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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.

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I answer my own question because I found the solution after observing the QGis Plugin "Discovery" from Lutraconsulting. See here : https://github.com/lutraconsulting/qgis-discovery-plugin/blob/master/Discovery/dbutils.py

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)

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.

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