定義抽象基類(Abstract Base Classes)

好比 web 框架中的 cache 部分的基類通常相似下面這樣:

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)

三種定義抽象基類的方法

一、使用 assert 語句

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

二、使用 NotImplementedError 異常

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

三、使用 abc 模塊

*python2

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

*python3

from abc import ABCMeta, abstractmethod

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

推薦使用 abc 模塊，NotImplementedError 也比較經常使用。

example:

class TestBase(object):
    def get_error(self):
        raise NotImplementedError

    def do(self):
        return self.get_error()


class Duo(TestBase):
    def __init__(self):
        super(Duo, self).__init__()

    def get_error(self):
        return "Do Here"


handler = Duo()

print handler.do()


# result : Do Here