QSqlQuery in PyQt4

What different QSqlQuery in Qt and PyQt ?

Code:

---

query=QSqlQuery()
query.prepare("select id, nama, id_group, groups from pemakai where nick=:a and groups=:b")
query.bindValue(":a", self._username)
query.bindValue(":b", self._grp)
if query.exec_():
	print "OK"
else:
	print query.lastError().text()

---

Result:

ERROR: wrong number of parameters for prepared statement "qpsqlpstmt_1"
DETAIL: Expected 2 parameters but got 0.
QPSQL: Unable to create query

You QSqlQuery should be related to open database connection, right?

Exactly. And I did

Then it should be something like: query=QSqlQuery(mydb)

I use one database (as default database). So not must use parameter in QSqlQuery

And what happens if you provide a hard-coded name instead of
self._username
Just to make sure the problem is really in the bindValue mechanism.