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

Mon Jun 17 10:33:06 PDT 2013

 plugins
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You don't need go use a while, I don't know why people keep using that
pattern when a simple for will do and is cleaner.

For f in layer.getFeatures:

No it just comes down to

If selected:
features = layers.selectedFeatures()[]
Else:
features = layer.getFeatures()
From: Ricardo Filipe Soares Garcia da
Sent: 17/06/2013 11:47 PM
To: qgis-developer at lists.osgeo.org
Subject: [Qgis-developer] iteration over a layer's features in python
plugins
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

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<html><head><meta content=3D"text/html; charset=3Dutf-8" http-equiv=3D"Cont=
ent-Type"></head><body><div><div style=3D"font-family: Calibri,sans-serif; =
font-size: 11pt;">You don't need go use a while, I don't know why people ke=
ep using that pattern when a simple for will do and is cleaner.<br><br>For =
f in layer.getFeatures:<br><br>No it just comes down to<br><br>If selected:=
<br>    features =3D layers.selectedFeatures()[]<br>Else:<br=
>    features =3D layer.getFeatures()<br></div></div><hr><sp=
an style=3D"font-family: Tahoma,sans-serif; font-size: 10pt; font-weight: b=
old;">From: </span><span style=3D"font-family: Tahoma,sans-serif; font-size=
: 10pt;">Ricardo Filipe Soares Garcia da</span><br><span style=3D"font-fami=
ly: Tahoma,sans-serif; font-size: 10pt; font-weight: bold;">Sent: </span><s=
pan style=3D"font-family: Tahoma,sans-serif; font-size: 10pt;">17/06/2013 1=
1:47 PM</span><br><span style=3D"font-family: Tahoma,sans-serif; font-size:=
 10pt; font-weight: bold;">To: </span><span style=3D"font-family: Tahoma,sa=
ns-serif; font-size: 10pt;">qgis-developer at lists.osgeo.org</span><br><span =
style=3D"font-family: Tahoma,sans-serif; font-size: 10pt; font-weight: bold=
;">Subject: </span><span style=3D"font-family: Tahoma,sans-serif; font-size=
: 10pt;">[Qgis-developer] iteration over a layer's features in python plugi=
ns</span><br><br></body></html><div dir=3D"ltr">Hi list<div><br></div><div>=
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).</div>

<div><br></div><div>However, I think the QGIS API does not make the impleme=
ntation of this pattern as easy as it could be. The QgsVectorLayer.getFeatu=
res() method returns an iterator but the QgsVectorLayer.selectedFeatures() =
returns a list. It would be nice if they both returned an iterator.=C2=A0</=
div>

<div><br></div><div>When working with all the features in a layer, I =C2=A0=
use:</div><div><br></div><div style>feat =3D QgsFeature()</div><div>feat_it=
erator =3D v_layer.getFeatures()</div><div style>while feat_iterator.nextFe=
ature(feat):</div>

<div style>=C2=A0 =C2=A0 # do stuff with the feature</div><div><br></div><d=
iv style>When working with only the currently selected features the code is=
 like:</div><div style><br></div><div style>selected_feats =3D v_layer.sele=
ctedFeatures()</div>

<div style>for feat in selected_feats:</div><div style>=C2=A0 =C2=A0 # do s=
tuff with the feature</div><div><div><br></div><div style>Since QgsVectorLa=
yer.getFeatures can take a QgsFeatureRequest as argument, I think a nice so=
lution could be to add one more filter to QgsFeatureRequest to allow iterat=
ing over a list of feature ids. It currently already allows to pass in a si=
ngle feature id, it would just be a matter of expanding that.</div>

<div style><br></div><div style>In the meantime I am using a custom iterato=
r class[1] in Python that wraps this different behaviour and lets me use th=
e 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.</div>

<div style><br></div><div style>Maybe I am asking for too much too late bec=
ause of the feature freeze for 2.0?</div><div style><br></div><div style><b=
r></div><div style>[1] - (see the FeatureIterator class) <a href=3D"https:/=
/github.com/ricardogsilva/coneforinputs/blob/master/coneforinputs.py">https=
://github.com/ricardogsilva/coneforinputs/blob/master/coneforinputs.py</a><=
/div>

<div style><br></div><div style><br></div>-- <br>__________________________=
_ ___ __<br>Ricardo Garcia Silva
</div></div>

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