Python 创建一级对象，包括所有对象'；s实例属性是只读的，类似于切片？

我的问题是如何创建像

slice

这样的类

slice

（内置类型）没有

\u dict\u

属性 甚至这个

切片的
元类
也是
类型

而且它没有使用
\uuuuuuuuuuuuuuuuuu
，并且它的所有属性都是只读的，并且它没有覆盖
\uuuuu setattr\uuuuuuuu
（我对此不确定，但看看我的代码，看看我是否正确）

检查此代码：

# how slice is removing the __dict__ from the class object
# and the metaclass is type!!

class sliceS(object):
    pass

class sliceS0(object):

    def __setattr__(self, name, value):
        pass

# this means that both have the same
# metaclass type.
print type(slice) == type(sliceS) # prints True

# from what i understand the metaclass is the one
# that is responsible for making the class object
sliceS2 = type('sliceS2', (object,), {})
# witch is the same
# sliceS2 = type.__new__(type, 'sliceS2', (object,), {})
print type(sliceS2) # prints type

# but when i check the list of attribute using dir
print '__dict__' in dir(slice)  # prints False
print '__dict__' in dir(sliceS) # prints True

# now when i try to set an attribute on slice
obj_slice = slice(10)
# there is no __dict__  here
print '__dict__' in dir(obj_slice) # prints False
obj_sliceS = sliceS()
try:
    obj_slice.x = 1
except AttributeError as e:
    # you get AttributeError
    # mean you cannot add new properties
    print "'slice' object has no attribute 'x'"

obj_sliceS.x = 1 # Ok: x is added to __dict__ of obj_sliceS
print 'x' in obj_sliceS.__dict__ # prints True

# and slice is not using __slots__ because as you see it's not here
print '__slots__' in dir(slice) # print False

# and this why i'm saying it's not overriding the __settattr__
print id(obj_slice.__setattr__) == id(obj_sliceS.__setattr__) # True: it's the same object
obj_sliceS0 = sliceS0()
print id(obj_slice.__setattr__) == id(obj_sliceS0.__setattr__) # False: it's the same object

# so slice have only start, stop, step and are all readonly attribute and it's not overriding the __setattr__
# what technique it's using?!!!!

如何制作此类一级对象它的所有属性都是只读的，您无法
添加新属性。
问题在于Python内置的
切片
类是用C语言编程的。当您使用C-Python API进行编码时，您可以编写等价的属性，这些属性可以通过
\uuuuuuu插槽
访问，而无需使用Python端可见的任何机制。（您甚至可以拥有“真正的”私有属性，这在纯Python代码中几乎是不可能的）

Python代码用来防止类实例出现
\uuuuuuuuuuuuuuuuuuuu
以及后续的“可以设置任何属性”的机制就是
\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu slots>属性。
但是，与实际使用该类时必须出现的magic dunder方法不同，在创建该类时才使用
\uuuuuuuuuuuuuuuu插槽
上的信息。因此，如果您关心的是在最后一个类中有一个可见的
\uuuuuuu\uuuuuu\uuuuu
，您可以在公开它之前将其从类中删除：

In [8]: class A:
   ...:     __slots__ = "b"
   ...:     

In [9]: del A.__slots__

In [10]: a = A()

In [11]: a.b = 5

In [12]: a.c = 5
------------------------
AttributeError   
...

In [13]: A.__slots__
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-13-68a69c802e74> in <module>()
----> 1 A.__slots__

AttributeError: type object 'A' has no attribute '__slots__'

或者，您可以使用元类自动创建和自动销毁Python visible
\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu

class AttrOnly(type):
   def __new__(metacls, name, bases, namespace, **kw):
        namespace["__slots__"] = list(namespace.keys())  # not sure if "list(" is needed
        cls = super().__new__(metacls, name, bases, namespace, **kw)
        del cls.__slots__
        return cls

class MyClass(metaclass=AttrOnly):
    x = int
    y = int

如果希望纯Python只读属性在实例本身中没有可见的对应项（如
\ux
由
属性
描述符使用以保持
x
属性的值），那么简单的方法是自定义
\uuu setattr\uuuu
。另一种方法是让元类在类创建阶段为每个属性自动添加只读属性。下面的元类就是这样做的，并使用
\uuuu slots\uuu
类属性来创建所需的描述符：

class ReadOnlyAttrs(type):
    def __new__(metacls, name, bases, namespace, **kw):
        def get_setter(attr):
            def setter(self, value):
                if getattr(self, "_initialized", False): 
                    raise ValueError("Can't set "  + attr)
                setattr(self, "_" + attr, value)
            return setter

        slots = namespace.get("__slots__", [])
        slots.append("initialized")
        def __new__(cls, *args, **kw):
            self = object.__new__(cls)  # for production code that could have an arbitrary hierarchy, this needs to be done more carefully
            for attr, value in kw.items():
                setattr(self, attr, value)
            self.initialized = True
            return self

        namespace["__new__"] = __new__
        real_slots = []
        for attr in slots:
            real_slots.append("_" + attr)
            namespace[attr] = property(
                (lambda attr: lambda self: getattr(self, "_" + attr))(attr), # Getter. Extra lambda needed to create an extra closure containing each attr
                get_setter(attr)
            )
        namespace["__slots__"] = real_slots
        cls = super().__new__(metacls, name, bases, namespace, **kw)
        del cls.__slots__
        return cls

请记住，您还可以自定义类“
\uuuuu dir\uuuuu
方法，以便在需要时不会看到
\ux
阴影属性。
。。。“一级对象”是什么意思？我想你指的是内置类型。但如果你指的是不重写
setattr
或使用
\uuuuuuuuuuuuuuuuuu
，则可以创建具有相同行为的类？写一个C扩展名。或者使用
属性
或其他描述符，而不实现
\uuuuuu set\uuuuuuuu
@juanpa.arrivillaga谢谢您的评论我编辑了我的问题以消除冲突。问题是，我如何创建一个行为类似于切片的类？它实际上什么都不做，只是让
\uu插槽\uu
不可见。我之所以添加它，是因为在所创建的类中，
\uuuuuu slots\uuuuu
的“不存在”是问题中的一个问题。对不起，刚才我看到您有“Python2.7”标记。然而，上面唯一应该改变的是如何使用元类（我猜，te元类“kw”属性将是无用的）；无论如何，现在研究Python 2中的高级Python功能毫无意义，我建议您更新到3.6。@jsbueno我不知道我可以从calss对象中删除插槽，因此基本上，如果class对象没有dict属性，也没有子类，您就不能向其添加新属性。我说的对吗？@jsbueno-sill这里缺少一件事：如何使该属性为只读。我认为如果不重写
\uuuu setattr\uuuu
class ReadOnlyAttrs(type):
    def __new__(metacls, name, bases, namespace, **kw):
        def get_setter(attr):
            def setter(self, value):
                if getattr(self, "_initialized", False): 
                    raise ValueError("Can't set "  + attr)
                setattr(self, "_" + attr, value)
            return setter

        slots = namespace.get("__slots__", [])
        slots.append("initialized")
        def __new__(cls, *args, **kw):
            self = object.__new__(cls)  # for production code that could have an arbitrary hierarchy, this needs to be done more carefully
            for attr, value in kw.items():
                setattr(self, attr, value)
            self.initialized = True
            return self

        namespace["__new__"] = __new__
        real_slots = []
        for attr in slots:
            real_slots.append("_" + attr)
            namespace[attr] = property(
                (lambda attr: lambda self: getattr(self, "_" + attr))(attr), # Getter. Extra lambda needed to create an extra closure containing each attr
                get_setter(attr)
            )
        namespace["__slots__"] = real_slots
        cls = super().__new__(metacls, name, bases, namespace, **kw)
        del cls.__slots__
        return cls