python:常规方法和classmethod之间的混合

Question

有时我需要用静态方法编写类,但有可能初始化它并保持状态(对象)

......喜欢:

class A:
  @classmethod
  def method(cls_or_self):
    # get reference to object when A().method() or to class when A.method()
    code

我现在拥有的是:

class A:
   def method(self = None, *params): code
   # or
   def method2(self = None, **params): code
   # but what I need is rather normal parameters, not optional and named args:
   def method3(self_or_cls, a, b=1, c=2, *p, **kw): code

请不要写关于staticmethod和classmethod之间的区别.我感兴趣的是,如果存在这样的装饰器(在或多或少的标准库中),而且如果上面适合于PEP.

Answer 1

函数和classmethod对象充当描述符 ; 两者都返回一个包装器,当被调用时将依次使用额外的参数调用底层函数.函数和classmethod对象的行为方式之间的唯一区别在于额外的参数是什么.

要创建两种方法的混合,请构建自己的描述符装饰器:

from functools import wraps

class hybridmethod(object):
    def __init__(self, func):
        self.func = func

    def __get__(self, obj, cls):
        context = obj if obj is not None else cls

        @wraps(self.func)
        def hybrid(*args, **kw):
            return self.func(context, *args, **kw)

        # optional, mimic methods some more
        hybrid.__func__ = hybrid.im_func = self.func
        hybrid.__self__ = hybrid.im_self = context

        return hybrid

在这里,我们返回一个包装器,它将使用类或实例作为第一个参数,具体取决于__get__调用描述符方法时可用的内容.

演示:

>>> class Foo(object):
...     @hybridmethod
...     def bar(cls_or_self):
...         print 'Called with cls_or_self={!r}'.format(cls_or_self)
... 
>>> Foo.bar()
Called with cls_or_self=<class '__main__.Foo'>
>>> Foo().bar()
Called with cls_or_self=<__main__.Foo object at 0x1043a4390>