PyQGIS – How to Create a Polygon from a CSV File with Coordinates

Question

I like a write a script which:

1. Imports a CSV file that contains a series of points (latitude and longitude) into QGIS

Lat	Long
56.99181358	-111.4801108
56.98924901	-111.4764563
56.98591437	-111.471848
56.98195783	-111.4725851
56.98184493	-111.4796676
56.98629477	-111.4847629

Here are the points when I import them into the QGIS. I want to create one polygon using these points.

2. Create a polygon from imported points and finally create a shapefile.

Here is how I did part 1:

uri = "file:///C:/Users/katala/Desktop/PyQGIS/Tailings Pond 1A.csv?encoding=%s&delimiter=%s&xField=%s&yField=%s&crs=%s" % ("UTF-8",",", "Lat", "Long","epsg:4326")

#Make a vector layer
eq_layer=QgsVectorLayer(uri,"eq-data","delimitedtext")

#Check if layer is valid
if not eq_layer.isValid():
    print ("Layer not loaded")

#Add CSV data    
QgsProject.instance().addMapLayer(eq_layer)

But I don't know how to do the second part and create a polygon. I found the following code, but I don't know how to adjust it to read the points from the generated vector layer.

polygon.setToGeometry(QgsGeometry.fromPolygonXY([**p_points**]),None) 

Answer 1

Read the attributes into a list of QGSPointXY's using getFeatures :

points = [QgsPointXY(f['long'], f['lat']) for f in iface.activeLayer().getFeatures()]
poly = QgsGeometry.fromPolygonXY([points]) #Create a polygon geometry

#Create a empty memory layer
vl = QgsVectorLayer("Polygon?crs=EPSG:4326&index=yes", "myLayer", "memory")
provider = vl.dataProvider()

newfeature = QgsFeature()
newfeature.setGeometry(poly)
provider.addFeature(newfeature)
QgsProject.instance().addMapLayer(vl)

(Nevermind my comment that a polygon has to start and stop in the same coordinate: Note that unlike some programs, QGIS will close the ring for you so there is no need to duplicate the first point as the last.)