[Qgis-user] How do I pass parameters to grass7 algorithms with Processing.run?

[Stephen]

Hi Nyall,

I'm revisiting this problem now after a month. Thanks for your help on this.

On 11.04.2018 02:03, Nyall Dawson wrote:
>> After correcting the statement to the following:
>>
>>> file_grid_r = QgsRasterLayer()
>>> processing.run("grass7:v.to.rast", {'input': grid_v, 'type': 0, 'use': 1,
>>>                                     'GRASS_REGION_PARAMETER': extent,
>>>                                     'output': file_grid_r,
>>>                                     'GRASS_REGION_CELLSIZE_PARAMETER':
>>> 0.008333,
>>>                                     'GRASS_SNAP_TOLERANCE_PARAMETER': -1,
>>>                                     'GRASS_MIN_AREA_PARAMETER': 0.000100},
>>>                              feedback=QgsProcessingFeedback())
>>
>>
>>  I get the message
>>
>>> Unable to execute algorithm
>>> Incorrect parameter value for output
> 
> The OUTPUT parameter should be set to the path to save the result,
> e.g. 'd:\blahblah.tif":
> 
>  processing.run("grass7:v.to.rast", {'input': grid_v, 'type': 0, 'use': 1,
>                                      'GRASS_REGION_PARAMETER': extent,
>                                      'output': 'd:\blah.tif',
>                                      'GRASS_REGION_CELLSIZE_PARAMETER':
>  0.008333,
>                                      'GRASS_SNAP_TOLERANCE_PARAMETER': -1,
>                                      'GRASS_MIN_AREA_PARAMETER': 0.000100},
>                               feedback=QgsProcessingFeedback())
> 
> processing.run will return a dict a results, and inside that you'll
> automatically have a QgsRasterLayer loaded corresponding to this
> output.
> 
> E.g.
> 
> params = { ....., 'OUTPUT': 'd:\blah.tif' }
> results = processing.run('grass7:v.to.rast', params, feedback )
> my_raster_layer = results['OUTPUT']
> assert my_raster_layer.isValid()
> 
> Here my_raster_layer will be a QgsRasterLayer, ready for further
> processing or adding to the project for visualisation.
> 
> I'm pondering whether we should automatically set output locations to
> temporary files if they aren't specified. E.g. if you don't set OUTPUT
> to 'd:\blah.tif', it'll fill it in automatically with a .tif file in
> your temp folder...
> 
> Nyall

Automatically setting output paths to temporary files would be a
sensible convenience, I think.

In any event, I used processing.tools.system.getTempFilename() to set a
filename and that gets me past the 'Incorrect parameter value for
output' error.

However, I am still getting "Unable to execute algorithm".

In AlgorithmExecutor, line 55:
results, ok = alg.run(parameters, context, feedback)

returns ({}, False)

parameters contains:

{'GRASS_REGION_CELLSIZE_PARAMETER': 0.008333, 'use': 1,
'GRASS_REGION_PARAMETER':
'33.91415594546105,41.91384412832699,-4.679722671256611,5.467319665842772',
'input': <qgis._core.QgsVectorLayer object at 0x7fdbbd02bc18>,
'GRASS_MIN_AREA_PARAMETER': 0.0001, 'GRASS_SNAP_TOLERANCE_PARAMETER':
-1, 'type': 0, 'output':
'/tmp/processing_da49d2e8c1bd4e249d1a5c6d42680149/5af9540a0b2212'}

context contains:

<qgis._core.QgsProcessingContext object at 0x7fdbbd04e3a8>

and feedback contains:

<qgis._core.QgsProcessingFeedback object at 0x7fdbbd02bee8>

Without a useful error message, it's very difficult to troubleshoot this
on my own.

Do you have any idea what might be wrong here?

Thanks

Stephen Bosch