Difference b/w creating a DeclarativeBase class vs assigning DeclarativeBase()

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satya dev

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Oct 10, 2023, 3:12:50 AM10/10/23
to sqlalchemy
What is the difference between
class Base(DeclarativeBase):
pass
vs
Base = DeclarativeBase()
I am following the SQLAlchemy Tutorial for declaring mapped classes when I inherit the Base class I can access the metadata and create my tables but when I assign it to Base and try to create the tables it throws an error saying
class User(Base):
TypeError: DeclarativeBase() takes no arguments


Code for reference:
from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column, relationship
from sqlalchemy import Integer, String, ForeignKey, MetaData, create_engine
from typing import List, Optional


engine = create_engine("postgresql://db_user:db_pw@localhost:5432/alembic_learning")
# class Base(DeclarativeBase):
# pass

Base = DeclarativeBase()

class User(Base):
__tablename__ = "users"

id: Mapped[int] = mapped_column(primary_key=True)
name:Mapped[str] = mapped_column(String(30))
fullname:Mapped[Optional[str]]

addresses:Mapped[List["Address"]] = relationship(back_populates="user")

def __repr__(self) -> str:
return f"User(id={self.id!r}, name={self.name!r}, fullname={self.fullname!r})"
class Address(Base):
__tablename__ = "address"

id:Mapped[int] = mapped_column(primary_key=True)
email_address:Mapped[str]
user_id = mapped_column(ForeignKey("users.id"))

user:Mapped[User] = relationship(back_populates="addresses")

def __repr__(self)-> str:
return f"Address(id={self.id!r}, email_address={self.email_address})"

Base.metadata.create_all(bind=engine)

Simon King

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Oct 10, 2023, 9:02:12 AM10/10/23
to sqlal...@googlegroups.com
I don't think this code was ever correct:

    Base = DeclarativeBase()

Before SQLAlchemy 2.0, there was a declarative_base() function that was used in the same way:

    from sqlalchemy.ext.declarative import declarative_base

    Base = declarative_base()

...but in SQLAlchemy 2.0, the preferred form is:

    from sqlalchemy.orm import DeclarativeBase

    class Base(DeclarativeBase):
        pass

The new approach works better with type-analysis tools like mypy.


Hope that helps,

Simon

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