fabric postgres password in command

Question

I have a fabric script that dumps database on server. And I can use it on multiple servers with the PostgreSQL database. The command is simple:

sudo("su postgres -c \"PGPASSWORD=%s pg_dump %s > /tmp/telemedia_newdb\""
% (HOST_SOURCE_DB_UPASS,HOST_SOURCE_DB))

But sometimes, Postgres does not ask for a password at all ...

Will this command fail without a password prompting from Postgres? (Or I know that it will not prompt and HOST_SOURCE_DB_UPASS=''). I want THIS code to work with or without password.