from qgis.core import
QgsApplication,QgsVectorLayer,QgsVectorDataProvider,QgsDataSourceUri
from PyQt5.QtCore import *
from qgis.PyQt.QtSql import QSqlDatabase, QSqlQuery

uri2 = QgsDataSourceUri()
uri2.setConnection("server", "5432", "table", "user", "pw")
uri2.setDataSource ("public", "table", "geom")
vlayer2=QgsVectorLayer (uri2.uri() ,"ww_manhole","postgres")
print (vlayer2.isValid())
if (vlayer2.startEditing()):
    print ("ok startEditing")
else:
    print("not ok")

my_field_name2 = 'new_field'
if (vlayer2.addAttribute(QgsField(my_field_name2, QVariant.String))):
    print ("ok AddAttribute")
else:
    print ("not ok2")

my_field_value2 = 'Hello world!'
for f in vlayer2.getFeatures():
    print ("ok getFeature")
    f[my_field_name2] = my_field_value2
    if (vlayer2.updateFeature(f)):
        print ("ok updatefeature")
    else:
        print ("not ok updatefeature")

if (vlayer2.updateFields()):
        print ("ok updateFields")
   else:
        print ("not ok updateFields")
if (vlayer2.commitChanges()):
    print ("ok commitChanges")
else:
    print("not ok commitChanges")

True
ok startEditing

ok AddAttribute

ok getFeature

ok updatefeature

not ok updateFields

not ok commitChanges

from qgis.PyQt.QtWidgets import QWidget,QLineEdit,QPushButton,QDialogButtonBox
from PyQt5.QtCore import *
from PyQt5.QtGui import *

def my_form_open(dialog, layer, feature):
    geom = feature.geometry()
    control = dialog.findChild(QLineEdit, "z")
    #layer = qgis.utils.iface.activeLayer()
    global myDialog
    myDialog = dialog
    global myFeature
    myFeature= feature
    buttonBox = dialog.findChild(QDialogButtonBox,"buttonBox")
    #buttonBox.accepted.disconnect(myDialog.accept)
    #Wire up our own signals.
    buttonBox.accepted.connect(validate)
    #buttonBox.rejected.connect(myDialog.reject)
    # Wire up our own signals.
    #pushButton.accepted.connect(validate)

def validate():

    qgis.utils.iface.messageBar().pushMessage("Autosave", u"Validalli : ")
    layer = qgis.utils.iface.activeLayer()
    #qgis.utils.iface.messageBar().pushMessage("Autosave", u"blba : "+z.text())
    z = myDialog.findChild(QLineEdit, "z")
    myFeature['z'] = z.text()
    layer.updateFeature(myFeature)

    layer.commitChanges()