python属性查找过程如何工作?

2022-01-13 00:00:00 python attributes python-datamodel

问题描述

当我说python 属性查找过程"时，我的意思是:当你编写 x.foo 时，python 做了什么??

When i say "python attribute lookup proccess" i mean: what python does when you write x.foo??

在网上搜索我没有找到太多关于此的文档，我发现的最好的论文之一将过程恢复到以下步骤(你可以看到完整的文章 这里)

Searching the web i didn't found to much docs about this, one of the best papers i found resumed the proccess to the following steps (you can see the full article here)

  1. 如果 attrname 是 objectname 的特殊(即 Python 提供的)属性，则返回它.
  2. 检查 objectname.__class__.__dict__ 的 attrname.如果存在并且是数据描述符，则返回描述符结果.搜索 objectname.__class__ 的所有基以查找相同的案例.
  3. 检查 objectname.__dict__ 的 attrname，如果找到则返回.如果 objectname 是一个类，也搜索它的基类.如果它是一个类并且描述符存在于它或其基类中，则返回描述符结果.
  4. 检查 objectname.__class__.__dict__ 的 attrname.如果存在且是非数据描述符，则返回描述符结果.如果它存在并且不是描述符，则返回它.如果它存在并且是一个数据描述符，我们就不应该在这里，因为我们会在第 2 点返回.搜索 objectname.__class__ 的所有基以查找相同的情况.
  5. 引发 AttributeError.

起初这似乎是正确的，但属性查找过程有点复杂，例如对于 x.foo，如果 x 是类或实例，它的行为就不一样了.

At first this might seem right, but the attribute lookup process is a little bit more complicated, for example for x.foo, it doesn't behave the same if x is a class or an instance.

我发现了一些无法通过这种方式解释的示例.考虑以下 python 代码:

I have a found some samples that can't be explained by this way. Consider the following python code:

class Meta(type):
    def __getattribute__(self, name):
        print("Metaclass getattribute invoked:", self)
        return type.__getattribute__(self, name)

    def __getattr__(self, item):
        print('Metaclass getattr invoked: ', item)
        return None

class C(object, metaclass=Meta):
    def __getattribute__(self, name):
        print("Class getattribute invoked:", args)
        return object.__getattribute__(self, name)

c=C()

现在检查以下行与相应的输出:

Now check the following lines with the corresponding output:

>> C.__new__
Metaclass getattribute invoked: <class '__main__.C'>
<built-in method __new__ of type object at 0x1E1B80B0>

>> C.__getattribute__
Metaclass getattribute invoked: <class '__main__.C'>
<function __getattribute__ at 0x01457F18>

>> C.xyz
Metaclass getattribute invoked: <class '__main__.C'>
Metaclass getattr invoked:  xyz
None

>> c.__new__
Class getattribute invoked: (<__main__.C object at 0x013E7550>, '__new__')
<built-in method __new__ of type object at 0x1E1B80B0>

>> c.__getattribute__
Class getattribute invoked: (<__main__.C object at 0x01438DB0>, '__getattribute__')
Metaclass getattribute invoked: <class '__main__.C'>
<bound method C.__getattribute__ of <__main__.C object at 0x01438DB0>>

>>

我的结论是(考虑到我们正在搜索 x.foo):

The conclusions i have been are (considering we're searching for x.foo):

  • __getattribute__ 对于 < 的实例是不同的.键入类型"> 和 <键入对象">.对于 C.foo()，首先在 C.__dict__ 上搜索 'foo'，如果找到则返回(而不是搜索 type(C))，对于 x.foo()，在 type(x).__dict__ 上搜索 'foo' 和在 x.__dict__ 上.
  • __getattribute__ 方法总是在 type(x) 上解析，我不明白的是最后一种情况:c.__getattribute__, is not object contains a method __getattribute__ (and C继承 from object)，那么为什么元类getattribute 方法被调用.

有人可以解释一下吗?或者告诉我在哪里可以找到有关此的文档，谢谢.

Can someone explain this please?? or at less tell me where can i find some documentation about this, thanks.

解决方案

如果你添加了 print("Metaclass getattribute invoked:", self, name) 你会看到:

If you added print("Metaclass getattribute invoked:", self, name) you'd see:

>>> c.__getattribute__
Class getattribute invoked: <__main__.C object at 0x2acdbb1430d0> __getattribute__
Metaclass getattribute invoked: <class '__main__.C'> __name__
<bound method C.__getattribute__ of <__main__.C object at 0x2acdbb1430d0>>

元类 __getattribute__ 被调用以构建表达式 c.__getattribute__ 的 repr，以便它可以打印 C 的 __name__.

The metaclass __getattribute__ is getting invoked in order to build the repr of the expression c.__getattribute__, so that it can print C's __name__.

btw，__getattribute__ 对类和元类的作用相同；首先在实例上查找属性，然后在实例的类型上查找.

btw, __getattribute__ works the same for classes and metaclasses; the attribute is looked up first on the instance then on the instance's type.

>>> Meta.foo = 1
>>> C.foo
('Metaclass getattribute invoked:', <class '__main__.C'>, 'foo')
1
>>> c.foo
('Class getattribute invoked:', <__main__.C object at 0x2acdbb1430d0>, 'foo')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in __getattribute__
AttributeError: 'C' object has no attribute 'foo'
>>> C.bar = 2
>>> c.bar
('Class getattribute invoked:', <__main__.C object at 0x2acdbb1430d0>, 'bar')
2

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