Python开发【笔记】： __get__和__getattr__和__getattribute__区别

引言：

1.object.__getattr__(self, name)
当一般位置找不到attribute的时候，会调用getattr，返回一个值或AttributeError异常。

2.object.__getattribute__(self, name)
无条件被调用，通过实例访问属性。如果class中定义了__getattr__()，则__getattr__()不会被调用（除非显示调用或引发AttributeError异常）

3.object.__get__(self, instance, owner)
如果class定义了它，则这个class就可以称为descriptor。owner是所有者的类，instance是访问descriptor的实例，如果不是通过实例访问，而是通过类访问的话，instance则为None。（descriptor的实例自己访问自己是不会触发__get__，而会触发__call__，只有descriptor作为其它类的属性才有意义。）（所以下文的d是作为C2的一个属性被调用）

示例1：

# __getattribute__、__getattr__、__get__
class C(object):
    a = 'abc'

    def __getattribute__(self, *args, **kwargs):
        print("__getattribute__() is called")
        return object.__getattribute__(self, *args, **kwargs)

    def __getattr__(self, name):
        print("__getattr__() is called ")
        return name + " from getattr"

    def __get__(self, instance, owner):
        print("__get__() is called", instance, owner)
        return self

    def foo(self, x):
        print(x)

class C2(object):
    d = C()

if __name__ == '__main__':
    c = C()
    c2 = C2()
    print(c.a)
    # __getattribute__() is called
    # abc
    print(c.b)
    # __getattribute__() is called
    # __getattr__() is called
    # b from getattr
    c2.d
    # __get__() is called <__main__.C2 object at 0x0000007D8B0E7D68> <class '__main__.C2'>
    print(c2.d.a)
    # __get__() is called <__main__.C2 object at 0x000000B6FDAE7D68> <class '__main__.C2'>
    # __getattribute__() is called
    # abc

示例2：

class LazyProperty:

    def __init__(self, method):
        print(method,method.__name__)
        self.method = method
        self.method_name = method.__name__
        # print('function overriden: {}'.format(self.method))
        # print("function's name: {}".format(self.method_name))

    def __get__(self, obj, cls):
        print('__get__',obj,cls)
        if not obj:
            return None
        value = self.method(obj)
        # print('value {}'.format(value))
        setattr(obj, self.method_name, value)
        return value

class Jefrey:

    def __init__(self):
        self.x = 'foo'
        self.y = 'bar'
        self._resource = None

    @LazyProperty
    def resource(self):
        print('initializing self._resource which is: {}'.format(self._resource))
        self._resource = tuple(range(5))    # 代价大的
        return self._resource

def main():
    t = Jefrey()
    # <function Jefrey.resource at 0x00000089690E6730> resource
    print(t.x)
    print(t.y)
    # foo
    # bar
    # 做更多的事情。。。
    print(t.resource)
    # __get__ <__main__.Jefrey object at 0x00000089690E7C50> <class '__main__.Jefrey'>
    # initializing self._resource which is: None
    # (0, 1, 2, 3, 4)
    print(t.resource)
    # (0, 1, 2, 3, 4)

if __name__ == '__main__':
    main()