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Type annotation strings

About the String in List["Hero"]

In the first Relationship attribute, we declare it with List["Hero"], putting the Hero in quotes instead of just normally there:

from typing import List, Optional

from sqlmodel import Field, Relationship, Session, SQLModel, create_engine


class Team(SQLModel, table=True):
    id: Optional[int] = Field(default=None, primary_key=True)
    name: str
    headquarters: str

    heroes: List["Hero"] = Relationship(back_populates="team")


class Hero(SQLModel, table=True):
    id: Optional[int] = Field(default=None, primary_key=True)
    name: str
    secret_name: str
    age: Optional[int] = None

    team_id: Optional[int] = Field(default=None, foreign_key="team.id")
    team: Optional[Team] = Relationship(back_populates="heroes")

# Code below omitted ๐Ÿ‘‡
๐Ÿ‘€ Full file preview
from typing import List, Optional

from sqlmodel import Field, Relationship, Session, SQLModel, create_engine


class Team(SQLModel, table=True):
    id: Optional[int] = Field(default=None, primary_key=True)
    name: str
    headquarters: str

    heroes: List["Hero"] = Relationship(back_populates="team")


class Hero(SQLModel, table=True):
    id: Optional[int] = Field(default=None, primary_key=True)
    name: str
    secret_name: str
    age: Optional[int] = None

    team_id: Optional[int] = Field(default=None, foreign_key="team.id")
    team: Optional[Team] = Relationship(back_populates="heroes")


sqlite_file_name = "database.db"
sqlite_url = f"sqlite:///{sqlite_file_name}"

engine = create_engine(sqlite_url, echo=True)


def create_db_and_tables():
    SQLModel.metadata.create_all(engine)


def create_heroes():
    with Session(engine) as session:
        team_preventers = Team(name="Preventers", headquarters="Sharp Tower")
        team_z_force = Team(name="Z-Force", headquarters="Sister Margaretโ€™s Bar")

        hero_deadpond = Hero(
            name="Deadpond", secret_name="Dive Wilson", team=team_z_force
        )
        hero_rusty_man = Hero(
            name="Rusty-Man", secret_name="Tommy Sharp", age=48, team=team_preventers
        )
        hero_spider_boy = Hero(name="Spider-Boy", secret_name="Pedro Parqueador")
        session.add(hero_deadpond)
        session.add(hero_rusty_man)
        session.add(hero_spider_boy)
        session.commit()

        session.refresh(hero_deadpond)
        session.refresh(hero_rusty_man)
        session.refresh(hero_spider_boy)

        print("Created hero:", hero_deadpond)
        print("Created hero:", hero_rusty_man)
        print("Created hero:", hero_spider_boy)

        hero_spider_boy.team = team_preventers
        session.add(hero_spider_boy)
        session.commit()


def main():
    create_db_and_tables()
    create_heroes()


if __name__ == "__main__":
    main()

What's that about? Can't we just write it normally as List[Hero]?

By that point, in that line in the code, the Python interpreter doesn't know of any class Hero, and if we put it just there, it would try to find it unsuccessfully, and then fail. ๐Ÿ˜ญ

But by putting it in quotes, in a string, the interpreter sees it as just a string with the text "Hero" inside.

But the editor and other tools can see that the string is actually a type annotation inside, and provide all the autocompletion, type checks, etc. ๐ŸŽ‰

And of course, SQLModel can also understand it in the string correctly. โœจ

That is actually part of Python, it's the current official solution to handle it.

Info

There's a lot of work going on in Python itself to make that simpler and more intuitive, and find ways to make it possible to not wrap the class in a string.