如何将 __len__ 添加到 sqlalchemy 声明性元？

Question

I have added the __len__ method to the User class as shown below:

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import func

Base = declarative_base()

class User(Base):
    __tablename__ = "user"
    id = Column(INTEGER(unsigned=True), autoincrement=True, primary_key=True)

    def __len__() -> int:
        try:
            session = Session()
            return session.query(func.count(User.id)).scalar()
        except:
            raise
        finally:
            session.close()

Calling User.__len__() works fine. However, calling len(User) yields the following error:

TypeError: object of type 'DeclarativeMeta' has no len()

I am not exactly sure how to adjust the class, such that len(User) is recognised as a valid statement.

Answer 1

I guess you are trying to find the length of the object of class User not the length of the User class itself?

If you do:

user = User()
len(user)

it will not have the same error

Answer 2

Finally works, needed to call __init__ .

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import func

Base = declarative_base()

class User(Base):
    __tablename__ = "user"
    id = Column(INTEGER(unsigned=True), autoincrement=True, primary_key=True)

    def __init__(self, *args, **kwargs) -> None:
        super(User, self).__init__(*args, **kwargs)

    def __len__(self) -> int:
        try:
            session = Session()
            return session.query(func.count(User.id)).scalar()
        except:
            raise
        finally:
            session.close()

And also instead of calling len(User) , I should have used len(User()) . (Thanks @Cheukting for this)