无法在 Python3 中解决多重继承问题

Question

文档是这样说的：

Python supports a form of multiple inheritance as well. A class 
definition with multiple base classes looks like this:

class DerivedClassName(Base1, Base2, Base3):
    <statement-1>
    .
    .
    .
    <statement-N>
For most purposes, in the simplest cases, you can think of the search 
for attributes inherited from a parent class as depth-first, left-to-
right, not searching twice in the same class where there is an overlap 
in the hierarchy. Thus, if an attribute is not found in 
DerivedClassName, it is searched for in Base1, then (recursively) in 
the base classes of Base1, and if it was not found there, it was 
searched for in Base2, and so on.

所以，我有这段代码来测试它：

class Class1:
        c1_1 = 1.1

class Class2:
        c2_1 = 2.1


class Blob(Class1, Class2):

    def dump():
        print('c1_1 = ' + str(c1_1))

Blob.dump()

但是，我明白了：

Traceback (most recent call last):
  File "classinherit.py", line 13, in <module>
    Blob.dump()
  File "classinherit.py", line 11, in dump
    print('c_1.1 = ' + str(c1_1))
NameError: name 'c1_1' is not defined

文档似乎说 Python 将首先在 Blob 类的范围内寻找一个（在这种情况下是类范围的）变量，而不是找到它会搜索 Class1 和 Class2 类......但这显然不会发生。

什么给了？

Answer 1

如果要访问类变量，则必须以某种方式引用该类：

class Class1:
        c1_1 = 1.1

class Class2:
        c2_1 = 2.1

class Blob(Class1, Class2):

    @classmethod
    def dump(cls):
        print('c1_1 = ' + str(cls.c1_1))

Blob.dump()

---

为了清楚起见，您必须始终引用类，而不仅仅是在继承的情况下。

例如，以下将不工作：

class Klass():
    v1 = 1

    def dump():
        print('class var = ' + str(v1))

Klass.dump()

NameError：名称“v1”未定义

同样，您必须引用该类才能使其工作：

class Klass():
    v1 = 1

    @classmethod
    def dump(cls):
        print('class var = ' + str(cls.v1))

Klass.dump()

---

如果您不想使用@classmethod，您还可以执行以下操作：

class Klass():
    v1 = 1

    def dump():
        print('class var = ' + str(Klass.v1))

Klass.dump()

但请记住，更改类名称 Klass 也需要更改 Klass.v1。

Answer 2

你正在做的是你试图访问类变量和（或）属性实际上没有告诉它属于哪个类或没有引用该类。 您可以查看@Mike Scotty 的答案，或者只是正确调用您的类变量，然后您就可以清楚地看到 Python 中的 MRO（方法解析顺序）是如何工作的。

class A:
    a = 8

class B:
    b = 9

class C(A, B):
    c = 99

输出

d = C() #After instantiating
print(d.a, d.b, d.c)
>> 8 9 99
# Without instantiating
print(C.a, C.b, C.c)
>> 8 9 99