以下是需要翻译的内容:
以下是解决方案应满足的要求列表：
- 实例化过时类应发出警告
- 子类化过时类应发出警告
- 支持
isinstance和issubclass检查
解决方案
这可以通过自定义元类来实现：
class DeprecatedClassMeta(type):
def __new__(cls, name, bases, classdict, *args, **kwargs):
alias = classdict.get('_DeprecatedClassMeta__alias')
if alias is not None:
def new(cls, *args, **kwargs):
alias = getattr(cls, '_DeprecatedClassMeta__alias')
if alias is not None:
warn("{} has been renamed to {}, the alias will be "
"removed in the future".format(cls.__name__,
alias.__name__), DeprecationWarning, stacklevel=2)
return alias(*args, **kwargs)
classdict['__new__'] = new
classdict['_DeprecatedClassMeta__alias'] = alias
fixed_bases = []
for b in bases:
alias = getattr(b, '_DeprecatedClassMeta__alias', None)
if alias is not None:
warn("{} has been renamed to {}, the alias will be "
"removed in the future".format(b.__name__,
alias.__name__), DeprecationWarning, stacklevel=2)
b = alias or b
if b not in fixed_bases:
fixed_bases.append(b)
fixed_bases = tuple(fixed_bases)
return super().__new__(cls, name, fixed_bases, classdict,
*args, **kwargs)
def __instancecheck__(cls, instance):
return any(cls.__subclasscheck__(c)
for c in {type(instance), instance.__class__})
def __subclasscheck__(cls, subclass):
if subclass is cls:
return True
else:
return issubclass(subclass, getattr(cls,
'_DeprecatedClassMeta__alias'))
解释
DeprecatedClassMeta.__new__ 方法被调用不仅针对它是元类的类，而且还针对这个类的每个子类。这就给了机会确保 DeprecatedClass 的实例永远不会被实例化或继承。
实例化很简单。元类重写了 __new__ 方法来总是返回一个 NewClass 实例。
继承并不难。 DeprecatedClassMeta.__new__ 接收一个基类列表，并需要将 DeprecatedClass 的实例替换为 NewClass 的实例。
最后，isinstance 和 issubclass 检查是通过在 PEP 3119 中定义的 __instancecheck__ 和 __subclasscheck__ 实现的。
测试
class NewClass:
foo = 1
class NewClassSubclass(NewClass):
pass
class DeprecatedClass(metaclass=DeprecatedClassMeta):
_DeprecatedClassMeta__alias = NewClass
class DeprecatedClassSubclass(DeprecatedClass):
foo = 2
class DeprecatedClassSubSubclass(DeprecatedClassSubclass):
foo = 3
assert issubclass(DeprecatedClass, DeprecatedClass)
assert issubclass(DeprecatedClassSubclass, DeprecatedClass)
assert issubclass(DeprecatedClassSubSubclass, DeprecatedClass)
assert issubclass(NewClass, DeprecatedClass)
assert issubclass(NewClassSubclass, DeprecatedClass)
assert issubclass(DeprecatedClassSubclass, NewClass)
assert issubclass(DeprecatedClassSubSubclass, NewClass)
assert isinstance(DeprecatedClass(), DeprecatedClass)
assert isinstance(DeprecatedClassSubclass(), DeprecatedClass)
assert isinstance(DeprecatedClassSubSubclass(), DeprecatedClass)
assert isinstance(NewClass(), DeprecatedClass)
assert isinstance(NewClassSubclass(), DeprecatedClass)
assert isinstance(DeprecatedClassSubclass(), NewClass)
assert isinstance(DeprecatedClassSubSubclass(), NewClass)
assert NewClass().foo == 1
assert DeprecatedClass().foo == 1
assert DeprecatedClassSubclass().foo == 2
assert DeprecatedClassSubSubclass().foo == 3
