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?

score 24 · Accepted Answer · edited Nov 14 '22 at 15:34

24

For QGIS 3

See a slightly modified answer from @MortenSickel below.

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()

edited Nov 14 '22 at 15:34

Taras

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answered Jul 25 '12 at 19:16

underdark

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Thank you for the reply, it was certainly helpful. However when using the the script i get the following error. "NameError: name 'QVariant' is not defined"
I left the "addAttributes" part out and managed to put a dot on the map, so it works, thank you. I am using Ubuntu 12.04 and QGIS 1.8.0-Lisboa But what can cause the above mentioned error?
– Theuns Heydenrych Jul 26 '12 at 07:54
1

Seems that setAttributeMap is no longer a method of QgsFeature classes, so this solution doesn't work for the most recent (its 2016) QGIS. – Spacedman Aug 08 '16 at 07:25
1

Just to mention a small mistake in the code provided. Either use a editing session or use the data provider. No need to use both at the same time, you can use data provider without editing session, or the reverse, you can add fields to the QgsVectorlayer directly in a editing session. – etrimaille Feb 02 '20 at 19:57

score 7 · Answer 2 · edited Nov 14 '22 at 15:35

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)

score 5 · Answer 3 · edited Nov 14 '22 at 15:36

5

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())

edited Nov 14 '22 at 15:36

Taras

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answered Aug 25 '13 at 07:38

G M

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1

But this is creating the empty .shpfile.If i want to save the existing shapefile to specified path how can i do that? – user99 Jan 12 '15 at 10:17

Creating new empty vector layer with PyQGIS

3 Answers3

For QGIS 3

For QGIS 2

create layer

Enter editing mode

add fields

add a feature

Commit changes

Enter editing mode

add fields

add a feature

To just create the layer and add features later, delete the four lines from here until Commit changes

Commit changes

Show in project

Linked