A brief explanation of python inheritance and multiple inheritance (code example)

The content of this article is a simple explanation (code example) of Python inheritance and multiple inheritance. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

Remember the following points:

Directly subclassing built-in types (such as dict, list or str) is prone to errors, because methods of built-in types usually ignore user-overridden methods, do not subclass Classifying built-in types, user-defined classes should inherit the collections module.

    def __setitem__(self, key, value):
        super().__setitem__(key, [value] * 2) # 错误案例
class AnswerDict(dict):
    def __getitem__(self, item): # 错误案例
        return 42
import collections
class DoppelDict2(collections.UserDict): # 正确案例
    def __setitem__(self, key, value):
        super().__setitem__(key, [value] * 2)
class AnswerDict2(collections.UserDict): # 正确案例
    def __getitem__(self, item):
        return 42

Another problem related to multiple inheritance is: if a superclass of the same level defines an attribute with the same name. How does Python determine which one to use?

class DoppelDict(dict):
    def __setitem__(self, key, value):
        super().__setitem__(key, [value] * 2)
class AnswerDict(dict):
    def __getitem__(self, item):
        return 42
import collections
class DoppelDict2(collections.UserDict):
    def __setitem__(self, key, value):
        super().__setitem__(key, [value] * 2)
class AnswerDict2(collections.UserDict):
    def __getitem__(self, item):
        return 42
class A:
    def ping(self):
        print('Ping:', self)
class B(A):
    def pong(self):
        print('pong:', self)
class C(A):
    def pong(self):
        print('PONG:', self)
class D(B, C):
    def ping(self):
        super().ping()
        print('post-ping:', self)
    def pingpong(self):
        self.ping()
        super().ping()
        self.pong()
        super().pong()
        C.pong(self)
if __name__ == '__main__':
    d = D()
    print(d.pong()) # 输出来源于B
    print(C.pong(d)) #输出来源于C 超类的方法都可以直接调用,此时要把实例作为显示参数传入.

Python can distinguish which method is called, by Method parsing order
>>> D.mro()
[f90f74cf2b5eef2157d27f4b07137685, d3d10dde313d07caf2874a0faa31cc27, 3c8ab9bdf9f4838c4d1910062f257eb2, d65e71c32c2035ca4f3e3c46100a7a5f, 0b6b3070adce8118f880cf43f8bc8037]
If you want to delegate method calls to the super class, the recommended way is to use the built-in super( ) function.

The following is the interpretation of the d.pingpong() method

>>> self.ping()

Ping: 51c9255e984783507746d24bd1018f2c post-ping: 51c9255e984783507746d24bd1018f2c The first call is self.ping(), which runs the ping method of class D.

The second call The one is super().ping(), skip the ping method of class D and find the ping method of class A. Ping: 51c9255e984783507746d24bd1018f2c

The third call is self .pong() method, based on __mro__, finds the pong method implemented by class B. pong: 51c9255e984783507746d24bd1018f2c

The fourth call is super().pong(), which is also based on __mro__, find the pong method implemented by class B. pong: 51c9255e984783507746d24bd1018f2c

The fifth call is C.pong(self), __mro__ is ignored, and C is found. The pong method implemented by the class. PONG: 51c9255e984783507746d24bd1018f2c

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