Re: [sqlalchemy] Difference b/w creating a DeclarativeBase class vs assigning DeclarativeBase()

[Simon King]

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.

https://docs.sqlalchemy.org/en/20/changelog/whatsnew_20.html#whatsnew-20-orm-declarative-typing

Hope that helps,

Simon

On Tue, Oct 10, 2023 at 8:12 AM satya dev  wrote:

> 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)
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
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> 
> .
>

-- 
SQLAlchemy - 
The Python SQL Toolkit and Object Relational Mapper

http://www.sqlalchemy.org/

To post example code, please provide an MCVE: Minimal, Complete, and Verifiable 
Example.  See  http://stackoverflow.com/help/mcve for a full description.
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[sqlalchemy] Difference b/w creating a DeclarativeBase class vs assigning DeclarativeBase()

[satya dev]

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)

-- 
SQLAlchemy - 
The Python SQL Toolkit and Object Relational Mapper

http://www.sqlalchemy.org/

To post example code, please provide an MCVE: Minimal, Complete, and Verifiable 
Example.  See  http://stackoverflow.com/help/mcve for a full description.
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