Hi Dan,
thanks for your response.
I have seen the example that you have attached and as Syntax require some parameters not optional, but the .sde contain itself all the parameter request:
server, port, istance, use and psw etc
Example if i set sde connection as workspace for a list table the script works so i suppose that the connection is open:
arcpy.env.workspace = ("Database Connections/TEST.sde")
tables = arcpy.ListTables()
for t in tables:
print(t)