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.

enter image description here

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:

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from PyQt5.QtCore import QSettings, QTranslator, qVersion, QCoreApplication, QVariant
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from qgis.core import QgsField
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 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):
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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.