AttributeError when creating list field with QVariant in QgsVectorLayer using PyQGIS

Question

I am trying to create a list-type field for the active layer using the code below, which is not working.

Is there any other way to create a list-type field and update the list([1,2,3]) as an attribute value to the created area?

layer = iface.activeLayer()

field = 'list_field'
if layer.fields().indexFromName(field) == -1:
    fieldz = QgsField(field , QVariant.list)
    layer.dataProvider().addAttributes([fieldz])
    layer.updateFields()

Answer 1

I could get the same error as you:

Traceback (most recent call last):
  File "C:\PROGRA~1\QGIS32~1.0\apps\Python39\lib\code.py", line 90, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
  File "<string>", line 5, in <module>
AttributeError: type object 'QVariant' has no attribute 'list'

This happened due to the wrong specification of the QVariant type. It should be QVariant.List instead of QVariant.list, see the documentation:

from qgis.core import QgsField, QgsVariantUtils
from PyQt5.QtCore import QVariant

new_field = QgsField("ListField", QVariant.List)
print(QgsVariantUtils.typeToDisplayString(new_field.type())) # List

So, your code can be adjusted as follows:

from qgis.gui import QgisInterface
from qgis.core import QgsField
from PyQt5.QtCore import QVariant

layer = iface.activeLayer()

field = 'list_field'
if layer.fields().indexFromName(field) == -1:
    fieldz = QgsField(field , QVariant.List)
    layer.dataProvider().addAttributes([fieldz])
    layer.updateFields()

However, I would probably proceed in a slightly different manner to create a new field:

from qgis.gui import QgisInterface
from qgis.core import QgsField
from PyQt5.QtCore import QVariant

layer = iface.activeLayer()

field = 'list_field'

if field not in layer.fields().names():
    layer_provider = layer.dataProvider()
    layer_provider.addAttributes([QgsField(field, QVariant.List)])
    layer.updateFields()

Also, keep in mind that you may encounter the following error e.g.:

Layer <LAYER_NAME>: type for field list_field not found

---

References:

• KeyError: '-1' when adding field and calculating expression with PyQGIS
• QGIS Python Tutorial: How to Add/Delete Field and Updating Attribute Value
• PyQGIS 101: Using expressions to compute new field values