我可以在__new__或__init__期间创建类属性吗？

Question

我想做这样的事情,但到目前为止我还没有取得多大成功.我想让每个attr只在访问时计算_lazy_eval的属性:

class Base(object):
    def __init__(self):
        for attr in self._myattrs:
            setattr(self, attr, property(lambda self: self._lazy_eval(attr)))

    def _lazy_eval(self, attr):
        #Do complex stuff here
        return attr


class Child(Base):
    _myattrs = ['foo', 'bar']


me = Child()
print me.foo
print me.bar

#desired output:
#"foo"
#"bar"

**更新**

这也不起作用:

class Base(object):
    def __new__(cls):
        for attr in cls._myattrs:
            setattr(cls, attr, property(lambda self: self._lazy_eval(attr)))
        return object.__new__(cls)

#Actual output (it sets both .foo and .bar equal to "bar"??)
#bar
#bar

**更新2**

使用了__metaclass__解决方案,但坚持使用它Base.__new__.看起来它需要一个更好定义的闭包 - "prop()" - 正确地形成属性:

class Base(object):
    def __new__(cls):
        def prop(x):
            return property(lambda self: self._lazy_eval(x))
        for attr in cls._myattrs:
            setattr(cls, attr, prop(attr))
        return object.__new__(cls)

#Actual output!  It works!
#foo
#bar

Answer 1

描述符(例如property类型的实例)仅在它们保存在类对象中时才有意义,而不是实例对象.因此,您需要更改类,而不是实例,并且(在Python 2.6或更高版本中)类装饰器非常方便用于此目的:

class Base(object):
    def _lazy_eval(self, attr):
        #Do complex stuff here
        return attr

def lazyclass(cls):
    for attr in cls._myattrs:
        setattr(cls, attr, property(lambda self: self._lazy_eval(attr)))
    return cls

@lazyclass
class Child(Base):
    _myattrs = ['foo', 'bar']

如果您坚持使用Python 2.5或更早版本,装饰器语法不适用于类,但很容易获得相同的效果,只需使用较少漂亮的语法 - 将最后3行更改为:

class Child(Base):
    _myattrs = ['foo', 'bar']
Child = lazyclass(Child)

它与类装饰器语法具有相同的语义.

Answer 2

从技术上讲，您需要一个元类：

class LazyMeta(type):
    def __init__(cls, name, bases, attr):
        super(LazyMeta, cls).__init__(name, bases, attr)
        def prop( x ):
            return property(lambda self: self._lazy_eval(x))
        for x in attr['lazyattrs']:
            setattr(cls, x, prop(x))

class Base(object):
    __metaclass__ = LazyMeta
    lazyattrs = []
    def _lazy_eval(self, attr):
        #Do complex stuff here
        return attr

class Child(Base):
    lazyattrs = ['foo', 'bar']

me = Child()

print me.foo
print me.bar