为了扩展Michael answer，如果你想使用__getattr__保持默认行为，你可以这样做:

class Foo(object):
    def __getattr__(self, name):
        if name == 'something':
            return 42

        # Default behaviour
        return self.__getattribute__(name)

现在异常消息更具描述性:

>>> foo.something
42
>>> foo.error
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in __getattr__
AttributeError: 'Foo' object has no attribute 'error'

为了扩展Michael answer，如果你想使用__getattr__保持默认行为，你可以这样做:

class Foo(object):
    def __getattr__(self, name):
        if name == 'something':
            return 42

        # Default behaviour
        return self.__getattribute__(name)

现在异常消息更具描述性:

>>> foo.something
42
>>> foo.error
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in __getattr__
AttributeError: 'Foo' object has no attribute 'error'

class A(object):
    def __init__(self):
        self.a = 42

    def __getattr__(self, attr):
        if attr in ["b", "c"]:
            return 42
        raise AttributeError("%r object has no attribute %r" %
                             (self.__class__.__name__, attr))

>>> a = A()
>>> a.a
42
>>> a.b
42
>>> a.missing
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 8, in __getattr__
AttributeError: 'A' object has no attribute 'missing'
>>> hasattr(a, "b")
True
>>> hasattr(a, "missing")
False

重写__getattr__应该没问题——__getattr__只会在最后的情况下调用，即在实例中没有匹配该名称的属性时调用。例如，如果你访问foo。Bar，那么__getattr__只会在foo没有名为Bar的属性时被调用。如果该属性是你不想处理的，则引发AttributeError:

class Foo(object):
    def __getattr__(self, name):
        if some_predicate(name):
            # ...
        else:
            # Default behaviour
            raise AttributeError

然而，与__getattr__不同，__getattribute__将首先被调用(仅适用于新样式类，即继承自object的类)。在这种情况下，你可以像这样保留默认行为:

class Foo(object):
    def __getattribute__(self, name):
        if some_predicate(name):
            # ...
        else:
            # Default behaviour
            return object.__getattribute__(self, name)

请参阅Python文档了解更多信息。