sqlalchemy.exc.ArgumentError: Can't load plugin: sqlalchemy.dialects:driver

I am trying to run alembic migration and when I run

alembic revision --autogenerate -m "Added initial tables"

It fails saying

sqlalchemy.exc.ArgumentError: Can't load plugin: sqlalchemy.dialects:driver

the database url is

postgresql+psycopg2://dev:passwd@localhost/db

and I even have psycopg2 installed in my virtualenv

$yolk -l
Flask-Login     - 0.1.3        - active
Flask-SQLAlchemy - 0.16         - active
Flask           - 0.9          - active
Jinja2          - 2.6          - active
Mako            - 0.7.3        - active
MarkupSafe      - 0.15         - active
Python          - 2.7.2        - active development (/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-dynload)
SQLAlchemy      - 0.8.0        - active
Werkzeug        - 0.8.3        - active
alembic         - 0.4.2        - active
antiorm         - 1.1.1        - active
appscript       - 1.0.1        - active
distribute      - 0.6.27       - active
envoy           - 0.0.2        - active
osascript       - 0.0.4        - active
pep8            - 1.4.5        - active
pip             - 1.1          - active
psycopg2        - 2.4.6        - active
wsgiref         - 0.1.2        - active development (/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7)
yolk            - 0.4.3        - active

Whay could be causing this issue?

Here's how to produce an error like that:

>>> from sqlalchemy import *
>>> create_engine("driver://")
Traceback (most recent call last):
... etc
sqlalchemy.exc.ArgumentError: Can't load plugin: sqlalchemy.dialects:driver

so I'd say you aren't actually using the postgresql URL you think you are - you probably are calling upon a default-generated alembic.ini somewhere.

For those who haven't noticed it, the "default-generated alembic.ini" zzzzeek refers to is in the root directory of the project.

The whole problem is one of setting the sqlalchemy.url config parameter in the alembic.ini file. Also, it can be set programmatically as explained in https://stackoverflow.com/a/15668175/973380.

Notice that the scheme doesn't actually specify the driver but the dialect: the scheme is of form dialect:// or dialect+driver://.

For example the correct urls to connect to a PostgreSQL database would start with for example postgres:// (which defaults to using psycopg2), or choosing a driver explicitly (postgres+psycopg2://, or with another driver).

If you happen to specify only psycopg2 you will get the error

sqlalchemy.exc.NoSuchModuleError: Can't load plugin: sqlalchemy.dialects:psycopg2

I resolved this by simply opening the alembic.ini in notepad++ and then amending the variable sqlachemy.url (on about line 38) to the url in my project file. The error is most likely caused because it has driver at the beginning.

i.e. renaming this line to

sqlalchemy.url = sqlite:///name_of_my_database.db