Using PyQGIS to get data contained in the "in-memory editing buffer" of layer that is currently being edited

Question

I am writing a Python plugin for QGIS and I need to be able to retrieve data about the current edits to a particular layer with in QGIS before the edits have been applied.

There does not seem to be any methods in the QgsVector layer or QgsMapLayer classes that allow me to retrieve such data. I am wondering if maybe such a method exists somewhere else.

Answer 1

For that you can use the QgsVectorLayerEditBuffer class which you can retrieve by calling editBuffer on your QgsVectorlayer.

In the following example I print all changed attribute values in my active layer, which is in edit mode:

from qgis.utils import iface
changed_values = iface.activeLayer().editBuffer().changedAttributeValues()
print(changed_values)

Similarly, you can print the changed geometries. Just take a look at the Documentation of QgsVectorLayerEditBuffer.

Answer 2

As @juto96’s answer indicates, you can use the QgsVectorLayerEditBuffer to query information about the attributes and geometries changes. In addition, until the changes are committed or rolled back, reading attributes and geometries for features from the QgsVectorLayer will return the modified values. If you need to get the original values while a transaction is open, you can read features from the QgsVectorLayer.dataProvider() (which not surprisingly returns a QgsVectorLayerDataProvider instance).