Getting active PostgreSQL connection username when stored through authentication configuration in QGIS 3

Question

I've been trying to get the username of a PostgreSQL connection in QGIS 3.6, using this Python code

layer = iface.activeLayer()
provider = layer.dataProvider()
username = QgsDataSourceUri(provider.dataSourceUri()).username()
print (username)

like in this question Getting active PostgreSQL connection's username as variable in QGIS?

It does give me an empty string and I think it is because the connection uses an authentication configuration, so the username is not available this way. Is there another way ? I found the QgsAuthManager class but as a newbie I really can't figure out what to do with it.

Answer 1

I found the solution after observing the QGIS Plugin "Discovery" from Lutraconsulting. See here.

The code is :

def get_postgres_conn_info(selected):
    """ Read PostgreSQL connection details from QSettings stored by QGIS
    """
    settings = QSettings()
    settings.beginGroup(u"/PostgreSQL/connections/" + selected)
    if not settings.contains("database"): # non-existent entry?
        return {}

    conn_info = dict()
    conn_info["host"] = settings.value("host", "", type=str)

    # password and username
    username = ''
    password = ''
    authconf = settings.value('authcfg', '')
    if authconf :
        # password encrypted in AuthManager
        auth_manager = QgsApplication.authManager()
        conf = QgsAuthMethodConfig()
        auth_manager.loadAuthenticationConfig(authconf, conf, True)
        if conf.id():
            username = conf.config('username', '')
            password = conf.config('password', '')
    else:
        # basic (plain-text) settings
        username = settings.value('username', '', type=str)
        password = settings.value('password', '', type=str)
    return username, password

###############

    myname, mypass = get_postgres_conn_info("your_connection_name")
    print(myname)

And as one can see they test for the two password storing possibilities and retrieve the login and password from the one that is used, after searching inside the QSettings.