[Qgis-developer] iteration over a layer's features in python plugins

[Matthias Kuhn]

Hi Ricardo,

On Mon 17 Jun 2013 15:47:08 CEST, Ricardo Filipe Soares Garcia da wrote:
> Hi list
>
> When building plugins that operate on vector layers, it is a common
> pattern to include a checkbox in the plugin's GUI where the user can
> choose to perform the plugin analysis on the whole layer or just using
> the currently selected features (see for example most fTools dialogs).
>
> However, I think the QGIS API does not make the implementation of this
> pattern as easy as it could be. The QgsVectorLayer.getFeatures()
> method returns an iterator but the QgsVectorLayer.selectedFeatures()
> returns a list. It would be nice if they both returned an iterator.
>
> When working with all the features in a layer, I  use:
>
> feat = QgsFeature()
> feat_iterator = v_layer.getFeatures()
> while feat_iterator.nextFeature(feat):
>     # do stuff with the feature
>
> When working with only the currently selected features the code is like:
>
> selected_feats = v_layer.selectedFeatures()
> for feat in selected_feats:
>     # do stuff with the feature
>
> Since QgsVectorLayer.getFeatures can take a QgsFeatureRequest as
> argument, I think a nice solution could be to add one more filter to
> QgsFeatureRequest to allow iterating over a list of feature ids. It
> currently already allows to pass in a single feature id, it would just
> be a matter of expanding that.

I've got that functionality ready in a separate branch but it will have 
to wait for 2.1 due to the feature freeze as you mentioned

>
> In the meantime I am using a custom iterator class[1] in Python that
> wraps this different behaviour and lets me use the same logic for
> processing the whole layer or just the selected features. I think it
> would be cleaner if this could be done on the API side.
>
> Maybe I am asking for too much too late because of the feature freeze
> for 2.0?
>
>
> [1] - (see the FeatureIterator class)
> https://github.com/ricardogsilva/coneforinputs/blob/master/coneforinputs.py
>
>
> --
> ___________________________ ___ __
> Ricardo Garcia Silva
>
>
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Best
Matthias