我正在扩展Python的webapp2网络框架，使其具备一些缺失的功能（以便快速、简便地创建应用程序）。
其中一个要求是每个子类需要有一些特定的静态类变量。最好的实现方法是，在使用这些变量时，如果它们缺失，就抛出异常，还是有更好的方法？
示例（非真实代码）：
子类：
class Bar(Foo):
page_name = 'New Page'
为了在此处进行处理，page_name 必须存在:
page_names = process_pages(list_of_pages)
def process_pages(list_of_pages)
page_names = []
for page in list_of_pages:
page_names.append(page.page_name)
return page_names
__call__中检查"required"属性即可。
元类
class ForceRequiredAttributeDefinitionMeta(type):
def __call__(cls, *args, **kwargs):
class_object = type.__call__(cls, *args, **kwargs)
class_object.check_required_attributes()
return class_object
__call__，我们创建了类对象，然后调用它的check_required_attributes()方法来检查是否定义了必需的属性。如果没有定义，我们就简单地抛出一个异常。
超类
class ForceRequiredAttributeDefinition(metaclass=ForceRequiredAttributeDefinitionMeta):
starting_day_of_week = None
def check_required_attributes(self):
if self.starting_day_of_week is None:
raise NotImplementedError('Subclass must define self.starting_day_of_week attribute. \n This attribute should define the first day of the week.')
三件事:
None，参见starting_day_of_week = Nonecheck_required_attributes方法，检查所需属性是否为None，如果是，则抛出一个带有合理错误消息的NotImplementedError给用户。工作和不工作子类的示例
class ConcereteValidExample(ForceRequiredAttributeDefinition):
def __init__(self):
self.starting_day_of_week = "Monday"
class ConcereteInvalidExample(ForceRequiredAttributeDefinition):
def __init__(self):
# This will throw an error because self.starting_day_of_week is not defined.
pass
输出
Traceback (most recent call last):
File "test.py", line 50, in <module>
ConcereteInvalidExample() # This will throw an NotImplementedError straightaway
File "test.py", line 18, in __call__
obj.check_required_attributes()
File "test.py", line 36, in check_required_attributes
raise NotImplementedError('Subclass must define self.starting_day_of_week attribute. \n This attribute should define the first day of the week.')
NotImplementedError: Subclass must define self.starting_day_of_week attribute.
This attribute should define the first day of the week.
NotImplementedError错误。def build_required_attributes_metaclass(*required_attrs):
class RequiredAttributesMeta(type):
def __init__(cls, name, bases, attrs):
if cls.mro() == [cls, object]:
return # don't require attrs on our base class
missing_attrs = ["'%s'" % attr for attr in required_attrs
if not hasattr(cls, attr)]
if missing_attrs:
raise AttributeError("class '%s' requires attribute%s %s" %
(name, "s" * (len(missing_attrs) > 1),
", ".join(missing_attrs)))
return RequiredAttributesMeta
class Base(metaclass=build_required_attributes_metaclass("a", "b" ,"c")):
pass
class Child(Base):
pass
AttributeError: class 'Child' requires attributes 'a', 'b', 'c'
RequiredAttributesMeta从那个元类派生，而不是type。这个方法有效。它可以防止子类被定义，更不用说实例化了。
class Foo:
page_name = None
author = None
def __init_subclass__(cls, **kwargs):
for required in ('page_name', 'author',):
if not getattr(cls, required):
raise TypeError(f"Can't instantiate abstract class {cls.__name__} without {required} attribute defined")
return super().__init_subclass__(**kwargs)
class Bar(Foo):
page_name = 'New Page'
author = 'eric'
getattr提供默认值，否则永远不会遇到TypeError，因为不存在的属性将引发AttributeError，即if not getattr(cls, required, None)。 - Jürgen Gmach抽象基类允许声明一个属性为抽象的，这将强制所有实现该属性的子类都必须拥有该属性。我只提供这个例子是为了完整性，许多 Python 爱好者认为您提出的解决方案更符合 Python 的风格。
import abc
class Base(object):
__metaclass__ = abc.ABCMeta
@abc.abstractproperty
def value(self):
return 'Should never get here'
class Implementation1(Base):
@property
def value(self):
return 'concrete property'
class Implementation2(Base):
pass # doesn't have the required property
print Implementation1()
Out[6]: <__main__.Implementation1 at 0x105c41d90>
尝试实例化第二个实现类：
print Implementation2()
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-4-bbaeae6b17a6> in <module>()
----> 1 Implementation2()
TypeError: Can't instantiate abstract class Implementation2 with abstract methods value
bases检查，以便您可以在mixin中使用它，而mixin本身不会触发它。可以添加类似的检查以跳过ABCs。def RequiredAttributes(*required_attrs):
class RequiredAttributesMeta(type):
def __init__(cls, name, bases, attrs):
if not bases:
return # No bases implies mixin. Mixins aren't the final class, so they're exempt.
if missing_attrs := [attr for attr in required_attrs if not hasattr(cls, attr)]:
raise AttributeError(f"{name!r} requires attributes: {missing_attrs}")
return RequiredAttributesMeta
class LicenseAccessMixin(metaclass=RequiredAttributes('access_control')):
... # define methods that safely refer to `self.access_control`.
def process_pages(list_of_pages): return [ page.page_name for page in list_of_pages ]的意思是将给定页面列表中每个页面的名称提取出来并返回一个名称列表。 - chepner