在Python中__slots__的目的是什么——特别是当我想要使用它时,什么时候不使用它?
当前回答
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).
其他回答
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。
除了其他答案,__slots__还通过将属性限制在预定义的列表中增加了一点排版安全性。这一直是JavaScript的一个问题,它还允许您向现有对象添加新属性,无论您是否有意。
下面是一个普通的unslot对象,它什么都不做,但是允许你添加属性:
class Unslotted:
pass
test = Unslotted()
test.name = 'Fred'
test.Name = 'Wilma'
由于Python是区分大小写的,所以拼写相同但大小写不同的两个属性是不同的。如果你怀疑其中一个是打字错误,那就太倒霉了。
使用插槽,你可以限制它:
class Slotted:
__slots__ = ('name')
pass
test = Slotted()
test.name = 'Fred' # OK
test.Name = 'Wilma' # Error
这一次,第二个属性(Name)是不允许的,因为它不在__slots__集合中。
我建议在可能的情况下使用__slots__更好,以保持对对象的更多控制。
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
"""
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).
除了其他答案,这里还有一个使用__slots__的例子:
>>> 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__,它只需要额外的一行(并使您的类成为一个新样式的类,如果它还不是的话)。通过这种方式,您可以将这些类的内存占用减少5倍,代价是必须编写自定义pickle代码(如果需要的话)。
推荐文章
- 如何在交互式Python中查看整个命令历史?
- 如何显示有两个小数点后的浮点数?
- 如何用OpenCV2.0和Python2.6调整图像大小
- 在每个列表元素上调用int()函数?
- 当使用代码存储库时,如何引用资源的相对路径
- 如何在Flask-SQLAlchemy中按id删除记录
- 在Python中插入列表的第一个位置
- Python Pandas只合并某些列
- 如何在一行中连接两个集而不使用“|”
- 从字符串中移除前缀
- 代码结束时发出警报
- 如何在Python中按字母顺序排序字符串中的字母
- 在matplotlib中将y轴标签添加到次要y轴
- 如何消除数独方块的凹凸缺陷?
- 为什么出现这个UnboundLocalError(闭包)?