在现代Python 3中,使用抽象基类实现接口要简单得多,它们的作用是作为插件扩展的接口契约。
创建接口/抽象基类:
from abc import ABC, abstractmethod
class AccountingSystem(ABC):
@abstractmethod
def create_purchase_invoice(self, purchase):
pass
@abstractmethod
def create_sale_invoice(self, sale):
log.debug('Creating sale invoice', sale)
创建一个普通的子类并覆盖所有抽象方法:
class GizmoAccountingSystem(AccountingSystem):
def create_purchase_invoice(self, purchase):
submit_to_gizmo_purchase_service(purchase)
def create_sale_invoice(self, sale):
super().create_sale_invoice(sale)
submit_to_gizmo_sale_service(sale)
您可以选择在抽象方法(如create_sale_invoice())中使用通用实现,在上面的子类中使用super()显式调用它。
没有实现所有抽象方法的子类实例化失败:
class IncompleteAccountingSystem(AccountingSystem):
pass
>>> accounting = IncompleteAccountingSystem()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: Can't instantiate abstract class IncompleteAccountingSystem with abstract methods
create_purchase_invoice, create_sale_invoice
通过将相应的注释与@abstractmethod组合,还可以拥有抽象属性、静态方法和类方法。
抽象基类对于实现基于插件的系统非常有用。类的所有导入子类都可以通过__subclasses__()访问,所以如果你用importlib.import_module()从插件目录加载所有类,如果它们是基类的子类,你可以通过__subclasses__()直接访问它们,并且你可以确保在实例化期间对所有类强制执行接口契约。
下面是上面AccountingSystem例子的插件加载实现:
...
from importlib import import_module
class AccountingSystem(ABC):
...
_instance = None
@classmethod
def instance(cls):
if not cls._instance:
module_name = settings.ACCOUNTING_SYSTEM_MODULE_NAME
import_module(module_name)
subclasses = cls.__subclasses__()
if len(subclasses) > 1:
raise InvalidAccountingSystemError('More than one '
f'accounting module: {subclasses}')
if not subclasses or module_name not in str(subclasses[0]):
raise InvalidAccountingSystemError('Accounting module '
f'{module_name} does not exist or does not '
'subclass AccountingSystem')
cls._instance = subclasses[0]()
return cls._instance
然后你可以通过AccountingSystem类访问计费系统插件对象:
>>> accountingsystem = AccountingSystem.instance()
(灵感来自这篇pymotw3帖子。)