如何在Python中创建只读类属性？

Question

基本上我想做这样的事情:

class foo:
    x = 4
    @property
    @classmethod
    def number(cls):
        return x

然后我希望以下工作:

>>> foo.number
4

不幸的是,上述方法无效.而不是给我4它给我<property object at 0x101786c58>.有没有办法实现上述目标？

Answer 1

这将创建Foo.number一个只读属性:

class MetaFoo(type):
    @property
    def number(cls):
        return cls.x

class Foo(object, metaclass=MetaFoo):
    x = 4

print(Foo.number)
# 4

Foo.number = 6
# AttributeError: can't set attribute

说明:使用@property时的常见情况如下所示:

class Foo(object):
    @property
    def number(self):
        ...
foo = Foo()

定义的属性Foo相对于其实例是只读的.那就是,foo.number = 6会提高一个AttributeError.

类似地,如果你想Foo.number提出一个,AttributeError你需要设置一个定义的属性type(Foo).因此需要一个元类.

请注意,这种只读不受黑客的影响.通过更改Foo的类可以使该属性可写:

class Base(type): pass
Foo.__class__ = Base

# makes Foo.number a normal class attribute
Foo.number = 6   
print(Foo.number)

版画

6

或者,如果你想建立Foo.number一个可设置的财产,

class WritableMetaFoo(type): 
    @property
    def number(cls):
        return cls.x
    @number.setter
    def number(cls, value):
        cls.x = value
Foo.__class__ = WritableMetaFoo

# Now the assignment modifies `Foo.x`
Foo.number = 6   
print(Foo.number)

还打印6.

Answer 2

在property从一类访问时描述符总是返回本身(即当instance是None在它的__get__方法).

如果这不是您想要的,您可以编写一个始终使用类object(owner)而不是实例的新描述符:

>>> class classproperty(object):
...     def __init__(self, getter):
...         self.getter= getter
...     def __get__(self, instance, owner):
...         return self.getter(owner)
... 
>>> class Foo(object):
...     x= 4
...     @classproperty
...     def number(cls):
...         return cls.x
... 
>>> Foo().number
4
>>> Foo.number
4

Answer 3

我同意unubtu的回答 ; 它似乎工作,然而,它不适用于Python 3上的这种精确语法(具体来说,Python 3.4是我所挣扎的).以下是在Python 3.4下必须形成模式以使事情有效的方法,似乎:

class MetaFoo(type):
   @property
   def number(cls):
      return cls.x

class Foo(metaclass=MetaFoo):
   x = 4

print(Foo.number)
# 4

Foo.number = 6
# AttributeError: can't set attribute

Answer 4

Mikhail Gerasimov的解决方案非常完整。不幸的是，这是一个缺点。如果您在使用他的classproperty有一类，没有子类可以使用它因 TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases有class Wrapper。

幸运的是，可以解决此问题。创建时，只需从给定类的元类继承即可class Meta。

def classproperty_support(cls):
  """
  Class decorator to add metaclass to our class.
  Metaclass uses to add descriptors to class attributes, see:
  http://stackoverflow.com/a/26634248/1113207
  """
  # Use type(cls) to use metaclass of given class
  class Meta(type(cls)): 
      pass

  for name, obj in vars(cls).items():
      if isinstance(obj, classproperty):
          setattr(Meta, name, property(obj.fget, obj.fset, obj.fdel))

  class Wrapper(cls, metaclass=Meta):
      pass
  return Wrapper

Answer 5

上面解决方案的问题是，它无法从实例变量访问类变量：

print(Foo.number)
# 4

f = Foo()
print(f.number)
# 'Foo' object has no attribute 'number'

而且，显式使用元类并不像使用常规property装饰器那样好。

我试图解决这个问题。现在，它是如何工作的：

@classproperty_support
class Bar(object):
    _bar = 1

    @classproperty
    def bar(cls):
        return cls._bar

    @bar.setter
    def bar(cls, value):
        cls._bar = value


# @classproperty should act like regular class variable.
# Asserts can be tested with it.
# class Bar:
#     bar = 1


assert Bar.bar == 1

Bar.bar = 2
assert Bar.bar == 2

foo = Bar()
baz = Bar()
assert foo.bar == 2
assert baz.bar == 2

Bar.bar = 50
assert baz.bar == 50
assert foo.bar == 50

如您所见，我们的@classproperty工作方式与@property类变量相同。我们唯一需要的是附加的@classproperty_support类装饰器。

解决方案也适用于只读类属性。

这是实现：

class classproperty:
    """
    Same as property(), but passes obj.__class__ instead of obj to fget/fset/fdel.
    Original code for property emulation:
    https://docs.python.org/3.5/howto/descriptor.html#properties
    """
    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        self.fget = fget
        self.fset = fset
        self.fdel = fdel
        if doc is None and fget is not None:
            doc = fget.__doc__
        self.__doc__ = doc

    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        if self.fget is None:
            raise AttributeError("unreadable attribute")
        return self.fget(obj.__class__)

    def __set__(self, obj, value):
        if self.fset is None:
            raise AttributeError("can't set attribute")
        self.fset(obj.__class__, value)

    def __delete__(self, obj):
        if self.fdel is None:
            raise AttributeError("can't delete attribute")
        self.fdel(obj.__class__)

    def getter(self, fget):
        return type(self)(fget, self.fset, self.fdel, self.__doc__)

    def setter(self, fset):
        return type(self)(self.fget, fset, self.fdel, self.__doc__)

    def deleter(self, fdel):
        return type(self)(self.fget, self.fset, fdel, self.__doc__)


def classproperty_support(cls):
    """
    Class decorator to add metaclass to our class.
    Metaclass uses to add descriptors to class attributes, see:
    http://stackoverflow.com/a/26634248/1113207
    """
    class Meta(type):
        pass

    for name, obj in vars(cls).items():
        if isinstance(obj, classproperty):
            setattr(Meta, name, property(obj.fget, obj.fset, obj.fdel))

    class Wrapper(cls, metaclass=Meta):
        pass
    return Wrapper

注意：代码没有经过太多测试，请注意它是否无法按预期工作。