[python] 定义抽象基类(Abstract Base Classes)

抽象基类一般用于规定子类必须重新定义某些方法。比如 web 框架中的 cache 部分的基类一般类似下面这样:

class BaseCache(object):
    def get(self, key):
        raise NotImplementedError('subclasses of BaseCache must provide a get() method')

    def set(self, key, value, timeout=60):
        raise NotImplementedError('subclasses of BaseCache must provide a set() method')


class MemcachedCache(BaseCache):
    def get(self, key):
        value = self._cache.get(key)
        return value

    def set(self, key, value, timeout=60):
        self._cache.set(key, value, timeout)

在插件、cache、session 等支持功能扩展的系统中，常用抽象基类来统一接口。

Why use Abstract Base Classes?

Abstract base classes are a form of interface checking more strict than individual hasattr() checks for particular methods. By defining an abstract base class, you can define a common API for a set of subclasses. This capability is especially useful in situations where a third-party is going to provide implementations, such as with plugins to an application, but can also aid you when working on a large team or with a large code-base where keeping all classes in your head at the same time is difficult or not possible.

下面介绍三种定义抽象基类的方法。

使用 assert 语句

class BaseClass(object):
    def action(self, foobar):
        assert False, 'subclasses of BaseClass must provide an action() method'

In [6]: BaseClass().action('a')
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-6-69f195c0ee1f> in <module>()
----> 1 BaseClass().action('a')

<ipython-input-3-25c84a2cb72e> in action(self, foobar)
      1 class BaseClass(object):
      2     def action(self, foobar):
----> 3         assert False, 'subclasses of BaseClass must provide an action() method'

AssertionError: subclasses of BaseClass must provide an action() method

使用 NotImplementedError 异常

class BaseClass(object):
    def action(self, foobar):
        raise NotImplementedError('subclasses of BaseClass must provide an action() method')

In [8]: BaseClass().action('a')
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-8-69f195c0ee1f> in <module>()
----> 1 BaseClass().action('a')

<ipython-input-7-81782a1e8377> in action(self, foobar)
      1 class BaseClass(object):
      2     def action(self, foobar):
----> 3         raise NotImplementedError('subclasses of BaseClass must provide an action() method')

NotImplementedError: subclasses of BaseClass must provide an action() method

使用 abc 模块

python 2.6, 2.7:

from abc import ABCMeta, abstractmethod

class BaseClass(object):
    __metaclass__ = ABCMeta

    @abstractmethod
    def action(self, foobar):
        pass

In [11]: BaseClass().action('a')
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-11-69f195c0ee1f> in <module>()
----> 1 BaseClass().action('a')

TypeError: Can't instantiate abstract class BaseClass with abstract methods action

python 3.x:

from abc import ABCMeta, abstractmethod

class BaseClass(metaclass=ABCMeta):
    @abstractmethod
    def action(self, foobar):
        pass

推荐使用 abc 模块，NotImplementedError 也比较常用。

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