的Python元類來生成

Question

類正確枚舉值我previous question被問及枚舉蟒蛇

比方說，我創建國家類：

class State(object): 
    """ 
    Abstract class representing a state. 
    """ 
    enum = 1 

    def __init__(self, instance): 
     self.instance = instance 

    def is_flag(self, flag): 
     return flag & self.enum == self.enum 

然後我創建不同的實際狀態

class New(State): 
    enum = 2 
    def pay(self): 
     ... 

class Paid(State): 
    enum = 4 
    def complete(self): 
     ... 

class Completed(State): 
    enum = 8 


ACCOUNTABLE = Paid.enum | Completed.enum 

雖然這個工作，我想自動生成的枚舉值，它似乎可以通過使用Meta類來完成，問題是如何？

Answer 1

雖然我不不認爲爲每個枚舉值定義一個單獨的類是a由於我評論中陳述的原因非常穩健的方法，這裏有一種方法可以完成，這將支持創建多個獨立的基礎狀態類。

class MetaState(type): 
    _baseclasses = {} # registry of instances & number of subclasses of each 
    def __new__(cls, name, bases, attrs): 
     cls = super(MetaState, cls).__new__(cls, name, bases, attrs) 
     if bases == (object,): # definition of a base class? 
      MetaState._baseclasses[cls] = 0 # create initial registry entry 
     else: # must be derived from a previously defined base state class 
      for base in bases: # find base state class 
       if base in MetaState._baseclasses: 
        setattr(cls, 'enum', 2 ** MetaState._baseclasses[base]) 
        MetaState._baseclasses[base] += 1 
        break 
      else: 
       raise TypeError('Class not derived from base state class') 
     return cls 

class BaseState(object): 
    """ Abstract base class for each derived state subclass. """ 
    __metaclass__ = MetaState 

    def is_flag(self, flag): 
     return flag & self.enum == self.enum 

class A(BaseState): pass 
class B(BaseState): pass 
class C(BaseState): pass 

print A.enum # -> 1 
print B.enum # -> 2 
print C.enum # -> 4 

Answer 2

Python 3.4有一個Enum data type，其中has been backported。

from enum import IntEnum 

States = IntEnum('States', [(n, 2**i) for i, n in enumerate('New Paid Complete'.split(), 1)]) 

list(States) # [<States.New: 2>, <States.Paid: 4>, <States.Complete: 8>] 

或

class States(IntEnum): 
    New = 2 
    Paid = 4 
    Complete = 8 
    def is_flag(self, flag): 
     return self & flag == flag 

Answer 3

我還拿出一個解決方案（需要修理），我的回答：

class StateMeta(type): 

    def __new__(cls, name, bases, attrs): 
     cls = super(StateMeta, cls).__new__(cls, name, bases, attrs) 
     if bases[-1] == object: 
      return cls 

     setattr(cls, 'enum', 2 ** (len(bases[-1].__subclasses__())-1)) 
     return cls 


class State(object): 
    """ 
    Abstract class representing a state. 
    """ 
    __metaclass__ = StateMeta 

    enum = 0 

    def __init__(self, instance): 
     self.instance = instance 

    def is_flag(self, flag): 
     return flag & self.enum == self.enum 

運行它：

>>> class A(State): 
...  pass 
... 
>>> A.enum 
1 
>>> class B(State): 
...  pass 
... 
>>> B.enum 
2