Python __slots__

Saving Space with the __slots__ Class Attribute

By default, Python stores instance attributes in a per-instance dict named __dict__.

为了使用底层的散列表提升访问速度，字典会消耗大量内存。如果处理上百万个属性不多的实例，通过__slots__类属性，能节省大量的内存，方法是让解释器在元组中存储实例属性，而不是用字典。

If you are dealing with millions of instances with few attributes, the __slots__ class attribute can save a lot of memory, by letting the interpreter store the instance attributes in a tuple instead of a dict.

To define __slots__ , you create a class with that name and assign it an iterable of str with identifiers for the instance attributes. I like to use a tuplefor that, because it conveys the message that the __slots__ definition cannot change.

By define __slots__ in the class, you are telling the interpreter : “These are all the instance attribute in this class.” Python then stores them in a tuple-like structure in each instance, avoiding the memory overhead of the per-instance __dict__. This can make a huge differnce in memory usage if you have millions of instances active at the same time.

在类中定义__slots__ 属性后，实例不能再有__slots__中所列名称之外的其他属性。 不要使用 __slots__ 属性禁止类的用户新增实例属性。__slots__ 是用于优化的，不是为了约束程序员。

The Problems with __slots__

• 每个子类都要定义 __slots__ 属性，因为解释器会忽略继承的 __slots__ 属性

• 实例只能拥有 __slots__ 中列出的属性，除非把 __dict__ 加入__slots__ 中，这样做就失去了节省内存的功效

• 如果不把 __weakref__ 加入 __slots__ ，实例就不能作为弱引用的目标。