Python Mixin and MRO

什么是 mixin ?

In object-oriented programming languages, a mixin is a class that provides a certain functionality to be inherited or just reused by a subclass, while not meant for instantiation (the generation of objects of that class). Mixins are synonymous with abstract base classes. Inheriting from a mixin is not a form of specialization but is rather a means of collecting functionality. A class or object may “inherit” most or all of its functionality from one or more mixins, therefore mixins can be thought of as a mechanism of multiple inheritance.

简单的说, mixin 是一种类的多继承的机制.

什么时候需要 mixin ?

就如stackoveflow 上的回答, 有两个主要的使用 mixin 的场景:

  • 你希望给一个类提供很多可选的特征(feature).
  • 你希望在很多不同的类中使用一个特定的特征(feature).

例如, werkzeug 的request, response系统. 我们可以实现简单request如下:

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from werkzeug import BaseRequest

class Request(BaseRequest):
    pass

如果我们希望支持accept header, 可以这样实现:

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from werkzeug import BaseRequest, AcceptMixin

class Request(BaseRequest, AcceptMixin):
    pass

如果我们希望request对象支持accept header, etags, authentication, user agent, 可以这样实现:

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from werkzeug import BaseRequest, AcceptMixin, ETagRequestMixin, UserAgentMixin, AuthorizationMixin

class Request(BaseRequest, AcceptMixin, ETagRequestMixin, UserAgentMixin, AuthorizationMixin):
    pass

Mixin 的实现

假定我们有下面两个类需要mixin:

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class Base(object):
    pass

class FeatureMixin(object):
    pass
  • 直接定义一个类 mixin 所有的 feature
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class MyClass(Base, FeatureMixin):
    pass
  • 通过闭包动态定义类来实现 Mixin
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def mixin(base, mixin, name):
    class MixinClass(base, mixin):
        pass
    MixinClass.__name__ = name
    return MixinClass

MyClass = mixin(Base, FeatureMixin, 'MyClass')
  • 使用type动态构造类来实现 Mixin
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MyClass = type('MyClass', (Base, FeatureMixin), {})
  • 更改__bases__来实现 Mixin. 只能 mixin classic class
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class Base(object):
    pass

class FeatureMixin: # not inherite from object
    pass

Base.__bases__ += (FeatureMixin,) # then Base should have FeatureMixin

MRO(Method Resolution Order)

In object-oriented programming languages with multiple inheritance, the diamond problem (sometimes referred to as the “deadly diamond of death”) is an ambiguity that arises when two classes B and C inherit from A, and class D inherits from both B and C. If D calls a method defined in A (and does not override the method), and B and C have overridden that method differently, then from which class does it inherit: B, or C?

多继承时, 运行环境必须知道调用哪一个父类的方法. Python2.3开始采用C3方法解析顺序(Method Resolution Order, 简称MRO)来动态解析调用的方法.

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>>> O = object
>>> class A(O): pass
>>> class B(O): pass
>>> class X(A,B): pass
>>> class Y(B,A): pass
>>> X.mro()
[<class '__main__.X'>, <class '__main__.A'>, <class '__main__.B'>, <type 'object'>]
>>> Y.mro()
[<class '__main__.Y'>, <class '__main__.B'>, <class '__main__.A'>, <type 'object'>]

参照上面代码执行的结果, 我们可以看出, class X的 MRO 是 <class '__main__.X'>, <class '__main__.A'>, <class '__main__.B'>, <type 'object'>. 当我们调用X实例对象的某个方法, 运行环境会按照X, A, B, object顺序解析该方法. 越左边的越优先, 也就是说如果只有A, B定义了这个方法, 系统会选择最先解析到的方法, 也就是A的方法定义. 同样, class Y的 MRO 是 <class '__main__.Y'>, <class '__main__.B'>, <class '__main__.A'>, <type 'object'>, 对于Y来说, B的方法要优先于A进行解析.

上面定义的类X和Y对A和B的方法解析顺序是刚好相反的, 如果有一个类继承于X和Y会出现什么结果?

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>>> class M(X, Y): pass
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Error when calling the metaclass bases
    Cannot create a consistent method resolution
order (MRO) for bases B, A

上面的错误说明, 类的继承必须保持一致的 MRO. 由于基类X和Y的 MRO 有冲突, 因此我们不可能从X和Y继承另外一个类.

关于 MRO 的算法可以参考官网上的Python 2.3 方法解析顺序.

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