Python __setattr__、 __getattr__、 __delattr__、__call__用法示例

getattr

`getattr`函数属于内建函数，可以通过函数名称获取

value = obj.attribute

value = getattr(obj, "attribute")

使用`getattr`来实现工厂模式

#一个模块支持html、text、xml等格式的打印，根据传入的formate参数的不同，调用不同的函数实现几种格式的输出
import statsout
def output(data, format="text"):                           

    output_function = getattr(statsout, "output_%s" %format) 

    return output_function(data)

__call__

`__call__`方法用于实例自身的调用：

class storage(dict):

    # __call__方法用于实例自身的调用

    #达到()调用的效果

    def __call__ (self, key):

         try:

             return self[key]

         except KeyError, k:

             return None
s = storage()

s['key'] = 'value'

print s(key) #调用__call__

__getattr__

从对象中读取某个属性时，首先需要从self.__dicts__中搜索该属性，再从__getattr__中查找。

class A(object):  

    def __init__(self):  

        self.name = 'from __dicts__: zdy'  

  

    def __getattr__(self, item):  

        if item == 'name':  

            return 'from __getattr__: zdy'  

        elif item == 'age':  

            return 26  

  

a = A()  

print a.name # 从__dict__里获得的  

print a.age # 从__getattr__获得的

__setattr__

`__setattr__`函数是用来设置对象的属性，通过object中的__setattr__函数来设置属性：

class A(object):

    def __setattr__(self, *args, **kwargs):  

        print 'call func set attr'  

        return object.__setattr__(self, *args, **kwargs)

__delattr__

`__delattr__`函数式用来删除对象的属性：

class A(object):

    def __delattr__(self, *args, **kwargs):  

        print 'call func del attr'  

        return object.__delattr__(self, *args, **kwargs) 

例子

完整例子可以参考微博API：http://github.liaoxuefeng.com/sinaweibopy/

class _Executable(object):
    def __init__(self, client, method, path):

        self._client = client

        self._method = method

        self._path = path

    #__call__函数实现_Executable函数对象为可调用的

    def __call__(self, **kw):

        method = _METHOD_MAP[self._method]

        if method==_HTTP_POST and 'pic' in kw:

            method = _HTTP_UPLOAD

        return _http_call('%s%s.json' % (self._client.api_url, self._path), method, self._client.access_token, **kw)
    def __str__(self):

        return '_Executable (%s %s)' % (self._method, self._path)
    __repr__ = __str__
class _Callable(object):
    def __init__(self, client, name):

        self._client = client

        self._name = name
    def __getattr__(self, attr):

        if attr=='get':



　#初始化_Executable对象，调用__init__函数

            return _Executable(self._client, 'GET', self._name)

        if attr=='post':

            return _Executable(self._client, 'POST', self._name)

        name = '%s/%s' % (self._name, attr)

        return _Callable(self._client, name)
    def __str__(self):

        return '_Callable (%s)' % self._name
    __repr__ = __str__

而在源码中，存在下面代码片段：

class APIClient(object):

    '''

    API client using synchronized invocation.

    '''

    ...
    def __getattr__(self, attr):

        if '__' in attr:

            return getattr(self.get, attr)

        return _Callable(self, attr)

因此，加入我们初始化对象，并调用某函数如下：

client = APIClient(...)

#会调用__getattr__函数，从而调用__call__函数

client.something.get()