Get number of changed features in a layer using pyqgis

Question

I'm looking for a method to find out how many features in a layer have changed during edit mode (specifically attribute values). For my example i'm using QgsVectorLayerEditBuffer object, method ChangeAttributeValue().

I use editBuffer() method to get the layer buffer and after making some changes, I use ChangedAttributesValues(). This returns 'None'. Example:

layer = iface.activeLayer()

buffer = layer.editBuffer()

..# make changes

buffer.changedAttributeValues()
..None

see also: Get number of changed features in a layer under edit mode using pyqgis

Answer 1

Link refers number of changed features in a layer under edit mode so, if you commit changes, buffer (QgsVectorLayerEditBuffer object) will be lost. Complete procedure is as follows:

1) Load layer and in Python Console:

>>>layer = iface.activeLayer()
>>>buffer = layer.editBuffer()
>>>buffer #for corroborating object was produced
<qgis._core.QgsVectorLayerEditBuffer object at 0x7f81da97c948>

2) Click in "Toggle Editing button", open attributes table and make some changes in it; don't commit them. In my case, I only changed in my layer the first id feature. So, following command produces:

>>>buffer.changedAttributeValues()
{0: {0: 1}}

If a new change is produced, but I commit it, result is as follows:

>>>buffer.changedAttributeValues()
Traceback (most recent call last):
  File "/usr/lib/python3.7/code.py", line 90, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
RuntimeError: wrapped C/C++ object of type QgsVectorLayerEditBuffer has been deleted

For producing several consecutive changes, I change layer id three times in two records:

buffer = layer.editBuffer()
buffer.changedAttributeValues()
{0: {0: 3}, 1: {0: 2}}

However, only two changes were computed because layer has only two features.