Using QgsExpression.evaluate() on vector layer closes QGIS

Question

I am trying to import a csv layer and run QgsExpression.evaluate() to make some computations on existing columns. However, QGIS 2.18, 3.2, and 3.3 all close when it reaches that point.

    layer.startEditing()

    context = QgsExpressionContext()
    scope = QgsExpressionContextScope()

    exp = QgsExpression(expression)
    if exp.hasParserError():
        raise Exception(exp.parserErrorString())

    for f in layer.getFeatures():
        QApplication.processEvents()
        scope.setFeature(f)
        context.appendScope(scope)

        value = exp.evaluate(context)

        computed_values[field_name].append(value)

        f.setAttribute(f.fieldNameIndex(field_name), value)

        layer.updateFeature(f)

    layer.commitChanges()

Is there any way to make it work for shapefile, or just ogr table layer?

Answer 1

Don't append the scope inside your loop, do it once outside the loop. Appending the scope transfers ownership of it to the context, and re-appending on the next iteration of the loop will cause undefined behaviour.