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/./pythonqgiscoreadditionsedit.py",
line 38, in enter
assert self.layer.startEditing() AssertionError

JULESG · Accepted Answer

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()

Setting attribute values using PyQGIS in an edit session

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