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

[Ricardo Filipe Soares Garcia da]

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.

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/qgis-developer/attachments/20130617/bc933af0/attachment.html>