[QGIS-Developer] QgsExpression python pattern

[Chris Crook]

Chris Crook wrote
> I am updating some plugins which use QgsExpression, and I'm wondering
> about the best way to use expressions in QGIS3.  It in the past I used a
> pattern like:
> 
>   exp=QgsExpression(expressionText)
>   exp.prepare(fields)
>   request=QgsFeatureRequest()
>   request.setSubsetOfAttributes( expression.referencedColumns(), fields )
>   for feat in layer.getFeatures(request)
>       value=expression.evaluate(feat)
> 
> However with QGIS3 this has clearly changed!  QgsExpression is not
> included in the QGIS3 API https://qgis.org/api/annotated.html.  The
> prepare and referencedColumns are no longer defined. I did have a look at
> the C++ code for QgsExpression, but I find I'm still looking for guidance.
> 
> I've searched the QEP issues  been able to locate a QEP, or other
> documentation.  Can anyone point me to some documentation or provide an
> example of current best practice please.

Correction: prepare and referencedColumns are still present, prepare takes
QgsExpressionContext rather than fields.  So I think the pattern is very
little changed





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