目标是创建一个行为类似db结果集的模拟类。

例如,如果一个数据库查询返回,使用dict表达式,{'ab':100, 'cd':200},那么我想看到:

>>> dummy.ab
100

一开始我想也许我可以这样做:

ks = ['ab', 'cd']
vs = [12, 34]
class C(dict):
    def __init__(self, ks, vs):
        for i, k in enumerate(ks):
            self[k] = vs[i]
            setattr(self, k, property(lambda x: vs[i], self.fn_readyonly))

    def fn_readonly(self, v)
        raise "It is ready only"

if __name__ == "__main__":
    c = C(ks, vs)
    print c.ab

但是cab返回一个属性对象。

用k = property(lambda x: vs[i])替换setattr行根本没有用。

那么,在运行时创建实例属性的正确方法是什么呢?

附注:我知道在如何使用__getattribute__方法中提出了一个替代方案?


当前回答

虽然给出了很多答案,但我没有找到一个让我满意的。我找到了自己的解决方案,使属性适用于动态情况。来源回答原来的问题:

#!/usr/local/bin/python3

INITS = { 'ab': 100, 'cd': 200 }

class DP(dict):
  def __init__(self):
    super().__init__()
    for k,v in INITS.items():
        self[k] = v 

def _dict_set(dp, key, value):
  dp[key] = value

for item in INITS.keys():
  setattr(
    DP,
    item,
    lambda key: property(
      lambda self: self[key], lambda self, value: _dict_set(self, key, value)
    )(item)
  )

a = DP()
print(a)  # {'ab': 100, 'cd': 200}
a.ab = 'ab100'
a.cd = False
print(a.ab, a.cd) # ab100 False

其他回答

最好的方法是定义__slots__。这样你的实例就不能有新的属性。

ks = ['ab', 'cd']
vs = [12, 34]

class C(dict):
    __slots__ = []
    def __init__(self, ks, vs): self.update(zip(ks, vs))
    def __getattr__(self, key): return self[key]

if __name__ == "__main__":
    c = C(ks, vs)
    print c.ab

打印12

    c.ab = 33

'C'对象没有属性'ab'

你可以使用下面的代码来使用字典对象更新类属性:

class ExampleClass():
    def __init__(self, argv):
        for key, val in argv.items():
            self.__dict__[key] = val

if __name__ == '__main__':
    argv = {'intro': 'Hello World!'}
    instance = ExampleClass(argv)
    print instance.intro

你不需要使用属性。只需重写__setattr__,使其为只读。

class C(object):
    def __init__(self, keys, values):
        for (key, value) in zip(keys, values):
            self.__dict__[key] = value

    def __setattr__(self, name, value):
        raise Exception("It is read only!")

Tada。

>>> c = C('abc', [1,2,3])
>>> c.a
1
>>> c.b
2
>>> c.c
3
>>> c.d
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'C' object has no attribute 'd'
>>> c.d = 42
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in __setattr__
Exception: It is read only!
>>> c.a = 'blah'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in __setattr__
Exception: It is read only!

扩展了kjfletch的想法

# This is my humble contribution, extending the idea to serialize
# data from and to tuples, comparison operations and allowing functions
# as default values.

def Struct(*args, **kwargs):
    FUNCTIONS = (types.BuiltinFunctionType, types.BuiltinMethodType, \
                 types.FunctionType, types.MethodType)
    def init(self, *iargs, **ikwargs):
        """Asume that unamed args are placed in the same order than
        astuple() yields (currently alphabetic order)
        """
        kw = list(self.__slots__)

        # set the unnamed args
        for i in range(len(iargs)):
            k = kw.pop(0)
            setattr(self, k, iargs[i])

        # set the named args
        for k, v in ikwargs.items():
            setattr(self, k, v)
            kw.remove(k)

        # set default values
        for k in kw:
            v = kwargs[k]
            if isinstance(v, FUNCTIONS):
                v = v()
            setattr(self, k, v)

    def astuple(self):
        return tuple([getattr(self, k) for k in self.__slots__])

    def __str__(self):
        data = ['{}={}'.format(k, getattr(self, k)) for k in self.__slots__]
        return '<{}: {}>'.format(self.__class__.__name__, ', '.join(data))

    def __repr__(self):
        return str(self)

    def __eq__(self, other):
        return self.astuple() == other.astuple()

    name = kwargs.pop("__name__", "MyStruct")
    slots = list(args)
    slots.extend(kwargs.keys())
    # set non-specific default values to None
    kwargs.update(dict((k, None) for k in args))

    return type(name, (object,), {
        '__init__': init,
        '__slots__': tuple(slots),
        'astuple': astuple,
        '__str__': __str__,
        '__repr__': __repr__,
        '__eq__': __eq__,
    })


Event = Struct('user', 'cmd', \
               'arg1', 'arg2',  \
               date=time.time, \
               __name__='Event')

aa = Event('pepe', 77)
print(aa)
raw = aa.astuple()

bb = Event(*raw)
print(bb)

if aa == bb:
    print('Are equals')

cc = Event(cmd='foo')
print(cc)

输出:

<Event: user=pepe, cmd=77, arg1=None, arg2=None, date=1550051398.3651814>
<Event: user=pepe, cmd=77, arg1=None, arg2=None, date=1550051398.3651814>
Are equals
<Event: user=None, cmd=foo, arg1=None, arg2=None, date=1550051403.7938335>

我想我应该扩展这个答案,现在我长大了,更聪明了,知道发生了什么。迟到总比不到好。

可以动态地向类添加属性。但这就是问题所在:你必须把它添加到类中。

>>> class Foo(object):
...     pass
... 
>>> foo = Foo()
>>> foo.a = 3
>>> Foo.b = property(lambda self: self.a + 1)
>>> foo.b
4

属性实际上是描述符的简单实现。它是一个对象,为给定类的给定属性提供自定义处理。有点像从__getattribute__中分解出一个巨大的if树。

When I ask for foo.b in the example above, Python sees that the b defined on the class implements the descriptor protocol—which just means it's an object with a __get__, __set__, or __delete__ method. The descriptor claims responsibility for handling that attribute, so Python calls Foo.b.__get__(foo, Foo), and the return value is passed back to you as the value of the attribute. In the case of property, each of these methods just calls the fget, fset, or fdel you passed to the property constructor.

描述符实际上是Python暴露其整个OO实现管道的方式。事实上,还有一种比属性更常见的描述符。

>>> class Foo(object):
...     def bar(self):
...         pass
... 
>>> Foo().bar
<bound method Foo.bar of <__main__.Foo object at 0x7f2a439d5dd0>>
>>> Foo().bar.__get__
<method-wrapper '__get__' of instancemethod object at 0x7f2a43a8a5a0>

简单的方法只是另一种描述符。它的__get__将调用实例作为第一个参数;实际上,它是这样做的:

def __get__(self, instance, owner):
    return functools.partial(self.function, instance)

不管怎样,我怀疑这就是为什么描述符只在类上起作用:它们是首先为类提供动力的东西的形式化。它们甚至是规则的例外:您可以明显地为类分配描述符,而类本身就是类型的实例!事实上,我试着读Foo。Bar仍然调用property.__get__;描述符在作为类属性访问时返回自己,这只是一种习惯。

我认为几乎所有Python的OO系统都可以用Python来表达,这非常酷。:)

哦,如果你感兴趣的话,我之前写过一篇关于描述符的冗长博文。