Python元类默认属性

我试图在Python（2.7）中创建元类，该元类将设置作为对象属性传递给对象的

\uuuu init\uuuu

的参数

class AttributeInitType(type):        
    def __call__(self, *args, **kwargs):
        obj = super(AttributeInitType, self).__call__(*args, **kwargs)
        for k, v in kwargs.items():
            setattr(obj, k, v)
        return obj

用法：

class Human(object):
    __metaclass__ = AttributeInitType

    def __init__(self, height=160, age=0, eyes="brown", sex="male"):
        pass

man = Human()

问题：我希望

man

实例将默认属性设置为类的

\uuuuu init\uuuu

中的属性。我怎么做

更新：我找到了更好的解决方案：

• 在类创建过程中只检查一次

  \uuuuuuu init\uuuuu

  方法
• 不重写由类的real

  \uuuuu init\uuuuu

代码如下：

import inspect
import copy

class AttributeInitType(type):
    """Converts keyword attributes of the init to object attributes"""
    def __new__(mcs, name, bases, d):
        # Cache __init__ defaults on a class-level
        argspec = inspect.getargspec(d["__init__"])
        init_defaults = dict(zip(argspec.args[-len(argspec.defaults):], argspec.defaults))
        cls = super(AttributeInitType, mcs).__new__(mcs, name, bases, d)
        cls.__init_defaults = init_defaults
        return cls

    def __call__(mcs, *args, **kwargs):
        obj = super(AttributeInitType, mcs).__call__(*args, **kwargs)
        the_kwargs = copy.copy(obj.__class__.__init_defaults)
        the_kwargs.update(kwargs)
        for k, v in the_kwargs.items():
            # Don't override attributes set by real __init__
            if not hasattr(obj, k):
                setattr(obj, k, v)
        return obj

---

如果在对象创建时传递参数，则情况会正常

>>> man
<test.Human object at 0x10a71e810>
>>> dir(man)
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__metaclass__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__']
>>> man=Human(height=10)
>>> dir(man)
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__metaclass__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'height']
>>> man.height
10

男人 >>>主任（男） “UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU","大小","str","子类hook","weakref" >>>人=人（身高=10） >>>主任（男） “UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU","大小","结构","子类钩","高度", >>>身高 10 但它不适用于默认参数。为此，您必须从

\uuuu init\uuu

函数对象中专门提取它们

---

另一种方法是修饰

\uuuu init\uuuu

。

您需要反思

\uuuu init\uuuu

方法并从中提取任何默认值。在那里工作会有帮助的

函数返回参数名列表和默认值列表。您可以将这些信息结合起来，找到给定函数的默认参数规范，然后使用这些信息设置对象的属性：

import inspect

class AttributeInitType(type):        
    def __call__(self, *args, **kwargs):
        obj = super(AttributeInitType, self).__call__(*args, **kwargs)
        argspec = inspect.getargspec(obj.__init__)
        defaults = dict(zip(argspec.args[-len(argspec.defaults):], argspec.defaults))
        defaults.update(kwargs)
        for key, val in defaults.items():
            setattr(obj, key, val)
        return obj

使用上述元类，您可以省略任何参数，它们将在新实例上设置，或者您可以通过显式传递它们来覆盖它们：

>>> man = Human()
>>> man.age
0
>>> man.height
160
>>> Human(height=180).height
180

---

在我的框架中，我使用*args/**kwargs、一个类默认字典以及对我的基本对象init中的属性设置器的调用来实现这一点。我觉得这比装饰更简单，比元类更简单

Class Base(object):
    defaults = {"default_attribute" : "default_value"}

    def __init__(self, *args, **kwargs):
        super(Base, self).__init__()
        self.attribute_setter(self.defaults.items(), *args, **kwargs)

    def attribute_setter(self, *args, **kwargs):
        if args: # also allow tuples of name/value pairs
            for attribute_name, attribute_value in args:
                setattr(self, attribute_name, attribute_value)
        [setattr(self, name, value) for name, value in kwargs.items()]
b = Base(testing=True)
# print b.testing
# True
# print b.default_attribute
# "default_value"

通过将任意属性指定为关键字参数（或名称/值对的位置参数元组），这种组合允许在运行时通过init分配任意属性

---

类默认值字典用于提供默认参数，而不是init的参数列表中显式命名的关键字参数。这使得创建新实例时使用的默认属性在运行时可修改。您可以通过dict.copy+dict.update“继承”类词典。

@ZaarHai：如果您有任何位置参数，它们将添加到

args

列表中，但不会添加到

defaults

元组中。我明白了。谢谢如果你能在答案中加上这个注释，那就太好了。@ZaarHai:我链接到了argspec文档，其中详细介绍了这一点。