CODEWITHSUNDEEP
pythonsqlalchemy
Barka
Barka

Reputation: 8922

how to access a class attribute from within the class in Python

Consider the following SQLAlchemy code

class Foo(Base):
    __tablename__ = 'currency'

    id = Column(Integer, primary_key=True)
    name = Column(String(40), nullable=False, unique=True)
    symbol = Column(String(4), nullable=False, unique=True)

    l =  session.query(Foo.symbol, Foo.id).all()

In the last line, I am trying to generate a list of the symbol-id pairs. It produces the following error:

NameError: name 'Foo' is not defined

I tried the following and got the errors as specified:

l = session.query(models.Foo.symbol, models.Foo.id).all()
#note: The Foo object is in the models.py file
#error: NameError: name 'models' is not defined

l = session.query(symbol, id).all()
#error: sqlalchemy.exc.CompileError: Cannot compile Column object until its 'name' is assigned.

l = session.query(self.symbol, self.id).all()
#error: NameError: name 'self' is not defined

So how do I pass in Foo object's column names to the SqlAlhemy query from inside the Foo class?

Why am I doing this? I am then converting the list to a dictionary and only access the dictionary from code throughout the program since its values are seldom changed. So I want to populate it once and then access it many many times without hitting the DB again. And I want to keep it in the Foo Class which I believe is where it belongs.

If there is a better way to do this please do let me know.

Upvotes: 1

Views: 612

Answers (1)

wim
wim

Reputation: 362717

You can't use the class before it has been defined. And you can't use symbol and id yet, even though the names can be correctly resolved from within the class block itself, because the ORM hasn't had a chance to prepare the columns yet. This is one of tasks of declarative Base, and Foo will inherit a metaclass which needs to prepare the model.

Assuming you set up the session correctly, you should be able to dynamically contribute this attribute on the class outside of the class definition block:

class Foo(Base):
    __tablename__ = 'currency'

    id = Column(Integer, primary_key=True)
    name = Column(String(40), nullable=False, unique=True)
    symbol = Column(String(4), nullable=False, unique=True)

Foo.l =  session.query(Foo.symbol, Foo.id).all()

Upvotes: 1

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