Previously, we created layers by loaded existing datasets into our project. In this example, we will create a new layer from scratch.

To get started, let’s create a new QgsVectorLayer object for point geometries with the layer name “temp” and a memory data provider:

vl = QgsVectorLayer("Point", "temp", "memory")

Then we can add some attribute fields using the data provider’s addAttributes() function:

from qgis.PyQt.QtCore import QVariant
pr = vl.dataProvider()
pr.addAttributes([QgsField("name", QVariant.String),
                  QgsField("age",  QVariant.Int),
                  QgsField("size", QVariant.Double)])
vl.updateFields() 

A common pattern in this example is “update-after-change”. When making changes to a layer’s data provider, these changes are not automatically reflected by the layer. Therefore, we need to call the vector layer’s updateFields() after adding the attributes.

With the layer set up, we can add a point feature and add the layer to the project:

f = QgsFeature()
f.setGeometry(QgsGeometry.fromPointXY(QgsPointXY(10,10)))
f.setAttributes(["Ada L.", 2, 0.3])
pr.addFeature(f)
vl.updateExtents() 
QgsProject.instance().addMapLayer(vl)

Similar to before, here we need to call updateExtents() after adding features to the data provider. Otherwise the layer’s extent will be incorrect.

Let’s have a look at some layer statistics now:

print("No. fields:", len(pr.fields()))
print("No. features:", pr.featureCount())
e = vl.extent()
print("Extent:", e.xMinimum(), e.yMinimum(), e.xMaximum(), e.yMaximum())

for f in vl.getFeatures():
    print("Feature:", f.id(), f.attributes(), f.geometry().asPoint())

This should print:

No. fields: 3
No. features: 1
Extent: 10.0 10.0 10.0 10.0
Feature: 1 ['Ada L.', 2, 0.3] <QgsPointXY: POINT(10 10)>

So far so good! But what if we decide that we want to add another attribute? Luckily QgsVectorLayers have a handy addAttribute() function. The documentation notes:

Calls to addAttribute() are only valid for layers in which edits have been enabled by a call to startEditing(). Changes made to features using this method are not committed to the underlying data provider until a commitChanges() call is made. Any uncommitted changes can be discarded by calling rollBack().

Ok, let’s do that! Pay attention to the combination of addAttribute() and updateFields().

vl.startEditing()

my_field_name = 'new field'
vl.addAttribute(QgsField(my_field_name, QVariant.String))
vl.updateFields()

for f in vl.getFeatures():
    print("Feature:", f.id(), f.attributes(), f.geometry().asPoint())

This should print:

Feature: 1 ['Ada L.', 2, 0.3, None] <QgsPointXY: POINT(10 10)>

So there is a new attribute field. We can populate it by assigning our desired value to the feature’s field. Again, we have to follow the update-after-changes pattern, this time using updateFeature():

my_field_value = 'Hello world!'
for f in vl.getFeatures():
    f[my_field_name] = my_field_value
    vl.updateFeature(f)

vl.commitChanges()

for f in vl.getFeatures():
    print("Feature:", f.id(), f.attributes(), f.geometry().asPoint())

Finally, this prints:

Feature: 1 ['Ada L.', 2, 0.3, 'Hello world!'] <QgsPointXY: POINT(10 10)>

Now, the only thing left to do is to stop the editing session:

iface.vectorLayerTools().stopEditing(vl) 

Excellent! … But wait!

There is an alternative to manually calling startEditing(), commitChanges(), stopEditing() and handling rollbacks. This alternative approach uses a context manager which will internally use vl.startEditing() and call vl.commitChanges() on success, and it will rollback if any exception occurs. This is how it’s done using with edit(vl):

my_field_name = 'new field'
my_field_value = 'Hello world!'

with edit(vl):
    vl.addAttribute(QgsField(my_field_name, QVariant.String))
    vl.updateFields()
    for f in vl.getFeatures():
        f[my_field_name] = my_field_value
        vl.updateFeature(f)

These are the basic steps to create a new layer from scratch. You saw how to define attribute fields and add features. Finally, we modified the layer and added another attribute field.

Next

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PyQGIS 101 is a work in progress. I’d appreciate any feedback, particularly from beginners!