Creating polygon and adding it as new layer with PyQGIS

Question

I'm attempting to add a polygon to a new layer using PyQGIS in QGIS 3.14. Adapting some examples from questions and answers here and here. I believe I have successfully created the polygon from an array of (four) input points and then a feature that contains the polygon.

I have then created a layer for the feature in the CRS I desire but cannot work how to get the feature into the layer. Code so far below.

# Attach modules 
from qgis.core import *                        # attach main QGIS library
from qgis.utils import *                       # attach main python library
import os                                      # attach operating system library

# Create an array [] object with the polygon vertices
vrtcs = []
vrtcs.append(QgsPointXY(396100,8969000))
vrtcs.append(QgsPointXY(396100,8973900))
vrtcs.append(QgsPointXY(397900,8973900))
vrtcs.append(QgsPointXY(397900,8969000))

# Create a polygon from the coordinates
ply_01 = QgsGeometry.fromPolygonXY([vrtcs])

# Create a feature object then put the polygon into the feature
ftr = QgsFeature()
ftr.setGeometry(ply_01)
print(ftr.geometry())

# Create a layer for the feature, in the desired CRS
lyr = QgsVectorLayer('Polygon?crs=epsg:29194', '200905_Bdy',"org")
Prj.addMapLayers([lyr]) 

# Set an object for the data provider for the layer
prv = lyr.dataProvider()                               

# Add the feature to the layer using this provider (fails)
prv.addFeatures([ftr])

Answer 1

Change 'org' into 'memory' and add Prj = QgsProject.instance() line before using Prj.

...

# Create a layer for the feature, in the desired CRS
lyr = QgsVectorLayer('Polygon?crs=epsg:29194', '200905_Bdy', "memory")
Prj = QgsProject.instance()  # ADD THIS LINE
Prj.addMapLayers([lyr])

Since you add one layer, you can use Prj.addMapLayer(lyr) instead of Prj.addMapLayers([lyr]).

...

Answer 2

You must start editing before adding the feature to the layer and commit the changes after you are done:

if feature.isValid():
    layer.startEditing()
    layer.addFeatures([feature])
    commited = layer.commitChanges()
    if commited:
        return True
    else:
        print(f'{layer.commitErrors()}')
        return False