QGIS Python, adding attributes and populating the fields

Question

I am writing a script to add attributes and then populate the fields. When I run the script in QGIS in the python console, it works. But when I put it in a script and run it through a plugin, it only adds the fields, but doesn't populate them.

Here is the script,

def addAtts(attName, varType):
    layer = iface.activeLayer()
    layer.dataProvider().addAttributes([QgsField(attName, varType)])
    layer.updateFields()



attDict = {'COLOR': 176, 'ICON': None, 'L_LIMIT': 0, 'LAYER': 50, 'MAJ_CAT': 'Nautical', 'MIN_CAT': '(AREA)Lake', 'MODE':  'N', 'VALUE': None, 'U_LIMIT': 34, 'FONT': '14B', 'P_LIMIT': 0, 'P_ICON': None, 'BFR_COLOR':  0, 'BFR_WIDTH':  0, 'RTP_LIMIT': 34, 'RTP_COLOR': 43, 'WIDTH': 1, 'BDR_COLOR': 43, 'BDR_WIDTH': 1, 'IMG_RENDER': 255, 'BFR_PATTERN': None, 'PATTERN': None, 'BDR_PATTERN': None, 'FILT_SRCH': None }

layer = iface.activeLayer()
layer.startEditing()

num = 0

for x in attDict.keys():
    num += 1
    it_is = str(attDict[x])
    try:
        int(it_is)
        it_is = True
        #qType = 'QVariant.Int'
        addAtts(x, QVariant.Int)
        layer.changeAttributeValue(0,num,attDict[x])

    except ValueError:
        it_is = False
        #qType = 'QVariant.String'
        addAtts(x,QVariant.String)
        layer.changeAttributeValue(0,num,attDict[x])

Answer 1

First, I tried your code out in Python Console of QGIS 3.8, with layer of following image, and it adds and populates the fields in attributes table.

Afterwards, I created a test plugin and put your code in it (slightly modified for working through plugin) with two functions before run method: addAtts and adding_attributes (this one connected to OK button). Code Snippet looks as follow:

.
.
.
from PyQt5.QtCore import QSettings, QTranslator, qVersion, QCoreApplication, QVariant
.
.
.
from qgis.core import QgsField
.
.
.
    def add_action(
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.
.
        okBtn = self.dlg.okButton
        okBtn.clicked.connect(self.adding_attributes)

        return action
.
.
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    def addAtts(self, attName, varType):
        layer = self.iface.activeLayer()
        layer.dataProvider().addAttributes([QgsField(attName, varType)])
        layer.updateFields()

    def adding_attributes(self):

        attDict = {'COLOR': 176, 'ICON': None, 'L_LIMIT': 0, 'LAYER': 50, 'MAJ_CAT': 'Nautical', 'MIN_CAT': '(AREA)Lake', 'MODE':  'N', 'VALUE': None, 'U_LIMIT': 34, 'FONT': '14B', 'P_LIMIT': 0, 'P_ICON': None, 'BFR_COLOR':  0, 'BFR_WIDTH':  0, 'RTP_LIMIT': 34, 'RTP_COLOR': 43, 'WIDTH': 1, 'BDR_COLOR': 43, 'BDR_WIDTH': 1, 'IMG_RENDER': 255, 'BFR_PATTERN': None, 'PATTERN': None, 'BDR_PATTERN': None, 'FILT_SRCH': None }

        layer = self.iface.activeLayer()
        layer.startEditing()

        num = 0

        for x in attDict.keys():
            num += 1
            it_is = str(attDict[x])
            try:
                int(it_is)
                it_is = True
                #qType = 'QVariant.Int'
                self.addAtts(x, QVariant.Int)
                layer.changeAttributeValue(0,num,attDict[x])

            except ValueError:
                it_is = False
                #qType = 'QVariant.String'
                self.addAtts(x,QVariant.String)
                layer.changeAttributeValue(0,num,attDict[x])

        layer.commitChanges()        

    def run(self):
.
.
.

After reloading plugin to save changes, I erased all layer fields created with script in Python Console and ran the plugin. Result was the same as in above image.