python __setattr__, __getattr__, __delattr__, __call__

 getattr

`getattr`函數屬於內建函數，可以通過函數名稱獲取

value = obj.attribute
value = getattr(obj, "attribute")

使用`getattr`來實現工廠模式

#一個模塊支持html、text、xml等格式的打印，根據傳入的formate參數的不同，調用不同的函數實現幾種格式的輸出

import statsout 

def output(data, format="text"):                           
    output_function = getattr(statsout, "output_%s" %format) 
    return output_function(data)

__call__

`__call__`方法用於實例自身的調用：

class storage(dict):
    # __call__方法用於實例自身的調用
    #達到()調用的效果
    def __call__ (self, key):
         try:
             return self[key]
         except KeyError, k:
             return None

s = storage()
s['key'] = 'value'
print s(key) #調用__call__

__getattr__

從對象中讀取某個屬性時，首先需要從self.__dicts__中搜索該屬性，再從__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 # 從__dict__里獲得的  
print a.age # 從__getattr__獲得的  

__setattr__

`__setattr__`函數是用來設置對象的屬性，通過object中的__setattr__函數來設置屬性：

class A(object):
    def __setattr__(self, *args, **kwargs):  
        print 'call func set attr'  
        return object.__setattr__(self, *args, **kwargs) 

__delattr__

`__delattr__`函數式用來刪除對象的屬性：

class A(object):
    def __delattr__(self, *args, **kwargs):  
        print 'call func del attr'  
        return object.__delattr__(self, *args, **kwargs)  

例子

完整例子可以參考微博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__函數實現_Executable函數對象為可調用的 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對象，調用__init__函數 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__

而在源碼中，存在下面代碼片段：

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，加入我們初始化對象，並調用某函數如下：

client = APIClient(...)
#會調用__getattr__函數，從而調用__call__函數
client.something.get()