Setting attribute values using PyQGIS in an edit session

Question

My PyQGIS script works fine if I am not in an edit session, but 99% of the time I will be, so I need it to work whilst in editing mode.

from PyQt5.QtGui import *
layer = QgsProject.instance().mapLayersByName("test")[0]
iface.setActiveLayer(layer)
value='222'
with edit(layer):
 for feature in layer.selectedFeatures():
 feature['flag1'] = value
 layer.updateFeature(feature)
msg = QMessageBox()
msg.setText("All done.")
msg.show()

So basically, it just updates the 'flag1' field with the assigned value for those features that have already been selected from the 'test' layer. How do I make this work inside an edit session? Currently I receive this error:

File "C:/PROGRA~1/QGIS3~1.16/apps/qgis/./python\qgis\core\additions\edit.py", line 38, in enter assert self.layer.startEditing() AssertionError

• djr_: I get: TypeError: 'QgsVectorLayer' object does not support indexing. As I said, the script works, but I need it to work within an edit session.

Answer 1

You can't start a new edit session if the layer is already in one.

You need to test if your layer is in an edit session and if it's the case, don't open a new one. QgsVectorLayer has a method isEditable() wich returns a boolean, true if the layer is in an edit session. You can use it to test your layer and know what to do.

This code should work :

from PyQt5.QtGui import *
layer = QgsProject.instance().mapLayersByName("test")[0]
iface.setActiveLayer(layer)
value='222'
if layer.isEditable() :
 for feature in layer.selectedFeatures():
 feature['flag1'] = value
 layer.updateFeature(feature)
else :
 with edit(layer):
 for feature in layer.selectedFeatures():
 feature['flag1'] = value
 layer.updateFeature(feature)
msg = QMessageBox()
msg.setText("All done.")
msg.show()

• nr_aus: additionally, I had a 'check if writable, make readonly' check in my code, whilst using the 'with edit()'. For some reason this really upset the with edit() command, when I removed the check if editable/makereadonly, the code worked. Not sure why though unfortunately, but worth mentioning.