于何时要使用它,何时不使用它?

What is the purpose of __slots__ in Python — especially with respect to when I would want to use it, and when not?


回答 0

在Python中,目的是__slots__什么?在什么情况下应该避免这种情况?

TLDR:

特殊属性__slots__允许您显式说明您希望对象实例具有哪些实例属性,并具有预期的结果:

  1. 更快的属性访问。
  2. 节省内存空间

节省的空间来自

  1. 将值引用存储在插槽中而不是中__dict__
  2. 如果父类拒绝它们并且您声明,则拒绝__dict____weakref__创建__slots__

快速警告

请注意,您只应在继承树中一次声明一个特定的插槽。例如:

class Base:
    __slots__ = 'foo', 'bar'

class Right(Base):
    __slots__ = 'baz', 

class Wrong(Base):
    __slots__ = 'foo', 'bar', 'baz'        # redundant foo and bar

遇到错误时,Python不会反对(它应该会),否则问题可能不会显现出来,但是您的对象将比原先占用更多的空间。Python 3.8:

>>> from sys import getsizeof
>>> getsizeof(Right()), getsizeof(Wrong())
(56, 72)

这是因为基准站的插槽描述符的插槽与错误的插槽分开。通常不应该这样,但是可以:

>>> w = Wrong()
>>> w.foo = 'foo'
>>> Base.foo.__get__(w)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: foo
>>> Wrong.foo.__get__(w)
'foo'

最大的警告是多重继承-无法将多个“具有非空插槽的父类”组合在一起。

为适应此限制,请遵循最佳做法:排除其父母(或他们的具体类)将共同继承的除一个或所有父代之外的所有抽象-给这些抽象留出空位(就像在父类中的抽象基类一样)标准库)。

有关示例,请参见下面有关多重继承的部分。

要求:

  • 要使名为in的属性__slots__实际上存储在插槽中而不是存储在插槽中__dict__,则类必须从继承object

  • 为防止创建__dict__,您必须继承,object并且继承中的所有类都必须声明,__slots__并且它们都不能具有'__dict__'条目。

如果您想继续阅读,有很多细节。

为什么使用__slots__:更快的属性访问。

Python的创建者Guido van Rossum 指出,他实际上是__slots__为了更快地访问属性而创建的。

证明可观的显着更快访问是微不足道的:

import timeit

class Foo(object): __slots__ = 'foo',

class Bar(object): pass

slotted = Foo()
not_slotted = Bar()

def get_set_delete_fn(obj):
    def get_set_delete():
        obj.foo = 'foo'
        obj.foo
        del obj.foo
    return get_set_delete

>>> min(timeit.repeat(get_set_delete_fn(slotted)))
0.2846834529991611
>>> min(timeit.repeat(get_set_delete_fn(not_slotted)))
0.3664822799983085

在Ubuntu 3.5上的Python 3.5中,插槽式访问的速度几乎快了30%。

>>> 0.3664822799983085 / 0.2846834529991611
1.2873325658284342

在Windows上的Python 2中,我测得的速度要快15%。

为何使用__slots__:内存节省

的另一个目的__slots__是减少每个对象实例占用的内存空间。

我自己对文档的贡献清楚地说明了背后的原因

通过使用节省的空间__dict__可能很大。

SQLAlchemy将大量内存节省归因__slots__

为了验证这一点,请在Ubuntu Linux上使用Python 2.7的Anaconda发行版(带有guppy.hpy(又是堆的)和)sys.getsizeof,不__slots__声明且没有其他声明的类实例的大小为64字节。但这包括__dict__。再次感谢Python的惰性求值,在__dict__引用它之前,显然不会调用,但是没有数据的类通常是无用的。当存在时,该__dict__属性另外至少为280个字节。

相反,__slots__声明为()(无数据)的类实例只有16个字节,插槽中有一项的总字节数为56个,插槽中有两项的总数为64个字节。

对于64位Python,我说明了dict在3.6中增长的每个点(0、1和2属性除外)的for __slots____dict__(未定义插槽)在Python 2.7和3.6中以字节为单位的内存消耗:

       Python 2.7             Python 3.6
attrs  __slots__  __dict__*   __slots__  __dict__* | *(no slots defined)
none   16         56 + 272   16         56 + 112 | if __dict__ referenced
one    48         56 + 272    48         56 + 112
two    56         56 + 272    56         56 + 112
six    88         56 + 1040   88         56 + 152
11     128        56 + 1040   128        56 + 240
22     216        56 + 3344   216        56 + 408     
43     384        56 + 3344   384        56 + 752

因此,尽管Python 3中的指令较小,但我们仍然可以看到__slots__实例可以很好地扩展以节省内存,这是您要使用的主要原因__slots__

只是为了完整起见,请注意,在类的命名空间中,每个插槽的一次性成本为Python 2中64字节,而在Python 3中为72字节,因为插槽使用数据描述符(如属性)称为“成员”。

>>> Foo.foo
<member 'foo' of 'Foo' objects>
>>> type(Foo.foo)
<class 'member_descriptor'>
>>> getsizeof(Foo.foo)
72

演示__slots__

要拒绝创建__dict__,必须子类化object

class Base(object): 
    __slots__ = ()

现在:

>>> b = Base()
>>> b.a = 'a'
Traceback (most recent call last):
  File "<pyshell#38>", line 1, in <module>
    b.a = 'a'
AttributeError: 'Base' object has no attribute 'a'

或子类化另一个定义的类 __slots__

class Child(Base):
    __slots__ = ('a',)

现在:

c = Child()
c.a = 'a'

但:

>>> c.b = 'b'
Traceback (most recent call last):
  File "<pyshell#42>", line 1, in <module>
    c.b = 'b'
AttributeError: 'Child' object has no attribute 'b'

要在对有槽位的__dict__对象进行子类化时允许创建,只需添加'__dict__'__slots__(请注意,槽位是有序的,并且您不应重复父类中已经存在的槽位):

class SlottedWithDict(Child): 
    __slots__ = ('__dict__', 'b')

swd = SlottedWithDict()
swd.a = 'a'
swd.b = 'b'
swd.c = 'c'

>>> swd.__dict__
{'c': 'c'}

或者甚至不需要__slots__在子类中声明,并且仍将使用父级的插槽,但不限制创建__dict__

class NoSlots(Child): pass
ns = NoSlots()
ns.a = 'a'
ns.b = 'b'

和:

>>> ns.__dict__
{'b': 'b'}

但是,__slots__可能会导致多重继承问题:

class BaseA(object): 
    __slots__ = ('a',)

class BaseB(object): 
    __slots__ = ('b',)

由于从具有两个非空插槽的父母创建子类失败:

>>> class Child(BaseA, BaseB): __slots__ = ()
Traceback (most recent call last):
  File "<pyshell#68>", line 1, in <module>
    class Child(BaseA, BaseB): __slots__ = ()
TypeError: Error when calling the metaclass bases
    multiple bases have instance lay-out conflict

如果遇到此问题,则可以将其__slots__从父级中移除,或者如果您可以控制父级,则给他们留空的空位,或重构为抽象:

from abc import ABC

class AbstractA(ABC):
    __slots__ = ()

class BaseA(AbstractA): 
    __slots__ = ('a',)

class AbstractB(ABC):
    __slots__ = ()

class BaseB(AbstractB): 
    __slots__ = ('b',)

class Child(AbstractA, AbstractB): 
    __slots__ = ('a', 'b')

c = Child() # no problem!

添加'__dict__'__slots__以获得动态分配:

class Foo(object):
    __slots__ = 'bar', 'baz', '__dict__'

现在:

>>> foo = Foo()
>>> foo.boink = 'boink'

因此,'__dict__'在具有插槽的情况下,我们将失去一些尺寸上的好处,因为它具有动态分配的优势,并且仍具有我们确实期望的名称的插槽。

当您从未插入槽的对象继承时,使用时会得到相同的语义__slots____slots__指向插入槽的值的名称,而其他所有值都放在实例的中__dict__

避免这样做__slots__是因为您不希望出现这种情况,因为它实际上并不是一个很好的理由- 如果需要,只需添加"__dict__"您的属性即可__slots__

如果需要该功能,可以类似地将__weakref____slots__显式添加。

子类化namedtuple时,设置为空tuple:

内置namedtuple使不可变的实例非常轻巧(本质上是元组的大小),但是要获得好处,如果您将它们子类化,则需要自己做:

from collections import namedtuple
class MyNT(namedtuple('MyNT', 'bar baz')):
    """MyNT is an immutable and lightweight object"""
    __slots__ = ()

用法:

>>> nt = MyNT('bar', 'baz')
>>> nt.bar
'bar'
>>> nt.baz
'baz'

尝试分配意外属性会引发,AttributeError因为我们已阻止创建__dict__

>>> nt.quux = 'quux'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'MyNT' object has no attribute 'quux'

可以__dict__通过设置off 允许创建__slots__ = (),但是不能__slots__对元组的子类型使用非空。

最大的警告:多重继承

即使多个父级的非空插槽相同,也不能一起使用:

class Foo(object): 
    __slots__ = 'foo', 'bar'
class Bar(object):
    __slots__ = 'foo', 'bar' # alas, would work if empty, i.e. ()

>>> class Baz(Foo, Bar): pass
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Error when calling the metaclass bases
    multiple bases have instance lay-out conflict

使用空__slots__父似乎提供了最大的灵活性,允许孩子选择阻止或允许(通过增加'__dict__'获得动态分配,见上面部分)创建的__dict__

class Foo(object): __slots__ = ()
class Bar(object): __slots__ = ()
class Baz(Foo, Bar): __slots__ = ('foo', 'bar')
b = Baz()
b.foo, b.bar = 'foo', 'bar'

你不具备有槽-因此,如果您添加它们,后来删除它们,它不应引起任何问题。

走出放在这里肢体:如果您撰写的混入或使用抽象基类,它不打算被实例化,空__slots__在那些父母似乎是在灵活性作为子类方面最好的一段路要走。

为了演示,首先,让我们创建一个我们希望在多重继承下使用的代码的类。

class AbstractBase:
    __slots__ = ()
    def __init__(self, a, b):
        self.a = a
        self.b = b
    def __repr__(self):
        return f'{type(self).__name__}({repr(self.a)}, {repr(self.b)})'

我们可以通过继承并声明预期的位置来直接使用以上内容:

class Foo(AbstractBase):
    __slots__ = 'a', 'b'

但是我们对此并不在意,这是微不足道的单一继承,我们需要另一个我们也可能继承的类,也许带有嘈杂的属性:

class AbstractBaseC:
    __slots__ = ()
    @property
    def c(self):
        print('getting c!')
        return self._c
    @c.setter
    def c(self, arg):
        print('setting c!')
        self._c = arg

现在,如果两个基地都有非空插槽,我们将无法进行以下操作。(实际上,如果我们愿意,我们可以给AbstractBase非空槽a和b,并将它们排除在下面的声明之外-将它们留在里面是错误的):

class Concretion(AbstractBase, AbstractBaseC):
    __slots__ = 'a b _c'.split()

现在,我们具有通过多重继承的功能,并且仍然可以拒绝__dict____weakref__实例化:

>>> c = Concretion('a', 'b')
>>> c.c = c
setting c!
>>> c.c
getting c!
Concretion('a', 'b')
>>> c.d = 'd'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Concretion' object has no attribute 'd'

其他避免插槽的情况:

  • __class__除非插槽布局相同,否则要在不具有它们的另一个类(并且不能添加它们)上执行分配时,请避免使用它们。(我对了解谁在做什么以及为什么这样做很感兴趣。)
  • 如果您想将诸如long,tuple或str之类的可变长度内建子类化,并想为其添加属性,请避免使用它们。
  • 如果您坚持通过实例变量的类属性提供默认值,请避免使用它们。

您也许可以从__slots__ 文档的其余部分(最新的3.7 dev文档)中找出更多的警告,我最近做出了很大的贡献。

对其他答案的批评

当前的最佳答案引用了过时的信息,而且非常容易波动,并且在某些重要方面未达到要求。

不要“仅__slots__在实例化许多对象时使用”

我引用:

__slots__如果要实例化大量(数百个,数千个)同一类的对象,则需要使用。”

例如,来自collections模块的抽象基类未实例化,但__slots__已为其声明。

为什么?

如果用户希望拒绝__dict____weakref__创建,则这些内容在父类中必须不可用。

__slots__ 创建接口或混入时有助于重用。

的确,许多Python用户并不是为可重用性而编写的,但是当您这样做时,可以选择拒绝不必要的空间使用是很有价值的。

__slots__ 不会破坏酸洗

腌制开槽的物体时,您可能会发现它带有误导性的抱怨TypeError

>>> pickle.loads(pickle.dumps(f))
TypeError: a class that defines __slots__ without defining __getstate__ cannot be pickled

这实际上是不正确的。此消息来自最早的协议,这是默认协议。您可以使用-1参数选择最新的协议。在Python 2.7中为2(在2.3中引入),在3.6中为4

>>> pickle.loads(pickle.dumps(f, -1))
<__main__.Foo object at 0x1129C770>

在Python 2.7中:

>>> pickle.loads(pickle.dumps(f, 2))
<__main__.Foo object at 0x1129C770>

在Python 3.6中

>>> pickle.loads(pickle.dumps(f, 4))
<__main__.Foo object at 0x1129C770>

所以我会牢记这一点,因为这是一个已解决的问题。

评论(至2016年10月2日)被接受

第一段是一半简短的解释,一半是预测的。这是真正回答问题的唯一部分

正确的用法__slots__是节省对象空间。静态结构不允许创建后添加任何内容,而不是具有允许随时向对象添加属性的动态命令。这样可以为使用插槽的每个对象节省一个指令的开销

后半部分是一厢情愿的想法,并且超出了预期:

尽管有时这是一个有用的优化,但是如果Python解释器足够动态,则仅在实际向对象添加内容时才需要dict,就完全没有必要了。

Python实际上做了类似的事情,只在__dict__访问时创建,但是创建许多没有数据的对象是相当荒谬的。

第二段过分简化,错过了避免的实际原因__slots__。以下不是避免使用插槽的真正原因(出于实际原因,请参阅上面我的回答的其余部分。):

它们以一种可被控制怪胎和静态类型临时表滥用的方式更改具有插槽的对象的行为。

然后,它继续讨论了使用Python实现该有害目标的其他方法,而不是讨论与之相关的任何方法__slots__

第三段是更多的如意算盘。答案者甚至根本没有写过这些杂乱的内容,而是为该网站的批评者弹药。

内存使用证据

创建一些普通对象和带槽对象:

>>> class Foo(object): pass
>>> class Bar(object): __slots__ = ()

实例化其中的一百万:

>>> foos = [Foo() for f in xrange(1000000)]
>>> bars = [Bar() for b in xrange(1000000)]

检查guppy.hpy().heap()

>>> guppy.hpy().heap()
Partition of a set of 2028259 objects. Total size = 99763360 bytes.
 Index  Count   %     Size   % Cumulative  % Kind (class / dict of class)
     0 1000000  49 64000000  64  64000000  64 __main__.Foo
     1     169   0 16281480  16  80281480  80 list
     2 1000000  49 16000000  16  96281480  97 __main__.Bar
     3   12284   1   987472   1  97268952  97 str
...

访问常规对象及其对象,__dict__然后再次检查:

>>> for f in foos:
...     f.__dict__
>>> guppy.hpy().heap()
Partition of a set of 3028258 objects. Total size = 379763480 bytes.
 Index  Count   %      Size    % Cumulative  % Kind (class / dict of class)
     0 1000000  33 280000000  74 280000000  74 dict of __main__.Foo
     1 1000000  33  64000000  17 344000000  91 __main__.Foo
     2     169   0  16281480   4 360281480  95 list
     3 1000000  33  16000000   4 376281480  99 __main__.Bar
     4   12284   0    987472   0 377268952  99 str
...

这与Python历史一致,来自Python 2.2中的Unifying类型和类。

如果您将内置类型作为子类,则多余的空间会自动添加到实例中以容纳__dict____weakrefs__。(__dict__尽管直到使用完,它才会被初始化,因此您不必担心空字典为您创建的每个实例所占用的空间。)如果不需要此多余的空间,则可以在短语中添加“ __slots__ = []”你的班。

In Python, what is the purpose of __slots__ and what are the cases one should avoid this?

TLDR:

The special attribute __slots__ allows you to explicitly state which instance attributes you expect your object instances to have, with the expected results:

  1. faster attribute access.
  2. space savings in memory.

The space savings is from

  1. Storing value references in slots instead of __dict__.
  2. Denying __dict__ and __weakref__ creation if parent classes deny them and you declare __slots__.

Quick Caveats

Small caveat, you should only declare a particular slot one time in an inheritance tree. For example:

class Base:
    __slots__ = 'foo', 'bar'

class Right(Base):
    __slots__ = 'baz', 

class Wrong(Base):
    __slots__ = 'foo', 'bar', 'baz'        # redundant foo and bar

Python doesn’t object when you get this wrong (it probably should), problems might not otherwise manifest, but your objects will take up more space than they otherwise should. Python 3.8:

>>> from sys import getsizeof
>>> getsizeof(Right()), getsizeof(Wrong())
(56, 72)

This is because the Base’s slot descriptor has a slot separate from the Wrong’s. This shouldn’t usually come up, but it could:

>>> w = Wrong()
>>> w.foo = 'foo'
>>> Base.foo.__get__(w)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: foo
>>> Wrong.foo.__get__(w)
'foo'

The biggest caveat is for multiple inheritance – multiple “parent classes with nonempty slots” cannot be combined.

To accommodate this restriction, follow best practices: Factor out all but one or all parents’ abstraction which their concrete class respectively and your new concrete class collectively will inherit from – giving the abstraction(s) empty slots (just like abstract base classes in the standard library).

See section on multiple inheritance below for an example.

Requirements:

  • To have attributes named in __slots__ to actually be stored in slots instead of a __dict__, a class must inherit from object.

  • To prevent the creation of a __dict__, you must inherit from object and all classes in the inheritance must declare __slots__ and none of them can have a '__dict__' entry.

There are a lot of details if you wish to keep reading.

Why use __slots__: Faster attribute access.

The creator of Python, Guido van Rossum, states that he actually created __slots__ for faster attribute access.

It is trivial to demonstrate measurably significant faster access:

import timeit

class Foo(object): __slots__ = 'foo',

class Bar(object): pass

slotted = Foo()
not_slotted = Bar()

def get_set_delete_fn(obj):
    def get_set_delete():
        obj.foo = 'foo'
        obj.foo
        del obj.foo
    return get_set_delete

and

>>> min(timeit.repeat(get_set_delete_fn(slotted)))
0.2846834529991611
>>> min(timeit.repeat(get_set_delete_fn(not_slotted)))
0.3664822799983085

The slotted access is almost 30% faster in Python 3.5 on Ubuntu.

>>> 0.3664822799983085 / 0.2846834529991611
1.2873325658284342

In Python 2 on Windows I have measured it about 15% faster.

Why use __slots__: Memory Savings

Another purpose of __slots__ is to reduce the space in memory that each object instance takes up.

My own contribution to the documentation clearly states the reasons behind this:

The space saved over using __dict__ can be significant.

SQLAlchemy attributes a lot of memory savings to __slots__.

To verify this, using the Anaconda distribution of Python 2.7 on Ubuntu Linux, with guppy.hpy (aka heapy) and sys.getsizeof, the size of a class instance without __slots__ declared, and nothing else, is 64 bytes. That does not include the __dict__. Thank you Python for lazy evaluation again, the __dict__ is apparently not called into existence until it is referenced, but classes without data are usually useless. When called into existence, the __dict__ attribute is a minimum of 280 bytes additionally.

In contrast, a class instance with __slots__ declared to be () (no data) is only 16 bytes, and 56 total bytes with one item in slots, 64 with two.

For 64 bit Python, I illustrate the memory consumption in bytes in Python 2.7 and 3.6, for __slots__ and __dict__ (no slots defined) for each point where the dict grows in 3.6 (except for 0, 1, and 2 attributes):

       Python 2.7             Python 3.6
attrs  __slots__  __dict__*   __slots__  __dict__* | *(no slots defined)
none   16         56 + 272†   16         56 + 112† | †if __dict__ referenced
one    48         56 + 272    48         56 + 112
two    56         56 + 272    56         56 + 112
six    88         56 + 1040   88         56 + 152
11     128        56 + 1040   128        56 + 240
22     216        56 + 3344   216        56 + 408     
43     384        56 + 3344   384        56 + 752

So, in spite of smaller dicts in Python 3, we see how nicely __slots__ scale for instances to save us memory, and that is a major reason you would want to use __slots__.

Just for completeness of my notes, note that there is a one-time cost per slot in the class’s namespace of 64 bytes in Python 2, and 72 bytes in Python 3, because slots use data descriptors like properties, called “members”.

>>> Foo.foo
<member 'foo' of 'Foo' objects>
>>> type(Foo.foo)
<class 'member_descriptor'>
>>> getsizeof(Foo.foo)
72

Demonstration of __slots__:

To deny the creation of a __dict__, you must subclass object:

class Base(object): 
    __slots__ = ()

now:

>>> b = Base()
>>> b.a = 'a'
Traceback (most recent call last):
  File "<pyshell#38>", line 1, in <module>
    b.a = 'a'
AttributeError: 'Base' object has no attribute 'a'

Or subclass another class that defines __slots__

class Child(Base):
    __slots__ = ('a',)

and now:

c = Child()
c.a = 'a'

but:

>>> c.b = 'b'
Traceback (most recent call last):
  File "<pyshell#42>", line 1, in <module>
    c.b = 'b'
AttributeError: 'Child' object has no attribute 'b'

To allow __dict__ creation while subclassing slotted objects, just add '__dict__' to the __slots__ (note that slots are ordered, and you shouldn’t repeat slots that are already in parent classes):

class SlottedWithDict(Child): 
    __slots__ = ('__dict__', 'b')

swd = SlottedWithDict()
swd.a = 'a'
swd.b = 'b'
swd.c = 'c'

and

>>> swd.__dict__
{'c': 'c'}

Or you don’t even need to declare __slots__ in your subclass, and you will still use slots from the parents, but not restrict the creation of a __dict__:

class NoSlots(Child): pass
ns = NoSlots()
ns.a = 'a'
ns.b = 'b'

And:

>>> ns.__dict__
{'b': 'b'}

However, __slots__ may cause problems for multiple inheritance:

class BaseA(object): 
    __slots__ = ('a',)

class BaseB(object): 
    __slots__ = ('b',)

Because creating a child class from parents with both non-empty slots fails:

>>> class Child(BaseA, BaseB): __slots__ = ()
Traceback (most recent call last):
  File "<pyshell#68>", line 1, in <module>
    class Child(BaseA, BaseB): __slots__ = ()
TypeError: Error when calling the metaclass bases
    multiple bases have instance lay-out conflict

If you run into this problem, You could just remove __slots__ from the parents, or if you have control of the parents, give them empty slots, or refactor to abstractions:

from abc import ABC

class AbstractA(ABC):
    __slots__ = ()

class BaseA(AbstractA): 
    __slots__ = ('a',)

class AbstractB(ABC):
    __slots__ = ()

class BaseB(AbstractB): 
    __slots__ = ('b',)

class Child(AbstractA, AbstractB): 
    __slots__ = ('a', 'b')

c = Child() # no problem!

Add '__dict__' to __slots__ to get dynamic assignment:

class Foo(object):
    __slots__ = 'bar', 'baz', '__dict__'

and now:

>>> foo = Foo()
>>> foo.boink = 'boink'

So with '__dict__' in slots we lose some of the size benefits with the upside of having dynamic assignment and still having slots for the names we do expect.

When you inherit from an object that isn’t slotted, you get the same sort of semantics when you use __slots__ – names that are in __slots__ point to slotted values, while any other values are put in the instance’s __dict__.

Avoiding __slots__ because you want to be able to add attributes on the fly is actually not a good reason – just add "__dict__" to your __slots__ if this is required.

You can similarly add __weakref__ to __slots__ explicitly if you need that feature.

Set to empty tuple when subclassing a namedtuple:

The namedtuple builtin make immutable instances that are very lightweight (essentially, the size of tuples) but to get the benefits, you need to do it yourself if you subclass them:

from collections import namedtuple
class MyNT(namedtuple('MyNT', 'bar baz')):
    """MyNT is an immutable and lightweight object"""
    __slots__ = ()

usage:

>>> nt = MyNT('bar', 'baz')
>>> nt.bar
'bar'
>>> nt.baz
'baz'

And trying to assign an unexpected attribute raises an AttributeError because we have prevented the creation of __dict__:

>>> nt.quux = 'quux'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'MyNT' object has no attribute 'quux'

You can allow __dict__ creation by leaving off __slots__ = (), but you can’t use non-empty __slots__ with subtypes of tuple.

Biggest Caveat: Multiple inheritance

Even when non-empty slots are the same for multiple parents, they cannot be used together:

class Foo(object): 
    __slots__ = 'foo', 'bar'
class Bar(object):
    __slots__ = 'foo', 'bar' # alas, would work if empty, i.e. ()

>>> class Baz(Foo, Bar): pass
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Error when calling the metaclass bases
    multiple bases have instance lay-out conflict

Using an empty __slots__ in the parent seems to provide the most flexibility, allowing the child to choose to prevent or allow (by adding '__dict__' to get dynamic assignment, see section above) the creation of a __dict__:

class Foo(object): __slots__ = ()
class Bar(object): __slots__ = ()
class Baz(Foo, Bar): __slots__ = ('foo', 'bar')
b = Baz()
b.foo, b.bar = 'foo', 'bar'

You don’t have to have slots – so if you add them, and remove them later, it shouldn’t cause any problems.

Going out on a limb here: If you’re composing mixins or using abstract base classes, which aren’t intended to be instantiated, an empty __slots__ in those parents seems to be the best way to go in terms of flexibility for subclassers.

To demonstrate, first, let’s create a class with code we’d like to use under multiple inheritance

class AbstractBase:
    __slots__ = ()
    def __init__(self, a, b):
        self.a = a
        self.b = b
    def __repr__(self):
        return f'{type(self).__name__}({repr(self.a)}, {repr(self.b)})'

We could use the above directly by inheriting and declaring the expected slots:

class Foo(AbstractBase):
    __slots__ = 'a', 'b'

But we don’t care about that, that’s trivial single inheritance, we need another class we might also inherit from, maybe with a noisy attribute:

class AbstractBaseC:
    __slots__ = ()
    @property
    def c(self):
        print('getting c!')
        return self._c
    @c.setter
    def c(self, arg):
        print('setting c!')
        self._c = arg

Now if both bases had nonempty slots, we couldn’t do the below. (In fact, if we wanted, we could have given AbstractBase nonempty slots a and b, and left them out of the below declaration – leaving them in would be wrong):

class Concretion(AbstractBase, AbstractBaseC):
    __slots__ = 'a b _c'.split()

And now we have functionality from both via multiple inheritance, and can still deny __dict__ and __weakref__ instantiation:

>>> c = Concretion('a', 'b')
>>> c.c = c
setting c!
>>> c.c
getting c!
Concretion('a', 'b')
>>> c.d = 'd'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Concretion' object has no attribute 'd'

Other cases to avoid slots:

  • Avoid them when you want to perform __class__ assignment with another class that doesn’t have them (and you can’t add them) unless the slot layouts are identical. (I am very interested in learning who is doing this and why.)
  • Avoid them if you want to subclass variable length builtins like long, tuple, or str, and you want to add attributes to them.
  • Avoid them if you insist on providing default values via class attributes for instance variables.

You may be able to tease out further caveats from the rest of the __slots__ documentation (the 3.7 dev docs are the most current), which I have made significant recent contributions to.

Critiques of other answers

The current top answers cite outdated information and are quite hand-wavy and miss the mark in some important ways.

Do not “only use __slots__ when instantiating lots of objects”

I quote:

“You would want to use __slots__ if you are going to instantiate a lot (hundreds, thousands) of objects of the same class.”

Abstract Base Classes, for example, from the collections module, are not instantiated, yet __slots__ are declared for them.

Why?

If a user wishes to deny __dict__ or __weakref__ creation, those things must not be available in the parent classes.

__slots__ contributes to reusability when creating interfaces or mixins.

It is true that many Python users aren’t writing for reusability, but when you are, having the option to deny unnecessary space usage is valuable.

__slots__ doesn’t break pickling

When pickling a slotted object, you may find it complains with a misleading TypeError:

>>> pickle.loads(pickle.dumps(f))
TypeError: a class that defines __slots__ without defining __getstate__ cannot be pickled

This is actually incorrect. This message comes from the oldest protocol, which is the default. You can select the latest protocol with the -1 argument. In Python 2.7 this would be 2 (which was introduced in 2.3), and in 3.6 it is 4.

>>> pickle.loads(pickle.dumps(f, -1))
<__main__.Foo object at 0x1129C770>

in Python 2.7:

>>> pickle.loads(pickle.dumps(f, 2))
<__main__.Foo object at 0x1129C770>

in Python 3.6

>>> pickle.loads(pickle.dumps(f, 4))
<__main__.Foo object at 0x1129C770>

So I would keep this in mind, as it is a solved problem.

Critique of the (until Oct 2, 2016) accepted answer

The first paragraph is half short explanation, half predictive. Here’s the only part that actually answers the question

The proper use of __slots__ is to save space in objects. Instead of having a dynamic dict that allows adding attributes to objects at anytime, there is a static structure which does not allow additions after creation. This saves the overhead of one dict for every object that uses slots

The second half is wishful thinking, and off the mark:

While this is sometimes a useful optimization, it would be completely unnecessary if the Python interpreter was dynamic enough so that it would only require the dict when there actually were additions to the object.

Python actually does something similar to this, only creating the __dict__ when it is accessed, but creating lots of objects with no data is fairly ridiculous.

The second paragraph oversimplifies and misses actual reasons to avoid __slots__. The below is not a real reason to avoid slots (for actual reasons, see the rest of my answer above.):

They change the behavior of the objects that have slots in a way that can be abused by control freaks and static typing weenies.

It then goes on to discuss other ways of accomplishing that perverse goal with Python, not discussing anything to do with __slots__.

The third paragraph is more wishful thinking. Together it is mostly off-the-mark content that the answerer didn’t even author and contributes to ammunition for critics of the site.

Memory usage evidence

Create some normal objects and slotted objects:

>>> class Foo(object): pass
>>> class Bar(object): __slots__ = ()

Instantiate a million of them:

>>> foos = [Foo() for f in xrange(1000000)]
>>> bars = [Bar() for b in xrange(1000000)]

Inspect with guppy.hpy().heap():

>>> guppy.hpy().heap()
Partition of a set of 2028259 objects. Total size = 99763360 bytes.
 Index  Count   %     Size   % Cumulative  % Kind (class / dict of class)
     0 1000000  49 64000000  64  64000000  64 __main__.Foo
     1     169   0 16281480  16  80281480  80 list
     2 1000000  49 16000000  16  96281480  97 __main__.Bar
     3   12284   1   987472   1  97268952  97 str
...

Access the regular objects and their __dict__ and inspect again:

>>> for f in foos:
...     f.__dict__
>>> guppy.hpy().heap()
Partition of a set of 3028258 objects. Total size = 379763480 bytes.
 Index  Count   %      Size    % Cumulative  % Kind (class / dict of class)
     0 1000000  33 280000000  74 280000000  74 dict of __main__.Foo
     1 1000000  33  64000000  17 344000000  91 __main__.Foo
     2     169   0  16281480   4 360281480  95 list
     3 1000000  33  16000000   4 376281480  99 __main__.Bar
     4   12284   0    987472   0 377268952  99 str
...

This is consistent with the history of Python, from Unifying types and classes in Python 2.2

If you subclass a built-in type, extra space is automatically added to the instances to accomodate __dict__ and __weakrefs__. (The __dict__ is not initialized until you use it though, so you shouldn’t worry about the space occupied by an empty dictionary for each instance you create.) If you don’t need this extra space, you can add the phrase “__slots__ = []” to your class.


回答 1

引用雅各布·哈伦Jacob Hallen)的话

正确的用法__slots__是节省对象空间。静态结构不允许创建后添加任何内容,而不是具有允许随时向对象添加属性的动态命令。[这种使用__slots__消除了每个对象一个字典的开销。]尽管这有时是有用的优化,但是如果Python解释器足够动态,以至仅在实际添加了dict时才需要该字典,则完全没有必要。宾语。

不幸的是,插槽有副作用。它们以一种可被控制怪胎和静态类型临时表滥用的方式更改具有插槽的对象的行为。这是不好的,因为控制怪胎应该滥用元类,而静态类型之间应该滥用装饰器,因为在Python中,应该只有一种明显的方法。

使CPython足够智能来处理节省的空间__slots__是一项艰巨的任务,这可能就是为什么它不在P3k更改列表中的原因(至今)。

Quoting Jacob Hallen:

The proper use of __slots__ is to save space in objects. Instead of having a dynamic dict that allows adding attributes to objects at anytime, there is a static structure which does not allow additions after creation. [This use of __slots__ eliminates the overhead of one dict for every object.] While this is sometimes a useful optimization, it would be completely unnecessary if the Python interpreter was dynamic enough so that it would only require the dict when there actually were additions to the object.

Unfortunately there is a side effect to slots. They change the behavior of the objects that have slots in a way that can be abused by control freaks and static typing weenies. This is bad, because the control freaks should be abusing the metaclasses and the static typing weenies should be abusing decorators, since in Python, there should be only one obvious way of doing something.

Making CPython smart enough to handle saving space without __slots__ is a major undertaking, which is probably why it is not on the list of changes for P3k (yet).


回答 2

你会想使用__slots__,如果你要实例化同一个类的对象很多(几百,几千)。__slots__仅作为内存优化工具存在。

不建议将其__slots__用于约束属性创建。

__slots__使用默认的(最旧的)泡菜协议将不能使用酸洗对象。有必要指定一个更高的版本。

python的其他一些自省功能也可能受到不利影响。

You would want to use __slots__ if you are going to instantiate a lot (hundreds, thousands) of objects of the same class. __slots__ only exists as a memory optimization tool.

It’s highly discouraged to use __slots__ for constraining attribute creation.

Pickling objects with __slots__ won’t work with the default (oldest) pickle protocol; it’s necessary to specify a later version.

Some other introspection features of python may also be adversely affected.


回答 3

每个python对象都有一个__dict__属性,该属性是包含所有其他属性的字典。例如,当您键入self.attrpython时实际上正在执行self.__dict__['attr']。您可以想象使用字典存储属性会花费一些额外的空间和时间来访问它。

但是,当您使用 __slots__,为该类创建的任何对象将没有__dict__属性。相反,所有属性访问都直接通过指针进行。

因此,如果要使用C样式结构而不是完整的类,则可以使用它__slots__来压缩对象的大小并减少属性访问时间。一个很好的例子是一个包含属性x&y的Point类。如果您有很多要点,可以尝试使用__slots__以节省一些内存。

Each python object has a __dict__ atttribute which is a dictionary containing all other attributes. e.g. when you type self.attr python is actually doing self.__dict__['attr']. As you can imagine using a dictionary to store attribute takes some extra space & time for accessing it.

However, when you use __slots__, any object created for that class won’t have a __dict__ attribute. Instead, all attribute access is done directly via pointers.

So if want a C style structure rather than a full fledged class you can use __slots__ for compacting size of the objects & reducing attribute access time. A good example is a Point class containing attributes x & y. If you are going to have a lot of points, you can try using __slots__ in order to conserve some memory.


回答 4

除了其他答案,这是使用的示例__slots__

>>> class Test(object):   #Must be new-style class!
...  __slots__ = ['x', 'y']
... 
>>> pt = Test()
>>> dir(pt)
['__class__', '__delattr__', '__doc__', '__getattribute__', '__hash__', 
 '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', 
 '__repr__', '__setattr__', '__slots__', '__str__', 'x', 'y']
>>> pt.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: x
>>> pt.x = 1
>>> pt.x
1
>>> pt.z = 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Test' object has no attribute 'z'
>>> pt.__dict__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Test' object has no attribute '__dict__'
>>> pt.__slots__
['x', 'y']

因此,要实现__slots__,只需要多花一行(如果还没有,请将您的类变成新样式的类)。这样,您可以将这些类的内存占用量减少5倍,而在必要时以及必须编写自定义的pickle代码的代价是。

In addition to the other answers, here is an example of using __slots__:

>>> class Test(object):   #Must be new-style class!
...  __slots__ = ['x', 'y']
... 
>>> pt = Test()
>>> dir(pt)
['__class__', '__delattr__', '__doc__', '__getattribute__', '__hash__', 
 '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', 
 '__repr__', '__setattr__', '__slots__', '__str__', 'x', 'y']
>>> pt.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: x
>>> pt.x = 1
>>> pt.x
1
>>> pt.z = 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Test' object has no attribute 'z'
>>> pt.__dict__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Test' object has no attribute '__dict__'
>>> pt.__slots__
['x', 'y']

So, to implement __slots__, it only takes an extra line (and making your class a new-style class if it isn’t already). This way you can reduce the memory footprint of those classes 5-fold, at the expense of having to write custom pickle code, if and when that becomes necessary.


回答 5

插槽对于库调用非常有用,以消除进行函数调用时的“命名方法分派”。SWIG 文档中提到了这一点。对于想要减少使用插槽的通常称为函数的函数开销的高性能库,速度要快得多。

现在,这可能与OP问题没有直接关系。它与构建扩展有关,而不是与在对象上使用slot语法有关。但这确实有助于完整了解插槽的使用情况以及它们背后的一些原因。

Slots are very useful for library calls to eliminate the “named method dispatch” when making function calls. This is mentioned in the SWIG documentation. For high performance libraries that want to reduce function overhead for commonly called functions using slots is much faster.

Now this may not be directly related to the OPs question. It is related more to building extensions than it does to using the slots syntax on an object. But it does help complete the picture for the usage of slots and some of the reasoning behind them.


回答 6

类实例的属性具有3个属性:实例,属性名称和属性值。

常规属性访问中,实例充当字典,而属性名称充当该字典中查找值的键。

instance(attribute)->值

__slots__访问,属性名称充当字典,实例充当字典中查找值的键。

属性(实例)->值

flyweight模式中,属性的名称充当字典,而值充当该字典中查找实例的键。

attribute(value)->实例

An attribute of a class instance has 3 properties: the instance, the name of the attribute, and the value of the attribute.

In regular attribute access, the instance acts as a dictionary and the name of the attribute acts as the key in that dictionary looking up value.

instance(attribute) –> value

In __slots__ access, the name of the attribute acts as the dictionary and the instance acts as the key in the dictionary looking up value.

attribute(instance) –> value

In flyweight pattern, the name of the attribute acts as the dictionary and the value acts as the key in that dictionary looking up the instance.

attribute(value) –> instance


回答 7

一个非常简单的__slot__属性示例。

问题:无 __slots__

如果__slot__我的Class中没有属性,则可以向对象添加新属性。

class Test:
    pass

obj1=Test()
obj2=Test()

print(obj1.__dict__)  #--> {}
obj1.x=12
print(obj1.__dict__)  # --> {'x': 12}
obj1.y=20
print(obj1.__dict__)  # --> {'x': 12, 'y': 20}

obj2.x=99
print(obj2.__dict__)  # --> {'x': 99}

如果您看上面的示例,可以看到obj1obj2都有自己的xy属性,而python还dict为每个对象创建了一个属性(obj1obj2)。

假设我的类Test是否有成千上万个此类对象?dict为每个对象创建一个附加属性将导致我的代码中大量开销(内存,计算能力等)。

解决方案: __slots__

现在,在下面的示例中,我的类Test包含__slots__属性。现在,我无法向对象添加新属性(attribute除外x),而python不再创建dict属性。这消除了每个对象的开销,如果您有很多对象,开销可能会变得很大。

class Test:
    __slots__=("x")

obj1=Test()
obj2=Test()
obj1.x=12
print(obj1.x)  # --> 12
obj2.x=99
print(obj2.x)  # --> 99

obj1.y=28
print(obj1.y)  # --> AttributeError: 'Test' object has no attribute 'y'

A very simple example of __slot__ attribute.

Problem: Without __slots__

If I don’t have __slot__ attribute in my class, I can add new attributes to my objects.

class Test:
    pass

obj1=Test()
obj2=Test()

print(obj1.__dict__)  #--> {}
obj1.x=12
print(obj1.__dict__)  # --> {'x': 12}
obj1.y=20
print(obj1.__dict__)  # --> {'x': 12, 'y': 20}

obj2.x=99
print(obj2.__dict__)  # --> {'x': 99}

If you look at example above, you can see that obj1 and obj2 have their own x and y attributes and python has also created a dict attribute for each object (obj1 and obj2).

Suppose if my class Test has thousands of such objects? Creating an additional attribute dict for each object will cause lot of overhead (memory, computing power etc.) in my code.

Solution: With __slots__

Now in the following example my class Test contains __slots__ attribute. Now I can’t add new attributes to my objects (except attribute x) and python doesn’t create a dict attribute anymore. This eliminates overhead for each object, which can become significant if you have many objects.

class Test:
    __slots__=("x")

obj1=Test()
obj2=Test()
obj1.x=12
print(obj1.x)  # --> 12
obj2.x=99
print(obj2.x)  # --> 99

obj1.y=28
print(obj1.y)  # --> AttributeError: 'Test' object has no attribute 'y'

回答 8

另一个晦涩的用法__slots__是从ProxyTypes包(以前是PEAK项目的一部分)中向对象代理添加属性。它ObjectWrapper允许您代理另一个对象,但拦截与代理对象的所有交互。它不是很常用(并且没有Python 3支持),但是我们已经使用它来实现基于龙卷风的异步实现周围的线程安全的阻塞包装,该龙卷风使用线程安全来反弹通过ioloop对代理对象的所有访问concurrent.Future对象以同步并返回结果。

默认情况下,对代理对象的任何属性访问都将为您提供代理对象的结果。如果需要在代理对象上添加属性,__slots__则可以使用。

from peak.util.proxies import ObjectWrapper

class Original(object):
    def __init__(self):
        self.name = 'The Original'

class ProxyOriginal(ObjectWrapper):

    __slots__ = ['proxy_name']

    def __init__(self, subject, proxy_name):
        # proxy_info attributed added directly to the
        # Original instance, not the ProxyOriginal instance
        self.proxy_info = 'You are proxied by {}'.format(proxy_name)

        # proxy_name added to ProxyOriginal instance, since it is
        # defined in __slots__
        self.proxy_name = proxy_name

        super(ProxyOriginal, self).__init__(subject)

if __name__ == "__main__":
    original = Original()
    proxy = ProxyOriginal(original, 'Proxy Overlord')

    # Both statements print "The Original"
    print "original.name: ", original.name
    print "proxy.name: ", proxy.name

    # Both statements below print 
    # "You are proxied by Proxy Overlord", since the ProxyOriginal
    # __init__ sets it to the original object 
    print "original.proxy_info: ", original.proxy_info
    print "proxy.proxy_info: ", proxy.proxy_info

    # prints "Proxy Overlord"
    print "proxy.proxy_name: ", proxy.proxy_name
    # Raises AttributeError since proxy_name is only set on 
    # the proxy object
    print "original.proxy_name: ", proxy.proxy_name

Another somewhat obscure use of __slots__ is to add attributes to an object proxy from the ProxyTypes package, formerly part of the PEAK project. Its ObjectWrapper allows you to proxy another object, but intercept all interactions with the proxied object. It is not very commonly used (and no Python 3 support), but we have used it to implement a thread-safe blocking wrapper around an async implementation based on tornado that bounces all access to the proxied object through the ioloop, using thread-safe concurrent.Future objects to synchronise and return results.

By default any attribute access to the proxy object will give you the result from the proxied object. If you need to add an attribute on the proxy object, __slots__ can be used.

from peak.util.proxies import ObjectWrapper

class Original(object):
    def __init__(self):
        self.name = 'The Original'

class ProxyOriginal(ObjectWrapper):

    __slots__ = ['proxy_name']

    def __init__(self, subject, proxy_name):
        # proxy_info attributed added directly to the
        # Original instance, not the ProxyOriginal instance
        self.proxy_info = 'You are proxied by {}'.format(proxy_name)

        # proxy_name added to ProxyOriginal instance, since it is
        # defined in __slots__
        self.proxy_name = proxy_name

        super(ProxyOriginal, self).__init__(subject)

if __name__ == "__main__":
    original = Original()
    proxy = ProxyOriginal(original, 'Proxy Overlord')

    # Both statements print "The Original"
    print "original.name: ", original.name
    print "proxy.name: ", proxy.name

    # Both statements below print 
    # "You are proxied by Proxy Overlord", since the ProxyOriginal
    # __init__ sets it to the original object 
    print "original.proxy_info: ", original.proxy_info
    print "proxy.proxy_info: ", proxy.proxy_info

    # prints "Proxy Overlord"
    print "proxy.proxy_name: ", proxy.proxy_name
    # Raises AttributeError since proxy_name is only set on 
    # the proxy object
    print "original.proxy_name: ", proxy.proxy_name

回答 9

您基本上没有用__slots__

在您认为自己可能需要的时间里__slots__,您实际上想使用轻量级轻量级设计模式。在这些情况下,您不再希望使用纯Python对象。相反,您希望在数组,结构或numpy数组周围使用类似Python对象的包装器。

class Flyweight(object):

    def get(self, theData, index):
        return theData[index]

    def set(self, theData, index, value):
        theData[index]= value

类似于类的包装器没有属性-它仅提供对基础数据起作用的方法。这些方法可以简化为类方法。实际上,可以将其简化为仅对底层数据数组进行操作的函数。

You have — essentially — no use for __slots__.

For the time when you think you might need __slots__, you actually want to use Lightweight or Flyweight design patterns. These are cases when you no longer want to use purely Python objects. Instead, you want a Python object-like wrapper around an array, struct, or numpy array.

class Flyweight(object):

    def get(self, theData, index):
        return theData[index]

    def set(self, theData, index, value):
        theData[index]= value

The class-like wrapper has no attributes — it just provides methods that act on the underlying data. The methods can be reduced to class methods. Indeed, it could be reduced to just functions operating on the underlying array of data.


回答 10

最初的问题是关于一般用例,而不仅仅是内存。因此,在这里应该提到的是,在实例化大量对象时,您也会获得更好的性能 -有趣的是,例如,将大型文档解析为对象或从数据库中解析时。

这是使用插槽和不使用插槽创建具有一百万个条目的对象树的比较。作为参考,在对树使用普通字典时的性能(在OSX上为Py2.7.10):

********** RUN 1 **********
1.96036410332 <class 'css_tree_select.element.Element'>
3.02922606468 <class 'css_tree_select.element.ElementNoSlots'>
2.90828204155 dict
********** RUN 2 **********
1.77050495148 <class 'css_tree_select.element.Element'>
3.10655999184 <class 'css_tree_select.element.ElementNoSlots'>
2.84120798111 dict
********** RUN 3 **********
1.84069895744 <class 'css_tree_select.element.Element'>
3.21540498734 <class 'css_tree_select.element.ElementNoSlots'>
2.59615707397 dict
********** RUN 4 **********
1.75041103363 <class 'css_tree_select.element.Element'>
3.17366290092 <class 'css_tree_select.element.ElementNoSlots'>
2.70941114426 dict

测试类(ident,来自插槽的appart):

class Element(object):
    __slots__ = ['_typ', 'id', 'parent', 'childs']
    def __init__(self, typ, id, parent=None):
        self._typ = typ
        self.id = id
        self.childs = []
        if parent:
            self.parent = parent
            parent.childs.append(self)

class ElementNoSlots(object): (same, w/o slots)

测试代码,详细模式:

na, nb, nc = 100, 100, 100
for i in (1, 2, 3, 4):
    print '*' * 10, 'RUN', i, '*' * 10
    # tree with slot and no slot:
    for cls in Element, ElementNoSlots:
        t1 = time.time()
        root = cls('root', 'root')
        for i in xrange(na):
            ela = cls(typ='a', id=i, parent=root)
            for j in xrange(nb):
                elb = cls(typ='b', id=(i, j), parent=ela)
                for k in xrange(nc):
                    elc = cls(typ='c', id=(i, j, k), parent=elb)
        to =  time.time() - t1
        print to, cls
        del root

    # ref: tree with dicts only:
    t1 = time.time()
    droot = {'childs': []}
    for i in xrange(na):
        ela =  {'typ': 'a', id: i, 'childs': []}
        droot['childs'].append(ela)
        for j in xrange(nb):
            elb =  {'typ': 'b', id: (i, j), 'childs': []}
            ela['childs'].append(elb)
            for k in xrange(nc):
                elc =  {'typ': 'c', id: (i, j, k), 'childs': []}
                elb['childs'].append(elc)
    td = time.time() - t1
    print td, 'dict'
    del droot

The original question was about general use cases not only about memory. So it should be mentioned here that you also get better performance when instantiating large amounts of objects – interesting e.g. when parsing large documents into objects or from a database.

Here is a comparison of creating object trees with a million entries, using slots and without slots. As a reference also the performance when using plain dicts for the trees (Py2.7.10 on OSX):

********** RUN 1 **********
1.96036410332 <class 'css_tree_select.element.Element'>
3.02922606468 <class 'css_tree_select.element.ElementNoSlots'>
2.90828204155 dict
********** RUN 2 **********
1.77050495148 <class 'css_tree_select.element.Element'>
3.10655999184 <class 'css_tree_select.element.ElementNoSlots'>
2.84120798111 dict
********** RUN 3 **********
1.84069895744 <class 'css_tree_select.element.Element'>
3.21540498734 <class 'css_tree_select.element.ElementNoSlots'>
2.59615707397 dict
********** RUN 4 **********
1.75041103363 <class 'css_tree_select.element.Element'>
3.17366290092 <class 'css_tree_select.element.ElementNoSlots'>
2.70941114426 dict

Test classes (ident, appart from slots):

class Element(object):
    __slots__ = ['_typ', 'id', 'parent', 'childs']
    def __init__(self, typ, id, parent=None):
        self._typ = typ
        self.id = id
        self.childs = []
        if parent:
            self.parent = parent
            parent.childs.append(self)

class ElementNoSlots(object): (same, w/o slots)

testcode, verbose mode:

na, nb, nc = 100, 100, 100
for i in (1, 2, 3, 4):
    print '*' * 10, 'RUN', i, '*' * 10
    # tree with slot and no slot:
    for cls in Element, ElementNoSlots:
        t1 = time.time()
        root = cls('root', 'root')
        for i in xrange(na):
            ela = cls(typ='a', id=i, parent=root)
            for j in xrange(nb):
                elb = cls(typ='b', id=(i, j), parent=ela)
                for k in xrange(nc):
                    elc = cls(typ='c', id=(i, j, k), parent=elb)
        to =  time.time() - t1
        print to, cls
        del root

    # ref: tree with dicts only:
    t1 = time.time()
    droot = {'childs': []}
    for i in xrange(na):
        ela =  {'typ': 'a', id: i, 'childs': []}
        droot['childs'].append(ela)
        for j in xrange(nb):
            elb =  {'typ': 'b', id: (i, j), 'childs': []}
            ela['childs'].append(elb)
            for k in xrange(nc):
                elc =  {'typ': 'c', id: (i, j, k), 'childs': []}
                elb['childs'].append(elc)
    td = time.time() - t1
    print td, 'dict'
    del droot