попробовать:
CON = cx_Oracle.connect ( "система / абв123")
Курсор = con.cursor ()
rno = int( entupRno.get() )
name = entupName.get()
marks = int( entupMarks.get() )
sql = "UPDATE student set name = '%s' and marks = '%d' where rno= '%d' "
args = (name,marks,rno)
cursor.execute(sql%args)
con.commit()
msg=str(cursor.rowcount) + "rows updated"
messagebox.showinfo("Success ",msg)
except cx_Oracle.DatabaseError as e:
con.rollback()
messagebox.showerror("Failure: ",e)