QGIS 3 - QSettings().value(r"[...]")[0:2] error - 'NoneType' object is not subscriptable

Question

I'm trying to find a solution to launch a plugin from outside the QGIS console

I've succeeded to call the plugin from the python console in the QGIS software but I'm now trying to do it outside of QGIS when it's already opened

for QGIS plugin in python console the following works to launch the plugin:

import sys
sys.path.append(r"path/to/plugin")
from plugin_name_folder import plugin_name
execute = plugin_name.plugin_class(iface)
execute.initGui()
execute.run()

in order to launch the plugin, here's the beginning of the code pre-made by the plugin builder :

from PyQt5.QtCore import QSettings, QTranslator, qVersion, QCoreApplication
from PyQt5.QtGui import QIcon
from PyQt5.QtWidgets import QAction

from .resources import *
from .zoom_snap_dialog import zoom_snapDialog
import os.path


class zoom_snap:
    def __init__(self, iface):
        self.iface = iface
        self.plugin_dir = os.path.dirname(__file__)
        locale = QSettings().value('locale/userLocale')[0:2]
        locale_path = os.path.join(
            self.plugin_dir,
            'i18n',
            'zoom_snap_{}.qm'.format(locale))

        if os.path.exists(locale_path):
            self.translator = QTranslator()
            self.translator.load(locale_path)

            if qVersion() > '4.3.3':
                QCoreApplication.installTranslator(self.translator)

        self.actions = []
        self.menu = self.tr(u'&(name)')

        # Check if plugin was started the first time in current QGIS session
        # Must be set in initGui() to survive plugin reloads
        self.first_start = None

When I try to launch my plugin with plugin_name.plugin_class(iface) I'm hitting the following error :

"locale_path=os.path.join("
TypeError: 'NoneType' object is not subscriptable

I've seen that another line before generates the same TypeError when launched separately in window command line :

QSettings().value('locale/userLocale')[0:2]
TypeError: 'NoneType' object is not subscriptable

My questions will be :

1- Where is QSettings().value([path])[0:2] extracting values from ? I can't see where locale/userLocale folder is and how is it taking its values

2- I've seen a lot of posts about the 'NoneType' TypeError posts but here I don't know if anybody had any clue about solving this type of error

Answer 1

For question 1:

The settings (QSettings) are read from QGIS itself. You can access the value in Preference > Options > Advanced. Click on the button to say that you will be careful and you access all the values. There is a locale/userLocale settings that define the language to use in QGIS.

For question 2:

The NoneType object is not subscritable error just mean that your are trying to subscript (with the slice [0:2]) a NoneType object which is not possible.

It seems that launching your plugin from console, doesn't allow it to access QSettings the proper way... (you get None when trying to access values). To make it work you can (as a workaround) delete the call to QSettings in your code. Note that if you do that you need to implement another way to load i18n if you want to use i18n in your plugin.