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python属性查找没有任何描述符魔法?

descriptor python-datamodel python

6

Matt Anderson · 技术社区 · 16 年前

在我编写的代码中,我已经开始更广泛地使用python描述符协议。通常,我希望使用默认的python查找魔法,但有时我发现我希望获得描述符对象本身,而不是其结果 __get__ 方法。想要知道描述符的类型,或者描述符中存储的访问状态,等等。

python发行版中是否有这样的功能(或类似的功能)?

谢谢

from inspect import isdatadescriptor

def namespaces(obj):
    obj_dict = None
    if hasattr(obj, '__dict__'):
        obj_dict = object.__getattribute__(obj, '__dict__')

    obj_class = type(obj)
    return obj_dict, [t.__dict__ for t in obj_class.__mro__]

def getattr_raw(obj, name):
    # get an attribute in the same resolution order one would normally,
    # but do not call __get__ on the attribute even if it has one
    obj_dict, class_dicts = namespaces(obj)

    # look for a data descriptor in class hierarchy; it takes priority over
    # the obj's dict if it exists
    for d in class_dicts:
        if name in d and isdatadescriptor(d[name]):
            return d[name]

    # look for the attribute in the object's dictionary
    if obj_dict and name in obj_dict:
        return obj_dict[name]

    # look for the attribute anywhere in the class hierarchy
    for d in class_dicts:
        if name in d:
            return d[name]

    raise AttributeError

Denis的回答给了我一个约定,在描述符类中使用它来获取描述符对象本身。但是,我有一个完整的描述符类的类层次结构,我不想开始 每一个 __得到__ 具有样板的函数

def __get__(self, instance, instance_type):
    if instance is None: 
        return self
    ...

为了避免这种情况,我让描述符类树的根继承自以下内容:

def decorate_get(original_get):
    def decorated_get(self, instance, instance_type):
        if instance is None:
            return self
        return original_get(self, instance, instance_type)
    return decorated_get

class InstanceOnlyDescriptor(object):
    """All __get__ functions are automatically wrapped with a decorator which
    causes them to only be applied to instances. If __get__ is called on a 
    class, the decorator returns the descriptor itself, and the decorated
    __get__ is not called.
    """
    class __metaclass__(type):
        def __new__(cls, name, bases, attrs):
            if '__get__' in attrs:
                attrs['__get__'] = decorate_get(attrs['__get__'])
            return type.__new__(cls, name, bases, attrs)

4 回复 | 直到 16 年前

1

13

Community Mohan Dere 9 年前

大多数描述符在仅作为实例属性访问时执行其工作。因此,在类中访问它时,可以方便地返回自身:

class FixedValueProperty(object):
    def __init__(self, value):
        self.value = value
    def __get__(self, inst, cls):
        if inst is None:
            return self
        return self.value

>>> class C(object):
...     prop = FixedValueProperty('abc')
... 
>>> o = C()
>>> o.prop
'abc'
>>> C.prop
<__main__.FixedValueProperty object at 0xb7eb290c>
>>> C.prop.value
'abc'
>>> type(o).prop.value
'abc'

请注意,这也适用于(大多数?)内置描述符:

>>> class C(object):
...     @property
...     def prop(self):
...         return 'abc'
... 
>>> C.prop
<property object at 0xb7eb0b6c>
>>> C.prop.fget
<function prop at 0xb7ea36f4>

当您需要在子类中扩展描述符时,访问描述符可能很有用,但有一个 better way 这样做。

2

3

uayebforever 9 年前

这个 inspect 库提供了一个无需任何描述符即可检索属性的函数: inspect.getattr_static .

https://docs.python.org/3/library/inspect.html#fetching-attributes-statically

(这是一个老问题,但当我试图记住如何做这件事时,我不断遇到它,所以我发布了这个答案,以便我能再次找到它!)

3

0

innov8 10 年前

上述方法

class FixedValueProperty(object):
    def __init__(self, value):
        self.value = value
    def __get__(self, inst, cls):
        if inst is None:
            return self
        return self.value

考虑一个具有简单属性的类:

class ClassWithProp:

    @property
    def value(self):
        return 3
>>>test=ClassWithProp()
>>>test.value
3
>>>test.__class__.__dict__.['value']
<property object at 0x00000216A39D0778>

从容器对象类访问时字典 ,跳过“描述符魔术”。还要注意的是,如果我们将属性分配给一个新的类变量,它的行为就像原始的“描述符魔术”,但如果分配给一个实例变量,该属性的行为就像任何普通对象一样,也会绕过“描述符魔术”。

>>> test.__class__.classvar =  test.__class__.__dict__['value']
>>> test.classvar
3
>>> test.instvar = test.__class__.__dict__['value']
>>> test.instvar
<property object at 0x00000216A39D0778>

4

0

extratype 8 年前

obj.prop 哪里 type(obj) is C .

C.prop 通常有效,因为描述符在通过访问时通常会返回自身 C (即,必须 ). 但是 prop 他们没有出席会议 obj , AttributeError 虽然 C.支柱 可能不会。所以最好使用 inspect.getattr_static(obj, 'prop') .

_PyObject_GenericGetAttrWithDict 在里面 Objects/object.c

import ctypes, _ctypes

_PyType_Lookup = ctypes.pythonapi._PyType_Lookup
_PyType_Lookup.argtypes = (ctypes.py_object, ctypes.py_object)
_PyType_Lookup.restype = ctypes.c_void_p

def type_lookup(ty, name):
    """look for a name through the MRO of a type."""
    if not isinstance(ty, type):
        raise TypeError('ty must be a type')

    result = _PyType_Lookup(ty, name)
    if result is None:
        raise AttributeError(name)

    return _ctypes.PyObj_FromPtr(result)

type_lookup(type(obj), 'prop') 以与CPython在上使用描述符相同的方式返回描述符 物体支柱 如果 obj 是一个常用对象(例如,不是类)。