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Is there any example about how to create a virtual field and subsequently change the attribute of a non-virtual field without wiping out the virtual field? Both are required to be done in the Python Console.

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    Good question as I use Virtual Fields quite a lot. I also noticed the Field Calculator in QGIS Modeler doesn't have an option to insert virtual fields so it would be interesting to see how to achieve this via the console.
    – Joseph
    Apr 28, 2015 at 8:57

1 Answer 1

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When you have a variable layer that is a reference to a vector layer you need

  • to specify the field definition

  • add the new field with an expression

     field = QgsField('twoTimesA', QVariant.LongLong)
     layer.addExpressionField(' 2 * "A" ', field)
    

You can now modify an attribute and when you query the layer next time the virtual field will be updated.

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