__new__和__init__的Python多重繼承與字符串和第二類

Question

我試圖創建一個從str類型和第二類繼承的派生類。這是問題的，因爲str類型並不簡單地調用__init__，但__new__方法由於其不變性。我知道，__init__和超工作得很好，你需要有相同的調用結構一路下滑。但是下面的實現失敗：

class base(object): 
    def __new__(cls, *args, **kwargs): 
     print "NEW BASE:", cls, args, kwargs 
     return super(base, cls).__new__(cls, *args, **kwargs) 

    def __init__(self, *args, **kwargs): 
     print "INIT BASE", args, kwargs 

class foo(base, str): 
    def __new__(cls, *args, **kwargs): 
     return super(foo, cls).__new__(cls, *args, **kwargs) 

    def __init__(self, *args, **kwargs): 
     super(foo, self).__init__(*args, **kwargs) 

這裏foo('cat')作品有：

>> NEW BASE: <class '__main__.foo'> ('cat',) {} 
>> INIT BASE ('cat',) {} 

但有一個說法foo('cat', x=3)，它失敗：

>> NEW BASE: <class '__main__.foo'> ('cat',) {'x': 3} 
Traceback (most recent call last): 
    File "inh.py", line 19, in <module> 
    foo('cat', x=3) 
    File "inh.py", line 12, in __new__ 
    return super(foo, cls).__new__(cls, *args, **kwargs) 
    File "inh.py", line 4, in __new__ 
    return super(base, cls).__new__(cls, *args, **kwargs) 
TypeError: str() takes at most 1 argument (2 given) 

我可以得到這個由工作改變base.__new__方法：

def __new__(cls, *args, **kwargs): 
    return super(base, cls).__new__(cls) 

但現在我已經改變了調用結構，我覺得會在晚些時候導致我的問題。

如何正確地從一個字符串和第二類繼承？

Answer 1

你不能只是做

def __new__(cls, *args, **kwargs): 
    return super(base, cls).__new__(cls) 

，因爲這會導致不正確的呼籲海峽的新（你會不會通過允許參數

>>> foo('t') 
NEW BASE: <class '__main__.foo'> ('t',) {} 
INIT BASE ('t',) {} 
'' 

你應該這樣做

def __new__(cls, *args, **kwargs): 
    return super(base, cls).__new__(cls, *args[:1]) 

但如果這能打碎了什麼東西您將使用base類是混入了類__new__方法接受多個參數。

作爲一個選項，也許你應該有類從str繼承但覆蓋新方法：

class CarelessStr(str): 
    def __new__(cls, *args, **kwargs): 
     return super(CarelessStr, cls).__new__(cls, *args[:1]) 

class foo(base, CarelessStr): 
    def __new__(cls, *args, **kwargs): 
     return super(foo, cls).__new__(cls, *args, **kwargs) 

    def __init__(self, *args, **kwargs): 
     super(foo, self).__init__(*args, **kwargs)