Python 3:对象如何成为类型的实例? [英] Python 3: How can object be instance of type?

问题描述

在 Python 3 中，object 是 type 的实例，type 也是 object 的实例！

In Python 3, object is an instance of type and type is also an instance of object!

每个类怎么可能派生自另一个?

How is it possible that each class is derived from the other?

任何实施细节?

我使用 isinstance(sub, base) 检查了这个，根据 Python 文档，它检查子类是否从基类派生:

I checked this using isinstance(sub, base), which, according to Python documentation, checks if sub class is derived from base class:

isinstance(object, type)
Out[1]: True

isinstance(type, object)
Out[2]: True

推荐答案

这是 Python 中的边缘情况之一:

This is one of the edge cases in Python:

• Python 中的一切都是对象，所以由于 object 是一切的基本类型，type(Python 中的东西)是 object.
• 由于object 是一切的基础type，object 也是type，这使得object type 的一个实例.

• Everything in Python is an object, so since object is the base type of everything, type (being something in Python) is an instance of object.
• Since object is the base type of everything, object is also a type, which makes object an instance of type.

请注意，这种关系是无法用 Python 中自己的事物复制的.这是语言中内置的一个例外.

Note that this relationship is nothing you can replicate with your own things in Python. It’s a single exception that is built into the language.

在实现方面，这两个名称分别用PyBaseObject_Type(对于object)和PyType_Type(对于type)表示代码>).

On the implementation side, the two names are represented by PyBaseObject_Type (for object) and PyType_Type (for type).

当您使用 isinstance 时，类型检查——在最后一步，在其他一切都失败后——由 type_is_subtype_base_chain:

When you use isinstance, the type check—in the very last step, after everything else has failed—is done by type_is_subtype_base_chain:

type_is_subtype_base_chain(PyTypeObject *a, PyTypeObject *b)
{
    do {
        if (a == b)
            return 1;
        a = a->tp_base;
    } while (a != NULL);

    return (b == &PyBaseObject_Type);
}

这本质上是在 a 的类型层次结构上继续向上，并根据 b 检查结果类型.如果它找不到，那么最后的方法是检查 b 是否实际上是 object 在这种情况下函数返回 true:因为一切都是对象.所以一切都是object的实例"部分实际上被硬编码到实例检查中.

This essentially keeps going up the type hierarchy of a and checks the resulting type against b. If it cannot find one, then the last resort is to check whether b is actually object in which case the function returns true: since everything is an object. So the "everything is an instance of object" part is actually hardcoded into the instance check.

至于为什么 object 是 type，这实际上更简单，因为它在 PyBaseObject_Type 的声明:

And as for why object is a type, this is actually even simpler because it’s simply defined that way in the declaration of PyBaseObject_Type:

PyTypeObject PyBaseObject_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "object",                                   /* tp_name */
    sizeof(PyObject),                           /* tp_basicsize */
    …

PyVarObject_HEAD_INIT 主要设置核心类型信息，包括基本类型，即PyType_Type.

The PyVarObject_HEAD_INIT essentially sets the core type information stuff, including the base type, which is PyType_Type.

这种关系实际上还有两个后果:

There are actually two more consequences of this relationship:

• 既然一切都是对象，object也是object的一个实例:isinstance(object, object)
• 由于PyType_Type 也是用相同的PyVarObject_HEAD_INIT 实现的，所以type 也是一种类型:isinstance(type, type).

• Since everything is an object, object is also an instance of object: isinstance(object, object)
• Since PyType_Type is also implemented with the same PyVarObject_HEAD_INIT, type is also a type: isinstance(type, type).

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