相关疑难解决方法(0)

在Python in a Nutshell(第2版)一书中,有一个示例使用
旧样式类来演示如何以经典分辨率顺序解析方法,
以及它与新顺序的不同之处.

我通过重写新样式的示例尝试了相同的示例,但结果与使用旧样式类获得的结果没有什么不同.我用来运行该示例的python版本是2.5.2.以下是示例:

class Base1(object):  
    def amethod(self): print "Base1"  

class Base2(Base1):  
    pass

class Base3(object):  
    def amethod(self): print "Base3"

class Derived(Base2,Base3):  
    pass

instance = Derived()  
instance.amethod()  
print Derived.__mro__  

调用instance.amethod()打印Base1,但根据我对MRO的理解,新类型的输出应该是Base3.通话Derived.__mro__打印:

(<class '__main__.Derived'>, <class '__main__.Base2'>, <class '__main__.Base1'>, <class '__main__.Base3'>, <type 'object'>)

我不确定我对新样式类的MRO的理解是不正确的还是我做了一个我无法察觉的愚蠢错误.请帮助我更好地了解MRO.

为什么是这样:

class MyType(type):
    def __init__(cls, name, bases, attrs):
        print 'created', cls
class MyMixin:
    __metaclass__ = MyType
class MyList(list, MyMixin): pass

好的,按预期工作:

created <class '__main__.MyMixin'>
created <class '__main__.MyList'>

但是这个:

class MyType(type):
    def __init__(cls, name, bases, attrs):
        print 'created', cls
class MyMixin:
    __metaclass__ = MyType
class MyObject(object, MyMixin): pass

不行,并且如此爆炸？:

created <class '__main__.MyMixin'>
Traceback (most recent call last):
  File "/tmp/junk.py", line 11, in <module>
    class MyObject(object, MyMixin): pass
TypeError: Error when calling the metaclass bases
    Cannot create a consistent method resolution
order …