python:当类属性、实例属性和方法都具有相同的名称时会发生什么? [英] python: What happens when class attribute, instance attribute, and method all have the same name?

问题描述

python如何在名称相同的情况下区分类属性、实例属性和方法?

How does python differentiate a class attribute, instance attribute, and method when the names are the same?

class Exam(object):

    test = "class var"

    def __init__(self, n):
        self.test = n

    def test(self):
        print "method : ",self.test

test_o = Exam("Fine")

print dir(test_o)

print Exam.test
print test_o.test
test_o.test()

输出:

['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__',    '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'test']
<unbound method load.test>
Fine
Traceback (most recent call last):
  File "example.py", line 32, in <module>
    test_o.test()
TypeError: 'str' object is not callable

如何调用

1. class 属性，Exam.test --> 输出显示方法
2. 实例属性 test_o.test --> "Fine"
3. method test_o.test() --> TypeError: 'str' object is not callable

1. class attribute, Exam.test --> <unbound method load.test> output shows method
2. instance attribute test_o.test --> "Fine"
3. method test_o.test() --> TypeError: 'str' object is not callable

推荐答案

类属性可通过类访问:

YourClass.clsattribute

或者通过实例(如果实例没有覆盖类属性):

or through the instance (if the instance has not overwritten the class attribute):

instance.clsattribute

方法，如 ecatmur 在他的回答中所述，是描述符并被设置为类属性.

Methods, as stated by ecatmur in his answer, are descriptors and are set as class attributes.

如果您通过实例访问方法，则该实例将作为 self 参数传递给描述符.如果你想从类中调用一个方法，那么你必须显式地传递一个实例作为第一个参数.所以这些是等价的:

If you access a method through the instance, then the instance is passed as the self parameter to the descriptor. If you want to call a method from the class, then you must explicitly pass an instance as the first argument. So these are equivalent:

instance.method()
MyClass.method(instance)

对实例属性和方法使用相同的名称会使方法通过实例隐藏，但方法仍然可以通过类使用:

Using the same name for an instance attribute and a method will make the method hidden via the instance, but the method is still available via the class:

#python3
>>> class C:
...     def __init__(self):
...         self.a = 1
...     def a(self):
...         print('hello')
... 
>>> C.a
<function a at 0x7f2c46ce3c88>
>>> instance = C()
>>> instance.a
1
>>> C.a(instance)
hello

结论:不要给实例属性和方法同名.我通过给出有意义的名字来避免这种情况.方法就是动作，所以我通常用动词或句子来表示它们.属性是数据，所以我用名词/形容词来表示它们，这样可以避免对方法和属性使用相同的名称.

Conclusion: do not give the same name to instance attributes and methods. I avoid this by giving meaningful names. Methods are actions, so I usually use verbs or sentences for them. Attributes are data, so I use nouns/adjectives for them, and this avoids using the same names for both methods and attributes.

请注意，您根本不能拥有与方法同名的类属性，因为该方法会完全覆盖它(最后，方法只是可调用的类属性，并且会自动接收类的实例作为第一个属性).

Note that you simply cannot have a class attribute with the same name as a method, because the method would completely override it (in the end, methods are just class attributes that are callable and that automatically receive an instance of the class as first attribute).

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