Creating new empty vector layer with PyQGIS

Question

I have looked at a few tutorials on Python scripts for QGIS. All of them create new vector and raster layers with some existing data source e.g. shapefile or GeoTIFF or PostGIS database table.

Is it possible to create a QGIS layer through a python script, where I can create/add/modify new features in the Python script, as the need arises?

It will be typically a vector layer with point data and custom symbols.

Will this be possible? Is there an example I can look at?

Answer 1

For QGIS 3

See a slightly modified answer from @MortenSickel below.

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For QGIS 2

Have a look at the Memory provider as described in PyQGIS Cookbook.

Memory provider is intended to be used mainly by plugin or 3rd party app developers. It does not store data on disk, allowing developers to use it as a fast backend for some temporary layers.

# To avoid 'QVariant' is not defined error
from PyQt4.QtCore import *

# create layer
vl = QgsVectorLayer("Point", "temporary_points", "memory")
pr = vl.dataProvider()
# Enter editing mode
vl.startEditing()
# add fields
pr.addAttributes( [ QgsField("name", QVariant.String),
               QgsField("age",  QVariant.Int),
               QgsField("size", QVariant.Double) ] )
# add a feature
fet = QgsFeature()
fet.setGeometry( QgsGeometry.fromPoint(QgsPoint(10,10)) )
fet.setAttributeMap( { 0 : QVariant("Johny"),
                  1 : QVariant(20),
                  2 : QVariant(0.3) } )
pr.addFeatures( [ fet ] )
# Commit changes
vl.commitChanges()

Answer 2

In QGIS 3 one need to do:

from qgis.core import QgsVectorLayer, QgsFeature, QgsField, QgsGeometry, QgsPointXY, QgsField, QgsProject

vl = QgsVectorLayer("Point", "temporary_points", "memory")
pr = vl.dataProvider()
# Enter editing mode
vl.startEditing()
# add fields
pr.addAttributes( [ QgsField("name", QVariant.String),
                QgsField("age",  QVariant.Int),
                QgsField("size", QVariant.Double) ] )
# add a feature
# To just create the layer and add features later, delete the four lines from here until Commit changes
fet = QgsFeature()
fet.setGeometry( QgsGeometry.fromPointXY(QgsPointXY(15,60)) )
fet.setAttributes(["Johny",20,0.3])
pr.addFeatures( [ fet ] )
# Commit changes
vl.commitChanges()
# Show in project
QgsProject.instance().addMapLayer(vl)

Answer 3

If you want to save a new vector layer (.shp) in your Hard Disk you have to use QgsVectorFileWriter().

It takes several parameters, the simplest way is to use the parameters from the project you are using. Have a look at this site.

cLayer = qgis.utils.iface.mapCanvas().currentLayer()
provider = cLayer.dataProvider()
writer = QgsVectorFileWriter("output_path_and_name.shp", provider.encoding(), provider.fields(), QGis.WKBPolygon, provider.crs())