My colleague Lukas and I banged our heads against this for much too long today. So, our SQLAlchemy is was configured like this:

ENV_DB_CONNECTION_DSN = postgresql://localhost:5432/mydatabase

And the database doesn't have a password (local) so I can log in to it like this on the command line:

$ psql mydatabase

Which assumes the username peterbe which is what I'm logged in. So, this is a shortcut for doing this:

$ psql mydatabase -U peterbe

Which, assumes a blank/empty password.

Anyway, you try to connect to the database and you get this error:

(OperationalError) fe_sendauth: no password supplied

So we tried again using this DSN:

ENV_DB_CONNECTION_DSN = postgresql://peterbe@localhost:5432/mydatabase

But then you get the same error. Tried this:

ENV_DB_CONNECTION_DSN = postgresql://peterbe:@localhost:5432/mydatabase

Now it says:

(OperationalError) FATAL:  password authentication failed for user "peterbe"

So (drum roll...) the answer is to not bother setting the host and port and it works. The right syntax is this:

ENV_DB_CONNECTION_DSN = postgresql:///mydatabase

It has nothing to with your pg_hda.conf or what type of trust you have set up. Just avoid the host and it doesn't do the password checking.

Even, better if you want to make sure it's using psycopg2 and not your old psycopg you can write this:

ENV_DB_CONNECTION_DSN = postgresql+psycopg2:///mydatabase
Marius Gedminas - 24 February 2011 [«« Reply to this]
postgresql:/// connects over a UDP socket and can use the Unix userid to authenticate.

postgresql://host:port/ connects over TCP, where the userid of the remote end is not known (or cannot be trusted).

It's a bit of a simplification to say pg_hba.conf doesn't matter: you need to allow Unix domain socket connections in that config file if you want paswordless logins. It's just that the default settings already allow that.
Rasjid Wilcox - 24 February 2011 [«« Reply to this]
I think there is a typo in Marius' comment. Pretty sure the first line should say 'unix socket' and not 'UDP socket'. They are different things and I don't think it is possible to connect to postgres via UDP.
Marius Gedminas - 24 February 2011 [«« Reply to this]

Thanks for correcting me. I don't know how I managed to type "UDP" when I meant "Unix domain".
Eliyahu - 06 October 2013 [«« Reply to this]

u so helped me. what dozens of sites didn't managed to do..
Dav Clark - 17 November 2013 [«« Reply to this]
Likewise, very helpful. Thanks!
Adrian - 25 March 2015 [«« Reply to this]
Have you ever done this without a password for an Amazon Redshift instance?

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