我可以像这样创建从元类派生的子类吗？

Question

我试图创建一个从父类的元类继承一个类，我不喜欢说：

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给了我下面的消息

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

我怎样才能做这样的事情？

预先感谢您。

Answer 1

在运行__init__之前评估课程正文。由于Base不依赖于__init__参数，你可以在评课时评价它：

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

请注意，您正在使用Base作为超，而不是一个元类;元类语法是__metaclass__ = Base或(metaclass=Base)，具体取决于版本。请参阅What is a metaclass in Python?