psycopg2: distinguishing connect failures through exception handling

Hi Folks,

My first email here. I am using psycopg2 to remotely verify a postgres
server is up, and separately that it is accessible given a particular
user/password/database. This is done using psycopg2.connect(args...). The
use case is automated deployment of the docker postgres container, where I
wish to distinguish postgres service is up from postgres service cannot be
accessed (e.g., with your user/password/database), from postgres service is
down, in order to verify the container deployment.

It seems the psycopg2.OperationalError exception is thrown in a variety of
circumstances: database does not exist, authentication failed, no password
supplied, etc. But, the exception pgcode and pgerror are not set,
evidently unless there is a successful connect. This leaves me in the
position of distinguishing failures based on the error message string,
e.g. fe_sendauth:
no password supplied or FATAL: password authentication failed for user
"<val>"

This does not seem like a reliable method, one certain to work in the
future, and known to work now across myriad versions of postgres (outside
the scope of my practical testing).

I have these two questions. Is this understanding correct? Is there a
reliable means to distinguish the types of connect failures using psycopg2?

Thank you for your time.

Best regards,

Stephen Quintero