[QGIS-Developer] QgsExpression python pattern

[Chris Crook]

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.

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