让我首先定义我的目标:
启用没有默认值(不是 None 或其他一些神奇值,但如果未实现则抛出错误)的抽象类成员(不是属性、方法或实例成员)的定义。
<为 (1) 服务,创建一个可重用的抽象机制,使创建具有抽象成员的类变得简单,并使执行此操作的代码最大限度地简洁。
能够将抽象类成员附加到父类(作为子类、元类或通过任何其他方式),其中在定义具体子类之前不需要定义抽象成员。
到目前为止我所得到的:
metaclass_lib.py:
class AbstractRequiredClassMembersMetaclass(type):
def __new__(cls, name, bases, attrs):
missed_members = []
for class_member in cls.REQUIRED_CLASS_MEMBERS:
if class_member not in attrs:
missed_members.append(class_member)
if missed_members:
raise NotImplementedError('Class missing required members %s.' % missed_members)
return super(AbstractRequiredClassMembersMetaclass, cls).__new__(cls, name, bases, attrs)
class _MakeRequiredClassMemebersMetaclass(object):
existing_classes = {}
def __call__(self, name, required_members):
if name in _MakeRequiredClassMemebersMetaclass.exisiting_classes:
if required_members != _MakeRequiredClassMemebersMetaclass.exisiting_classes[name].REQUIRED_CLASS_MEMBERS:
raise RuntimeError('Class of name %s already defined with different required_members.' % name)
else:
NewClass = type(name, (AbstractRequiredClassMembersMetaclass,), {'REQUIRED_CLASS_MEMBERS' : required_members})
_MakeRequiredClassMemebersMetaclass.exisiting_classes[name] = NewClass
return _MakeRequiredClassMemebersMetaclass.exisiting_classes[name]
make_required_class_members_metaclass = _MakeRequiredClassMemebersMetaclass()
goods.py(用于说明的实现):
from metaclass_lib import make_required_class_members_metaclass
class AbstractGood(object):
__metaclass__ = make_required_class_members_metaclass('AbstractGoodMeta', ('unit_measure',))
def __init__(self):
self.unit_price = None
def total_cost(self, number_of_units):
return self.unit_price * self.number_of_units
class DeliGood(AbstractGood):
unit_measure = 'lbs'
def __init__(self):
self.sandwich_counter_product = False
class RefridgeratedGood(AbstractGood):
unit_measure = 'package'
def __init__(self):
self.freezer_safe = False
self.optimal_temp = '35'
这不起作用,因为元类在创建 AbstractGood
type
对象期间被阻塞。问题是我希望所有具体商品定义类成员,但我不想在任何抽象基础中定义类成员。我能想到的就是让元类只执行 in attrs
检查关键字是否不在 name
中(例如 if 'Abstract' 不在 name
),但这看起来很笨拙且脆弱。
有更好的方法吗?
最佳答案
也是一种 hack,但比检查名称更可靠。
您知道抽象基类没有基类
(或者在 python 2 中只有 object
类型)。所以你可以检查一下:)
class AbstractBase(object):
class __metaclass__(type):
def __new__(mcs, name, bases, dict_):
if bases[0] is not object: # hack to not apply these on AbstractBase
assert 'unit_measure' in dict_
return type.__new__(mcs, name, bases, dict_)
class DeliGood(AbstractBase):
unit_measure = 'lbs'
关于python - 将元类规则应用于所有后代而不是直接元类,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/10726676/