Re: [QGIS-Developer] QgsExpression python pattern

[Nyall Dawson]

On 1 February 2018 at 05:45, 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.

That's a bug with the generated docs (fix incoming ) - it's still
around. BTW you may prefer the work-in-progress Python API docs here:
http://python.qgis.org/api/

> 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.

Best place to check first is the API breaks docs:

https://qgis.org/api/api_break.html

This details the changes in QgsExpression (amongst all the other classes).

Hope that helps!

Nyall
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Re: [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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[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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