Python:如何实现__getattr __()？

Question

我的班级有一个词典,例如:

class MyClass(object):
    def __init__(self):
        self.data = {'a': 'v1', 'b': 'v2'}

然后我想使用dict的密钥和MyClass实例来访问dict,例如:

ob = MyClass()
v = ob.a   # Here I expect ob.a returns 'v1'

我知道这应该由__getattr__实现,但我是Python的新手,我不知道如何实现它.

Answer 1

class MyClass(object):

    def __init__(self):
        self.data = {'a': 'v1', 'b': 'v2'}

    def __getattr__(self, attr):
        return self.data[attr]

>>> ob = MyClass()
>>> v = ob.a
>>> v
'v1'

实施时要小心__setattr__,你需要做一些修改:

class MyClass(object):

    def __init__(self):
        # prevents infinite recursion from self.data = {'a': 'v1', 'b': 'v2'}
        # as now we have __setattr__, which will call __getattr__ when the line
        # self.data[k] tries to access self.data, won't find it in the instance 
        # dictionary and return self.data[k] will in turn call __getattr__
        # for the same reason and so on.... so we manually set data initially
        super(MyClass, self).__setattr__('data', {'a': 'v1', 'b': 'v2'})

    def __setattr__(self, k, v):
        self.data[k] = v

    def __getattr__(self, k):
        # we don't need a special call to super here because getattr is only 
        # called when an attribute is NOT found in the instance's dictionary
        try:
            return self.data[k]
        except KeyError:
            raise AttributeError

>>> ob = MyClass()
>>> ob.c = 1
>>> ob.c
1

如果您不需要设置属性,只需使用namedtuple例如.

>>> from collections import namedtuple
>>> MyClass = namedtuple("MyClass", ["a", "b"])
>>> ob = MyClass(a=1, b=2)
>>> ob.a
1

如果你想要默认参数,你可以围绕它编写一个包装类:

class MyClass(namedtuple("MyClass", ["a", "b"])):

    def __new__(cls, a="v1", b="v2"):
        return super(MyClass, cls).__new__(cls, a, b)

或者它看起来更好看作为一个功能:

def MyClass(a="v1", b="v2", cls=namedtuple("MyClass", ["a", "b"])):
    return cls(a, b)

>>> ob = MyClass()
>>> ob.a
'v1'

Answer 2

迟到了,但找到了两个非常好的资源,可以更好地解释这一点(恕我直言).

正如解释在这里,你应该使用self.__dict__从内部访问的字段__getattr__,以避免无限递归.提供的示例是:

def __getattr__(self, attrName):
  if not self.__dict__.has_key(attrName):
     value = self.fetchAttr(attrName)    # computes the value
     self.__dict__[attrName] = value
  return self.__dict__[attrName]

注意:在第二行(上图)中,更多Pythonic方式(has_key显然甚至在Python 3中删除):

if attrName not in self.__dict__:

在其他资源解释说,__getattr__只有当对象没有找到,则调用,并且hasattr总是返回True如果有一个实现__getattr__.它提供了以下示例,以演示:

class Test(object):
    def __init__(self):
        self.a = 'a'
        self.b = 'b'

    def __getattr__(self, name):
        return 123456

t = Test()
print 'object variables: %r' % t.__dict__.keys()
#=> object variables: ['a', 'b']
print t.a
#=> a
print t.b
#=> b
print t.c
#=> 123456
print getattr(t, 'd')
#=> 123456
print hasattr(t, 'x')
#=> True