[Qgis-user] Create virtual fields with pyQGIS with standalone scripts

[Roland Berger]

Hi

I would like to add a virtual field to a PostGIS vector layer with 
PyQGIS with standalone scripts 
(https://docs.qgis.org/3.34/en/docs/pyqgis_developer_cookbook/intro.html#python-applications)
All examples I have found so far with google, chatgpt4 and JetBrains AI 
Assistant gave me solutions that do not work. They only work in the 
Python Console inside QGIS.
Does anybody know of a solution that works with PyQGIS with standalone 
scripts?
See below what I have tried so far in different variations.

Thanks for any help
Roland

Example created by chatgpt:
...

field_name = "virtual_field"

expression = '"attribute_name" * 2'  # Example expression

# Create and add the virtual field

layer.startEditing()

layer.addExpressionField(expression, QgsField(field_name, QVariant.Double))

layer.commitChanges()

...

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