[Qgis-user] pyQGIS - set attributes by expression

[Wouter Impens]

Thanks for the help Nyall, but I still don't get any result.
I'll put the whole script here, maybe I'm doing something wrong on another
level (I'm rather new in coding):

from PyQt5.QtCore import *
from qgis.core import *

canvas = qgis.utils.iface.mapCanvas()
my_layers = canvas.layers()

for a_layer in my_layers:
    a_layer.startEditing()
    a_layer.addAttribute (QgsField('perc_nr', QVariant.String))
    a_layer.commitChanges()

    my_features = a_layer.getFeatures()
    an_expr = QgsExpression('''to_string(  to_int( substr(  "CAPAKEY" , 7,
4)))+' ' +  CASE WHEN substr(  "CAPAKEY" , 14, 1) = '_'  THEN '' else
lower(substr(  "CAPAKEY" , 14, 1)) END + ' ' +  CASE WHEN to_int( substr(
"CAPAKEY" , 15, 3)) = '0'  THEN ''  else  to_string(  to_int( substr(
"CAPAKEY" , 15, 3)))  END  + ' ' +  CASE WHEN to_int( substr(  "CAPAKEY" ,
12, 2)) = '0'   THEN '' else '/' + to_string( to_int( substr(  "CAPAKEY" ,
12, 2))) END''')
    context = QgsExpressionContext()
    for a_feat in my_features:
        my_index = a_feat.fieldNameIndex('perc_nr')
        context.setFeature(a_feat)
        value = an_expr.evaluate(context)
        a_feat.setAttribute(my_index, value)

    a_layer.commitChanges()

The first part - ading the attribute - works fine, but filling in the
fields don't give any result (nor an error either).
I'm working in QGIS 3.

Thanks!
Wouter


Op do 7 feb. 2019 om 08:08 schreef Nyall Dawson <nyall.dawson at gmail.com>:

> On Wed, 6 Feb 2019 at 18:14, Wouter Impens <wouter.impens at gmail.com>
> wrote:
> >> > I'm trying to iterate over the fields of a vector layer (using the
> index) and fill in the values by an expression
> >> >
> >> >  my_features = a_layer.getFeatures()
> >> >     for a_feat in my_features:
> >> >         mijn_index = a_feat.fieldNameIndex('perc_nr')
> >> >         value = an_expr
>
> Change this to:
>
> my_features = a_layer.getFeatures()
> context = QgsExpressionContext()
> for a_feat in my_features:
>     mijn_index = a_feat.fieldNameIndex('perc_nr')
>     context.setFeature(a_feat)
>     value = an_expr.evalute(context)
>     ...
>
> Nyall
>
>
> >>
> >> What's "an_expr" here? Is it a QgsExpression object?
> >>
> >> Nyall
> >>
> >> >          a_feat.setAttribute(mijn_index, value)
> >> >
> >> >     a_layer.commitChanges()
> >> >
> >> > I don't get an error but nothing is filled in. I evaluated the
> expression and it's true. It also works in the field calculator.
> >> >
> >> > Anyone an idea?
> >> >
> >> >
> >> > Kind regards
> >> > Wouter
> >> > _______________________________________________
> >> > Qgis-user mailing list
> >> > Qgis-user at lists.osgeo.org
> >> > List info: https://lists.osgeo.org/mailman/listinfo/qgis-user
> >> > Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
>
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