Python __slots__ 的用法笔记 | 隔叶黄莺 Yanbin Blog

2023-02-28 | 阅读(392)

Python 是一个动态语言，可以动态的给实例或类增减属性或方法，给类添加的属性会影响到前后所有创建的实例。但是使用 __slots__ 属性可以限定类或实例属性和方法，如果没有 __slots__ 的话实例的属性和方法包含在实例的 __dict__ 字典中，类的属性和方法包含在类的 __dict__ 字典中。

在使用 __slots__ 按常规写法可能会出现的问题大概有

AttributeError: 'Xxx' object has no attribute 'yyy'
AttributeError: 'Xxx' object attribute 'yyy' is read-only
ValueError: 'yyy' in __slots__ conflicts with class variable

我们来看下面的例子

>>> class Cat:
...     lags = 4
...     def __init__(self):
...         self.eyes = 2
...
...     def walk(self):
...         pass
...
...
>>> Cat.__dict__
mappingproxy({'__module__': '__main__', 'lags': 4, '__init__': <function Cat.__init__ at 0x106816700>, 'walk': <function Cat.walk at 0x106817920>, '__dict__': <attribute
'__dict__' of 'Cat' objects>, '__weakref__': <attribute '__weakref__' of 'Cat' objects>, '__doc__': None})
>>> c1 = Cat()
>>> c1.__dict__
{'eyes': 2}
>>> c1.miaow = lambda: 'hello'
>>> c1.__dict__
{'eyes': 2, 'miaow': <function <lambda> at 0x106d6fce0>}
>>> Cat.ears = 2
>>> c1.ears
2
>>> c1.__dict__
{'eyes': 2, 'miaow': <function <lambda> at 0x106d6fce0>}
>>> Cat.__dict__
mappingproxy({'__module__': '__main__', 'lags': 4, '__init__': <function Cat.__init__ at 0x106816700>, 'walk': <function Cat.walk at 0x106817920>, '__dict__': <attribute
'__dict__' of 'Cat' objects>, '__weakref__': <attribute '__weakref__' of 'Cat' objects>, '__doc__': None, '__getattribute__': <slot wrapper '__getattribute__' of 'object'
 objects>, 'ears': 2})

>>> class Cat:

... lags = 4

... def __init__(self):

... self.eyes = 2

...

... def walk(self):

... pass

...

>>> Cat.__dict__

mappingproxy({'__module__': '__main__', 'lags': 4, '__init__': <function Cat.__init__ at 0x106816700>, 'walk': <function Cat.walk at 0x106817920>, '__dict__': <attribute

'__dict__' of 'Cat' objects>, '__weakref__': <attribute '__weakref__' of 'Cat' objects>, '__doc__': None})

>>> c1 = Cat()

>>> c1.__dict__

{'eyes': 2}

>>> c1.miaow = lambda: 'hello'

>>> c1.__dict__

{'eyes': 2, 'miaow': <function <lambda> at 0x106d6fce0>}

>>> Cat.ears = 2

>>> c1.ears

>>> c1.__dict__

{'eyes': 2, 'miaow': <function <lambda> at 0x106d6fce0>}

>>> Cat.__dict__

mappingproxy({'__module__': '__main__', 'lags': 4, '__init__': <function Cat.__init__ at 0x106816700>, 'walk': <function Cat.walk at 0x106817920>, '__dict__': <attribute

'__dict__' of 'Cat' objects>, '__weakref__': <attribute '__weakref__' of 'Cat' objects>, '__doc__': None, '__getattribute__': <slot wrapper '__getattribute__' of 'object'

objects>, 'ears': 2})

类或实例可以随意的添加属性和方法

如果我们引入 __slots__ 来限定属性或方法

>>> class Cat:
...     __slots__ = ('lags', 'eyes')
...     def __init__(self):
...         self.eyes = 2
...
...
...
>>> Cat.__dict__
mappingproxy({'__module__': '__main__', '__slots__': ('lags', 'eyes'), '__init__': <function Cat.__init__ at 0x10699ef20>, 'eyes': <member 'eyes' of 'Cat' objects>, 'lags
': <member 'lags' of 'Cat' objects>, '__doc__': None})
>>> c1 = Cat()
>>> c1.__dict__
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    c1.__dict__
AttributeError: 'Cat' object has no attribute '__dict__'
>>> c1.lags
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    c1.lags
AttributeError: 'Cat' object has no attribute 'lags'
>>> c1.eyes
2
>>> c1.lags = 4
>>> c1.ears = 2
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    c1.ears = 2
     ^^^^^^^
AttributeError: 'Cat' object has no attribute 'ears'
>>> c1.miaow = lambda: 'hello'
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    c1.miaow = lambda: 'hello'
     ^^^^^^^^
AttributeError: 'Cat' object has no attribute 'miaow'
>>> c1.lags = lambda: 'hello'
>>> c1.lags
<function <lambda> at 0x106d6c7c0>

>>> class Cat:

... __slots__ = ('lags', 'eyes')

... def __init__(self):

... self.eyes = 2

...

>>> Cat.__dict__

mappingproxy({'__module__': '__main__', '__slots__': ('lags', 'eyes'), '__init__': <function Cat.__init__ at 0x10699ef20>, 'eyes': <member 'eyes' of 'Cat' objects>, 'lags

': <member 'lags' of 'Cat' objects>, '__doc__': None})

>>> c1 = Cat()

>>> c1.__dict__

Traceback (most recent call last):

File "<input>", line 1, in <module>

c1.__dict__

AttributeError: 'Cat' object has no attribute '__dict__'

>>> c1.lags

Traceback (most recent call last):

File "<input>", line 1, in <module>

c1.lags

AttributeError: 'Cat' object has no attribute 'lags'

>>> c1.eyes

>>> c1.lags = 4

>>> c1.ears = 2

Traceback (most recent call last):

File "<input>", line 1, in <module>

c1.ears = 2

^^^^^^^

AttributeError: 'Cat' object has no attribute 'ears'

>>> c1.miaow = lambda: 'hello'

Traceback (most recent call last):

File "<input>", line 1, in <module>

c1.miaow = lambda: 'hello'

^^^^^^^^

AttributeError: 'Cat' object has no attribute 'miaow'

>>> c1.lags = lambda: 'hello'

>>> c1.lags

引入了 __slots__ 后实例不再有 __dict__ 属性，只能添加在 __slots__ 中列出的属性或方法。添加没在 __slots__ 中的属性或方法时会报错误

AttributeError: 'Xxx' object has no attribute 'yyy'

在初始化函数中 __init__(self) 中也是一样的，如

>>> class Cat:
...     __slots__ = ('lags', 'eyes')
...     def __init__(self):
...         self.ears = 2
...
...
...
>>> c1 = Cat()
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    c1 = Cat()
          ^^^^^
  File "<input>", line 4, in __init__
    self.ears = 2
    ^^^^^^^^^
AttributeError: 'Cat' object has no attribute 'ears'

>>> class Cat:

... __slots__ = ('lags', 'eyes')

... def __init__(self):

... self.ears = 2

...

>>> c1 = Cat()

Traceback (most recent call last):

File "<input>", line 1, in <module>

c1 = Cat()

^^^^^

File "<input>", line 4, in __init__

self.ears = 2

^^^^^^^^^

AttributeError: 'Cat' object has no attribute 'ears'

__slots__ 中没有 ears, 所以不能在初始化方法或外部动态添加该属性

但是定义类时声明的方法不在 __slots__ 约束内

>>> class Cat:
...     __slots__ = ('lags', 'eyes')
...     def walk(self):
...         pass
...
...
>>> c1 = Cat()
>>> c1.walk()

>>> class Cat:

... __slots__ = ('lags', 'eyes')

... def walk(self):

... pass

...

>>> c1 = Cat()

>>> c1.walk()

__slots__ 也不约束通过类动态添加属性或方法

>>> class Cat:
...     __slots__ = ('lags', 'eyes')
...
...
>>> Cat.ears = 2
>>>

>>> class Cat:

... __slots__ = ('lags', 'eyes')

...

>>> Cat.ears = 2

>>>

在 __slots__ 中不能包含类变量，比如

>>> class Cat:
...     __slots__ = ('lags')
...     lags = 4
...
...
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    class Cat:
ValueError: 'lags' in __slots__ conflicts with class variable

>>> class Cat:

... __slots__ = ('lags')

... lags = 4

...

Traceback (most recent call last):

File "<input>", line 1, in <module>

class Cat:

ValueError: 'lags' in __slots__ conflicts with class variable

声明的没定义在 __slots__ 中的类变量对实例方法是只读的

>>> class Cat:
...     __slots__ = ('lags')
...     eyes = 2
...     def __init__(self):
...         self.eyes = 3
...
...
...
>>> c1 = Cat()
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    c1 = Cat()
          ^^^^^
  File "<input>", line 5, in __init__
    self.eyes = 3
    ^^^^^^^^^
AttributeError: 'Cat' object attribute 'eyes' is read-only

>>> class Cat:

... __slots__ = ('lags')

... eyes = 2

... def __init__(self):

... self.eyes = 3

...

>>> c1 = Cat()

Traceback (most recent call last):

File "<input>", line 1, in <module>

c1 = Cat()

^^^^^

File "<input>", line 5, in __init__

self.eyes = 3

^^^^^^^^^

AttributeError: 'Cat' object attribute 'eyes' is read-only

但是声明在没定义在 __slots__ 中的类变量通过实例来修改也不行，但可以通过类属性来修改

>>> class Cat:
...     __slots__ = ('lags')
...     eyes = 2
...
...
>>> c1 = Cat()
>>> c1.eyes = 3
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    c1.eyes = 3
     ^^^^^^^
AttributeError: 'Cat' object attribute 'eyes' is read-only
>>> Cat.eyes = 3
>>> c1.eyes
3

>>> class Cat:

... __slots__ = ('lags')

... eyes = 2

...

>>> c1 = Cat()

>>> c1.eyes = 3

Traceback (most recent call last):

File "<input>", line 1, in <module>

c1.eyes = 3

^^^^^^^

AttributeError: 'Cat' object attribute 'eyes' is read-only

>>> Cat.eyes = 3

>>> c1.eyes

定义在 __slots__ 中的属性或方法在 IDE 中会有智能提示。

__slots__ 只能作用在当前类中，不会影响到子类，子类需定义自己的 __slots__.

>>> class Animal:
...     __slots__ = ('lags')
...
...
>>> class Cat(Animal):
...     pass
...
>>> Animal.__slots__
'lags'
>>> Cat.__slots__
'lags'
>>> c = Cat()
>>> c.eyes = 2

>>> class Animal:

... __slots__ = ('lags')

...

>>> class Cat(Animal):

... pass

...

>>> Animal.__slots__

'lags'

>>> Cat.__slots__

'lags'

>>> c = Cat()

>>> c.eyes = 2

__slots__ 对于实例是只读的，通过类的 __slots__ 属性可修改，但不改变原有的约束

>>> class Cat:
...     __slots__ = ('lags', 'eyes')
...
...
>>> c.__slots__ = ('lags', 'eyes', 'ears')
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    c.__slots__ = ('lags', 'eyes', 'ears')
     ^^^^^^^^^^^
AttributeError: 'Cat' object attribute '__slots__' is read-only
>>> Cat.__slots__ = ('lags', 'eyes', 'ears')
>>> c = Cat()
>>> c.ears = 2
Traceback (most recent call last):
  File "<input>", line 1, in <module>
    c.ears = 2
     ^^^^^^
AttributeError: 'Cat' object has no attribute 'ears'
>>> c.__slots__
('lags', 'eyes', 'ears')
>>> Cat.__slots__
('lags', 'eyes', 'ears')

>>> class Cat:

... __slots__ = ('lags', 'eyes')

...

>>> c.__slots__ = ('lags', 'eyes', 'ears')

Traceback (most recent call last):

File "<input>", line 1, in <module>

c.__slots__ = ('lags', 'eyes', 'ears')

^^^^^^^^^^^

AttributeError: 'Cat' object attribute '__slots__' is read-only

>>> Cat.__slots__ = ('lags', 'eyes', 'ears')

>>> c = Cat()

>>> c.ears = 2

Traceback (most recent call last):

File "<input>", line 1, in <module>

c.ears = 2

^^^^^^

AttributeError: 'Cat' object has no attribute 'ears'

>>> c.__slots__

('lags', 'eyes', 'ears')

>>> Cat.__slots__

('lags', 'eyes', 'ears')

__slots__ 还可声明为 list, 或省略圆括号的 tuple 形式

>>> class Cat:
...     __slots__ = ['lags', 'eyes']
...
...
>>> class Cat:
...     __slots__ = 'lags', 'eyes'
...
...
>>> type(Cat.__slots__)
<class 'tuple'>