[QGIS-Developer] QGIS3 - processing python script. Read features from output layer

[Nyall Dawson]

On 16 February 2018 at 17:29, kimaidou <kimaidou at gmail.com> wrote:
> Thanks Nyall,
>
> Our posts crossed !
>
> Ok for the shortcut and the getMapLayer method.
>
> One question : in my alg, if I use QgsProcessingParameterFeatureSource
> instead of QgsProcessingParameterVectorLayer for my input, can I still use
> mapLayerFromString and the getFeatures, or is there a
> featureSourceFromString which allows me to loop through the features ?

There's no method like that. At the moment all vector outputs from
processing algorithms are full vector layers (either disk-based or
memory layers). That's because currently only vector layers and vector
data providers are QgsFeatureSources. It's possible (likely?) that in
future we'll see this change at may need to add that method. [1]

Nyall

[1] E.g. I'd like to see QgsFeatureStore become a QgsFeatureSource,
and act as a simple dump for features without the complexity of using
a memory layer. I think there's a lot of value in QgsVectorLayerCache
also becoming a QgsFeatureSource... I think some processing algorithms
could greatly benefit from using a QgsVectorLayerCache, when they are
iterating multiple times of a source layer....







> Michaël
>
> 2018-02-16 7:22 GMT+01:00 Nyall Dawson <nyall.dawson at gmail.com>:
>>
>> On 16 February 2018 at 04:36, kimaidou <kimaidou at gmail.com> wrote:
>> > Hi all,
>> >
>> > I have a simple processing python alg which
>> > * generates a buffer with qgis:buffer
>> > * use qgis:fieldcalculator to add an buf_area field -> the output vector
>> > of
>> > my alg is this buffer+new field.
>> >
>> > I would like to parse the features of the output vector in order to
>> > gather
>> > some information and put it in a new string output. I have not been able
>> > to
>> > get the layer object to perform a getFeatures on it.
>> >
>> > See the code
>> > http://paste.debian.net/1010463/
>> >
>> > In line 32, my blayer is None
>> > I think I am not using the good way to get the output vector layer
>> > features?
>> > I tried to use the same way as for reading an input layer, but oviously
>> > it
>> > is different.
>>
>> blayer = QgsProcessingUtils.mapLayerFromString( area_result[ 'OUTPUT'
>> ]), context )
>>
>> I'm thinking of adding a shortcut for this to the context, e.g.:
>>
>> blayer = context.getMapLayer( area_result['OUTPUT'] )
>>
>> What do you think?
>>
>> Nyall
>
>