Local MySQLdb connection fails with AttributeError for paramstyle when running GAE development server

Question

I'm building a GAE Flask application with Flask-Alchemy, against Cloud SQL, and running dev_appserver to test the application as I build it.

However, if I set the SQLALCHEMY_DATABASE_URI to a mysql+gaerdbms:///appname?instance=instanceid URL, I get the following traceback when trying to call db.create_all():

Traceback (most recent call last):
  # earlier lines omitted for brevity
  File "/Project/app/foo.bar/foo/bar/admin/__init__.py", line 26, in init_db
    db.create_all()
  File "/Project/app/distlib/flask_sqlalchemy/__init__.py", line 856, in create_all
    self._execute_for_all_tables(app, bind, 'create_all')
  File "/Project/app/distlib/flask_sqlalchemy/__init__.py", line 848, in _execute_for_all_tables
    op(bind=self.get_engine(app, bind), tables=tables)
  File "/Project/app/distlib/flask_sqlalchemy/__init__.py", line 797, in get_engine
    return connector.get_engine()
  File "/Project/app/distlib/flask_sqlalchemy/__init__.py", line 473, in get_engine
    self._engine = rv = sqlalchemy.create_engine(info, **options)
  File "/Project/app/distlib/sqlalchemy/engine/__init__.py", line 332, in create_engine
    return strategy.create(*args, **kwargs)
  File "/Project/app/distlib/sqlalchemy/engine/strategies.py", line 69, in create
    dialect = dialect_cls(**dialect_args)
  File "/Project/app/distlib/sqlalchemy/dialects/mysql/base.py", line 1986, in __init__
    default.DefaultDialect.__init__(self, **kwargs)
  File "/Project/app/distlib/sqlalchemy/engine/default.py", line 124, in __init__
    self.paramstyle = self.dbapi.paramstyle
AttributeError: 'module' object has no attribute 'paramstyle'

What gives? Why is the (DB-API 2.0 required) paramstyle attribute missing?

Answer 1

This means the MySQLdb module is missing and failed to import. The GAE SDK does not itself come with the MySQLdb client library; install MySQLdb (as instructed in the SDK documentation):

venv/bin/pip install mysql-python

or use your OS package manager to install MySQLdb in your system python.

The error is caused by the Google google.appengine.api.rdbms_mysqldb module, acting as a stub, not having a paramstyle attribute when import MySQLdb fails. A stub connect() function is provided that'll raise a more helpful exception, but due to an unfortunate interaction with SQLAlchemy the error there is a lot less informative.