[Qgis-user] pyQGIS - set attributes by expression

[Nyall Dawson]

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
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