Calculating number of values in list in attribute using PyQGIS

Question

I just want to count the values in a list (NEIGHBORS) for each object in my layer and put the result in a column (NUMBER).

My code is not working

from qgis.PyQt.QtCore import QVariant
test = iface.activeLayer()
v1 =  QgsVectorLayer("NUMBER")
with edit (v1):
    for f in v1.getFeatures():
        f ["NUMBER"] = f[len('NEIGHBORS')]
        v1.updateFeature(f)

and has the following error.

Traceback (most recent call last):
  File "C:\PROGRA~1\QGIS31~1.15\apps\Python39\lib\code.py", line 90, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
  File "<string>", line 36, in <module>
  File "C:\PROGRA~1/QGIS31~1.15/apps/qgis-ltr/./python\qgis\core\additions\edit.py", line 38, in __enter__
    assert self.layer.startEditing()
AssertionError

Do you have an idea?

Answer 1

The error is because you are creating and trying to edit an invalid layer. You do not need to create a new layer object at all. All you need is this:

lyr = iface.activeLayer()
fld_ix = lyr.fields().lookupField('NUMBER')
atts_map = {f.id(): {fld_ix: len(f['NEIGHBORS'].split(','))} for f in lyr.getFeatures() if f['NEIGHBORS'] != NULL}
#print(atts_map)
lyr.dataProvider().changeAttributeValues(atts_map)

Select your layer in the TOC and Run.

Test result:

You can find more information in the Pyqgis Developer Cookbook.