[FIXED] Flask SQLAlchemy can't set attribute error in a venv

Issue

I'm learning authentication functionality with Flask-SQLAlchemy, so building out very basic apps where a user (attributes: name, email and password) is able to sign-up and login, get redirected to a dummy profile page, and that's it. The puzzling behaviour is the tutorials I've done won't work all the way through if I use a virtual environment.

To show this behaviour, follow this digital ocean tutorial. At step 6, we have:

>>> from project import db, create_app
>>> db.create_all(app=create_app())

If I have the auth environment active (as the tutorial states I should), I get the following error message:

/.../flask_auth_app/venv/lib/python3.8/site-packages/flask_sqlalchemy/__init__.py:833: FSADeprecationWarning: SQLALCHEMY_TRACK_MODIFICATIONS adds significant overhead and will be disabled by default in the future.  Set it to True or False to suppress this warning.
  warnings.warn(FSADeprecationWarning(
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/.../flask_auth_app/venv/lib/python3.8/site-packages/flask_sqlalchemy/__init__.py", line 1039, in create_all
    self._execute_for_all_tables(app, bind, 'create_all')
  File "/.../flask_auth_app/venv/lib/python3.8/site-packages/flask_sqlalchemy/__init__.py", line 1031, in _execute_for_all_tables
    op(bind=self.get_engine(app, bind), **extra)
  File "/.../flask_auth_app/venv/lib/python3.8/site-packages/flask_sqlalchemy/__init__.py", line 962, in get_engine
    return connector.get_engine()
  File "/.../flask_auth_app/venv/lib/python3.8/site-packages/flask_sqlalchemy/__init__.py", line 555, in get_engine
    options = self.get_options(sa_url, echo)
  File "/.../flask_auth_app/venv/lib/python3.8/site-packages/flask_sqlalchemy/__init__.py", line 570, in get_options
    self._sa.apply_driver_hacks(self._app, sa_url, options)
  File "/.../flask_auth_app/venv/lib/python3.8/site-packages/flask_sqlalchemy/__init__.py", line 914, in apply_driver_hacks
    sa_url.database = os.path.join(app.root_path, sa_url.database)
AttributeError: can't set attribute

I'll get the same error if I try to sign up a new user while running the app on localhost - that is, I can only successfully sign up a new user if I don't have a virtual environment running, so that the app reads from global versions of installed packages instead.

This error goes away if I deactivate the environment before running python3. I'll get the same behaviour if I follow this tutorial, example code given here.

Package Versions

This is the same in venv and ~/miniconda3/lib/python3.8/site-packages. I'm on a MacOS Big Sur v11.2.3.

Flask==1.1.2
Flask-Login==0.5.0
Flask-SQLAlchemy==2.4.4
SQLAlchemy==1.4.0

Solution

I recently delt with that same error message. It is actually due to an upgrade in SQLAlchemy, which gets installed as a dependency of flask-sqlalchemy.

You can find the question I posted on stackoverflow for the issue here:

The problem gets solved by uninstalling SQLAlchemy==1.4.0 and installing the previous version SQLAlchemy==1.3.23.

Try doing that in your virtual environ and see if it helps.

Answered By - Ange Uwase