PyQGIS function (version 3.4) to delete all fields in a vector Layer except the required one

Question

I wrote a function that is supposed to delete all fields of a layer except a required one.

My idea was to iterate over all fields of the layer and save all fieldnames except the required one in a List.

NextI want to execute the tool qgis:deletecolumn and pass the list as fields to drop. I'm using QGIS 3.4 that's why I chose 'qgis:deletecolumn' and not "qgis:dropfields'.

This is the code:

def Channel2(self):
    channel1 = self.Channel1()
    fieldname = self.dlg.Channel_ID.currentText()
    fieldNameList = []
    for field in channel1.fields():
        if not (field.name == fieldname):
            fieldNameList.append(field.name)
    param = {'INPUT': channel1,
             'COLUMN': fieldNameList,
             'OUTPUT': 'memory:'}
    channel2 = processing.run('qgis:deletecolumn', param)
    vlayer = channel2['OUTPUT']
    QgsProject.instance().addMapLayer(vlayer)
    return vlayer

I don't get any error message and I do get a new layer as output but the new layer still has all the fields. Checking the documentation it seems that I don't pass a list to the Parameter 'COLUMN' but a "tablefield". But I don't know find what a "tablefield" is supposed to be and how to create it. Or is the error somewhere else?

Answer 1

field.name is a method, you need to call it with (): field.name() is the field name.

Same as:

somelist = ['a','b']
[letter.upper for letter in somelist]
[<built-in method upper of str object at 0x000002CDB16DCDF8>, <built-in method upper of str object at 0x000002CDB16DC768>]

[letter.upper() for letter in somelist]
['A', 'B']

So your fieldnamelist is a list of methods, not fieldnames. Try:

if field.name() != fieldname:
    fieldNameList.append(field.name())