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" ?

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.