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都是一个只在访问时计算_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__metaclass__解决方案，但将其粘贴在Base.__new__中。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']

如果您遇到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，你需要一个元类：

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

Answer 3

您可以通过__ class __ dict

访问类属性

self.__class__.attr

Answer 4

您可以考虑使用__getattr__()代替：

class Base(object):
    def __getattr__(self, attr):
        if attr not in self._myattrs:
            raise AttributeError
        return 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