如何在Python中创建只读类属性?

property 描述符总是在从类访问时返回自身(即 instance 是 None 在它的 __get__

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

>>> 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

这将使 Foo.number 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

解释 :使用 @property 看起来像这样:

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

中定义的属性 Foo 对于其实例是只读的。也就是说, foo.number = 6 会引起 AttributeError .

类似地,如果你想 食品编号 提出异议 属性错误 您需要设置中定义的属性 type(Foo) . 因此需要一个元类。

请注意,这种只读性不能免受黑客的攻击。 可以通过更改Foo的

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

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

6

或者,如果你想 可设置属性,

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)

同时打印

6

我同意你的看法 unubtu's answer 蟒蛇3 (具体来说,python3.4是我一直在努力解决的问题)。下面是如何在Python3.4下形成模式以使事情正常工作的:

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

print(Foo.number)
# 4

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

property 装饰工。

我试图解决这个问题。下面是它现在的工作原理:

@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

@classproperty 这和 @property 对于类变量。我们唯一需要的是额外的 @classproperty_support 班级装饰师。

以下是实现:

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: 代码没有经过太多的测试,如果没有如您所期望的那样工作,请随时注意。

米哈伊尔·格拉西莫夫的解决方案是相当完整的。不幸的是,这是一个缺点。如果有一个类使用他的classproperty,那么由于 TypeError: metaclass conflict: the metaclass of a derived class must be a (non-strict) subclass of the metaclasses of all its bases class Wrapper .

幸运的是,这个问题可以解决。创建时只从给定类的元类继承 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