python - SQLAlchemy - what is declarative_base

Question

I am learning sqlalchemy.
Here is my initial code :

user.py

from sqlalchemy import Column, Integer, Sequence, String
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()

class User(Base):
   __tablename__ = 'users'
   id = Column(Integer,Sequence('user_seq'), primary_key=True)
   username = Column(String(50), unique=True)
   fullname = Column(String(150))
   password = Column(String(50))
   def __init__(self, name, fullname, password):
      self.name = name
      self.fullname = fullname
      self.password = password

main.py

from sqlalchemy.orm import sessionmaker
from sqlalchemy import create_engine
from user import User
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()

if __name__ == '__main__':
   engine = create_engine('mysql://root:[email protected]:3306/test', echo=True)
   Base.metadata.create_all(engine, checkfirst=True)
   Session = sessionmaker(bind=engine)
   session = Session()
   ed_user = User('ed', 'Ed Jones', 'edspassword')
   session.add(ed_user)
   session.commit()

When I run main.py, it won't create tables automatically and gives me an exception on session.commit().

Also, when I move line Base = declarative_base() to any different module and use the same Base variable in main.py and in user.py - it creates the table.

My question: "What is declarative_base" ?

question from:https://stackoverflow.com/questions/15175339/sqlalchemy-what-is-declarative-base

Answer 1

declarative_base() is a factory function that constructs a base class for declarative class definitions (which is assigned to the Base variable in your example). The one you created in user.py is associated with the User model, while the other one (in main.py) is a different class and doesn't know anything about your models, that's why the Base.metadata.create_all() call didn't create the table. You need to import Base from user.py

from user import User, Base

instead of creating a new Base class in main.py.