def METHOD_NAME(self):
    """
    Set choices for add_user_field select and edit_user_field select
    """
    touse = []
    fields = self.layer.qgis_layer.fields()
    for f in fields:
        type = QVariant.typeToName(f.type()).upper()
        if type == 'QSTRING': # and f.length() > 200:
            touse.append(f.name())
    self.fields['edit_user_field'].choices = \
        self.fields['add_user_field'].choices = [(None, '--------')] + [(f, f) for f in touse]