Can I create a subclass derived from metaclass like this?

Question

I tried to create a class that inherit from a metaclass declared in the superclass, I did like that:

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)

Python gives me the following message

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

How can I do something like this?

Thank you in advance.

There is no `self` in the class scope when the outer class is defined. Why do you want to do something like this, anyway? — Keith, Feb 14 '13 at 16:58
How about using the usual method and put all the classes in a module? — Keith, Feb 14 '13 at 17:27
I was thinking about that seriously, this could be the best way. — zorro, Feb 14 '13 at 20:05

score 0 · Accepted Answer · edited May 23 '17 at 12:12

A class body is evaluated before __init__ runs. Since Base doesn't depend on __init__ parameters, you can evaluate it at class evaluation time:

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

Note that you're using Base as a superclass, not a metaclass; metaclass syntax is __metaclass__ = Base or (metaclass=Base) depending on version. See What is a metaclass in Python?

Can I create a subclass derived from metaclass like this?

1 Answers1