Rename field QgsField is not valid

Question

In pyQGIS i tried this code (contains also debug prints):

def joinLayers(inputlayer, layer, index_field, name = "JOINED", dataProvider = "ESRI Shapefile"):
 a = []
 b = []
 outpath = getPath(inputlayer.source(), name)
 inputfields = inputlayer.pendingFields()
 nameoflayer = layer.name()
 toappend = layer.pendingFields()
 for fild in toappend:
    a.append(fild.name() + "j")
    fname = fild.name()
    newfield = QgsField(unicode(fname+ "j"), fild.type(), fild.typeName(), fild.length(), fild.precision(), fild.comment())
    b.append(newfield.name())
    inputfields.append(newfield, QgsFields.OriginJoin)
 print a
 print b
 output = QgsVectorFileWriter(outpath, dataProvider, inputfields, inputlayer.crs(), layer.wkbType(), inputlayer.dataProvider().encoding())
 del output

Main problem is renaming fields in for loop over QgsFields.

a

• [u'UWIj', u'Wellj', u'UBHIj', u'Boreholej', u'Statej', u'Jurisdictij', u'Offshorej', u'Basinj', u'Sub-basinj', u'Latitudej', u'Longitudej', u'Legislatioj', u'Titlej', u'Operatorj', u'Classj', u'Structurej', u'Seismic loj', u'Drilled byj', u'Rigsj', u'Spud/Kickj', u'TD Datej', u'Rig releasj', u'Start deptj', u"Driller'sj", u'Oil showj', u'Gas showj', u'Condensatej', u'Oil fieldj', u'Discoveryj', u'Statusj', u'Well headj', u'Basic relej', u'WCRj']

b

• [u'UWIj', u'Wellj', u'UBHIj', u'Boreholej', u'Statej', u'Jurisdictij', u'Offshorej', u'Basinj', u'Sub-basinj', u'Latitudej', u'Longitudej', u'Legislatioj', u'Titlej', u'Operatorj', u'Classj', u'Structurej', u'Seismic loj', u'Drilled byj', u'Rigsj', u'Spud/Kickj', u'TD Datej', u'Rig releasj', u'Start deptj', u"Driller'sj", u'Oil showj', u'Gas showj', u'Condensatej', u'Oil fieldj', u'Discoveryj', u'Statusj', u'Well headj', u'Basic relej', u'WCRj']

After write this fields into QgsVectorFileWriter, some fields doesn´t have "j" suffix.

For example attribute Condensate, see attribute table in qgis:

Answer 1

The attribute field names only allow a maximum of 10 characters due to a limitation in the .dbf storage file (thanks to @MatthiasKuhn for highlighting this).

Your code adds the letter "j" to the end of each name, some of which contain exactly 10 or more characters (such as "Condensate" as you mentioned). Hence why you are not seeing it in the attribute table. You would need to change the for loop to add the "j" letter after the 9th character.

Try adding [:9] to fname when creating a new field:

newfield = QgsField(unicode(fname[:9] + "j"), fild.type(), fild.typeName(), fild.length(), fild.precision(), fild.comment())

The letter "j" will still be added to the end of field names which have less characters (e.g. "Well" will still become "Wellj").