TypeError: Cannot create a consistent method resolution order (MRO) for bases P1, P2

Posted 2020-10-12

新式类多重继承原则：广度优先，从左到右
==================正确==================
In [19]: class P1(object):
    ...:     def foo(self):
    ...:         print(‘called P1-foo()‘)
    ...: class P2(object):
    ...:     def foo(self):
    ...:         print(‘called P2-foo()‘)
    ...:     def bar(self):
    ...:         print(‘called P2-bar()‘)
    ...: class C1(P1,P2):   ###P1和P2顺序一致
    ...:     pass
    ...: class C2(P1,P2):   ###P1和P2顺序一致
    ...:     def bar(self):
    ...:         print(‘called C2-bar()‘)
    ...: class GC(C2,C1):
    ...:     pass
In [20]: gc.foo()
called P1-foo() ### GC--->C2--->C1--->P2
In [21]: gc.bar()
called C2-bar()  ### GC--->C2--->C1
==================正确==================
In [31]: class P1(object):
    ...:     def foo(self):
    ...:         print(‘called P1-foo()‘)
    ...: class P2(object):
    ...:     def foo(self):
    ...:         print(‘called P2-foo()‘)
    ...:     def bar(self):
    ...:         print(‘called P2-bar()‘)
    ...: class C1(P2,P1):  ###P2和P1顺序一致
    ...:     pass
    ...: class C2(P2,P1):  ###P2和P1顺序一致
    ...:     def bar(self):
    ...:         print(‘called C2-bar()‘)
    ...: class GC(C2,C1):
    ...:     pass
In [33]: gc.foo()
called P2-foo() ### GC--->C2--->C1--->P2
In [34]: gc.bar()
called C2-bar()   ### GC--->C2--->C1
==================正确==================
In [29]: class P1(object):
    ...:     def foo(self):
    ...:         print(‘called P1-foo()‘)
    ...: class P2(object):
    ...:     def foo(self):
    ...:         print(‘called P2-foo()‘)
    ...:     def bar(self):
    ...:         print(‘called P2-bar()‘)
    ...: class C1(P2,P1):
    ...:     pass
    ...: class C2(P1,P2):
    ...:     def bar(self):
    ...:         print(‘called C2-bar()‘)
    ...: 
==================错误==================
In [26]: class P1(object):
    ...:     def foo(self):
    ...:         print(‘called P1-foo()‘)
    ...: class P2(object):
    ...:     def foo(self):
    ...:         print(‘called P2-foo()‘)
    ...:     def bar(self):
    ...:         print(‘called P2-bar()‘)
    ...: class C1(P2,P1):   ###P2和P1
    ...:     pass
    ...: class C2(P1,P2):   ###P1和P2
    ...:     def bar(self):
    ...:         print(‘called C2-bar()‘)
    ...: class GC(C2,C1):
    ...:     pass
    ...: 
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-26-7eeba137b395> in <module>()
     12     def bar(self):
     13         print(‘called C2-bar()‘)
---> 14 class GC(C2,C1):
     15     pass
TypeError: Cannot create a consistent method resolution
order (MRO) for bases P2, P1

原因分析：
子类GC在继承C2和C1时，无法分辨C1和C2中P1和P2的继承关系，一会(P1,P2)一会又是(P2,P1)，导致GC继承MRO出错。
因为继承顺序发生了改变，所以gc.foo()和gc.bar()调用的结果也发生了改变。

自己试出来的不知道对不对

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