从Python 3.9开始，字典可用于通过__slots__向属性添加描述。没有描述的属性可以使用None，即使给出了描述，私有变量也不会出现。

class Person:

    __slots__ = {
        "birthday":
            "A datetime.date object representing the person's birthday.",
        "name":
            "The first and last name.",
        "public_variable":
            None,
        "_private_variable":
            "Description",
    }


help(Person)
"""
Help on class Person in module __main__:

class Person(builtins.object)
 |  Data descriptors defined here:
 |
 |  birthday
 |      A datetime.date object representing the person's birthday.
 |
 |  name
 |      The first and last name.
 |
 |  public_variable
"""

__slot__属性的一个非常简单的例子。

问题:没有__slots__

如果我的类中没有__slot__属性，我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}

如果你看上面的例子，你可以看到obj1和obj2有它们自己的x和y属性，python还为每个对象(obj1和obj2)创建了一个dict属性。

假设我的类Test有数千个这样的对象?在我的代码中，为每个对象创建一个额外的属性字典将导致大量的开销(内存，计算能力等)。

解决方案:使用__slots__

现在在下面的例子中，我的类Test包含__slots__属性。现在我不能添加新的属性到我的对象(属性x除外)和python不再创建dict属性。这消除了每个对象的开销，如果您有许多对象，这将变得非常重要。

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'

类实例的属性有3个属性:实例、属性名和属性值。

在常规属性访问中，实例充当字典，属性名充当字典查找值中的键。

实例(属性)——>值

在__slots__访问中，属性的名称充当字典，实例充当字典查找值中的键。

属性(实例)——>值

在flyweight模式中，属性的名称充当字典，值充当查找实例的字典中的键。

属性(value)——>实例

如果你要实例化很多(成百上千)同一个类的对象，你会想要使用__slots__。__slots__仅作为内存优化工具存在。

强烈建议使用__slots__来约束属性创建。

使用__slots__ pickle对象将无法使用默认的(最古老的)pickle协议;有必要指定一个更高的版本。

python的其他一些自省特性也可能受到不利影响。

引用雅各布·海伦的话:

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).

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.

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。如果你需要在代理对象上添加一个属性，可以使用__slots__。

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