我可以像這樣創建從元類派生的子類嗎？

Question

我試圖創建一個從父類的元類繼承一個類，我不喜歡說：

from sqlalchemy import Column, String, create_engine 
from sqlalchemy.ext.declarative import declarative_base 
from sqlalchemy.orm import sessionmaker 


class Database(object): 
    """ 
     Class that holds all data models classes and functions 
    """ 
    def __init__(self, wenDbPath = "test.db"): 
     self.engine = create_engine('sqlite:///' + "c:/"+wenDbPath, echo=False) 
     self.Session = sessionmaker(bind=self.engine) 
     **self.Base** = declarative_base() # This returns a metaclass. 
     self.Base.metadata.create_all(self.engine) 
     self.session = self.Session() 

    class Element(**self.Base**): 
     __tablename__ = 'elements' 
     idNumber = Column(String(255), primary_key=True) 
     smallDiscription = Column(String(50), nullable=False) 
     longDiscription = Column(String()) 
     webLink = Column(String()) 

     def __init__(self, idNumber, smallDiscription, longDiscription, webLink): 
      self.idNumber = idNumber 
      self.longDiscription = longDiscription 
      self.smallDiscription = smallDiscription 
      self.webLink = webLink 
     def __repr__(self): 
      return "<Element ('%s : %s')>" % (self.idNumber, self.smallDiscription) 

的Python給了我下面的消息

class Element(self.Base): NameError: name 'self' is not defined

我怎樣才能做這樣的事情？

預先感謝您。

Answer 1

在運行__init__之前評估課程正文。由於Base不依賴於__init__參數，你可以在評課時評價它：

class Database(object): 
    ... 
    Base = declarative_base() # This returns a metaclass. 
    class Element(Base): 
     ... 

請注意，您正在使用Base作爲超，而不是一個元類;元類語法是__metaclass__ = Base或(metaclass=Base)，具體取決於版本。請參閱What is a metaclass in Python?