How to make Processing algorithms use default parameter values in QGIS 2.14?

Question

I would like to know how to get processing algorithms in general, and warp (reproject) specifically, to run with default values. I am running a script in the QGIS Python console. In the QGIS 2.14 documentation it is stated that:

"Input parameters such as strings, booleans, or numerical values have default values. To use them, specify None in the corresponding parameter entry."

However, when I run the following code, I receive an error.

processing.runalg("gdalogr:warpreproject",
                   path+"land_use.tif", #input
                   "ESPG:3035", #source crs
                   "ESPG:4326", #destination crs
                   "0", #no data value
                   0, #target resolution: 0=unchanged
                   0, # method: 0=near
                   5, # output raster type: 5=Float32
                   0, #compression: 0=none
                   None, #jpeg compression: default
                   None, #zlevel: default
                   None, #predictor: default
                   None, #tiled: default
                   2, #bigtiff: 2=no
                   None, #TFW: default
                   None, #extra: default
                   path+"land_use_reprojected.tif")

The error is simply

Error: Wrong parameter value: None

I feel like QGIS is throwing an error instead of referring to default values, have I misunderstood the documentation?

I also find that the error message is uninformative.

Germán Carrillo · Accepted Answer · 2016-09-06T12:26:23.000

7

It turns out you can actually use default parameter values calling Processing algorithms from PyQGIS using a different syntax. I didn't find it in the docs, but that's what we have GIS.SE for :D.

Just call the algorithm providing parameters as a Python dictionary with keys being parameter names. A minimal example:

import processing
processing.runalg( "gdalogr:warpreproject", {"INPUT":"/docs/geodata/raster.tif", "DEST_SRS":"EPSG:32618", "METHOD":0} )

You can get parameter names calling:

processing.alghelp("gdalogr:warpreproject")

So, in your case, the call could be:

processing.runalg("gdalogr:warpreproject",
                   {"INPUT":path+"land_use.tif",
                   "SOURCE_SRS":"ESPG:3035",
                   "DEST_SRS":"ESPG:4326",
                   "NO_DATA":"0", 
                   "METHOD":0, 
                   "RTYPE":5,
                   "COMPRESS":0,
                   "BIGTIFF":2,
                   "OUTPUT":path+"land_use_reprojected.tif"})

You only need to include mandatory parameters and those for which you want to set your own values. The rest will take default values. If you miss any mandatory parameter, QGIS-Processing will let you know about that with an error message.

edited Sep 06 '16 at 12:26

answered Sep 05 '16 at 18:03

Germán Carrillo

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thanks for this! Interesting, so those parameters that should use default values can simply be left out of the call? – Jon R Sep 06 '16 at 06:38
I'm marking this as the accepted answer because it more thoroughly addresses the key issue: accessing default parameters. – Jon R Sep 06 '16 at 07:01
@JonR - I agree! I also did not know it was possible using dictionary keys, great solution :) – Joseph Sep 06 '16 at 09:16
Right @JonR, you include mandatory parameters and those for which you want to set your own values. The rest will take default values. If you miss any mandatory parameter, QGIS-Processing will let you know about that with an error message. I've included this in the answer. – Germán Carrillo Sep 06 '16 at 12:23

score 2 · Answer 2 · answered Sep 05 '16 at 10:36

I would suggest to not use None as an input parameter value (except for when you want to create a temporary output) as a number of algorithms will report the

Error: Wrong parameter value: None.

Personally, I use "" which should essentially mean the same thing. Also note that the following parameters seem to require a minimum value of 1:

JPEGCOMPRESSION
ZLEVEL
PREDICTOR

So you could try using something like:

processing.runalg("gdalogr:warpreproject",
                   path+"land_use.tif", #input
                   "ESPG:3035", #source crs
                   "ESPG:4326", #destination crs
                   "0", #no data value
                   0, #target resolution: 0=unchanged
                   0, # method: 0=near
                   5, # output raster type: 5=Float32
                   0, #compression: 0=none
                   1, #jpeg compression: default
                   1, #zlevel: default
                   1, #predictor: default
                   "", #tiled: default
                   2, #bigtiff: 2=no
                   "", #TFW: default
                   "", #extra: default
                   path+"land_use_reprojected.tif")

This answer is working, thanks. Can you recommend a good resource for learning more of the details around what values are suitable for processing algorithms, such as the minimum value requirements on zlevel, etc that you mention? — Jon R, Sep 05 '16 at 15:06
@JonR - Most welcome, glad it helped! You ask a very good question, personally I think best resource for this is linked in your question where you can call the parameters of an algorithm from the Python Console. I also gave an answer fairly recently describing briefly how to use it here. — Joseph, Sep 05 '16 at 15:11
And the minimum requirements were just trial and error, I don't think there is documents for that. Note that the Processing plugin is continually being updated so changes are being made to various algorithms in each version. Which is why is probably easier to call the help parameters from the plugin itself instead of searching for online documentation :) — Joseph, Sep 05 '16 at 15:14

How to make Processing algorithms use default parameter values in QGIS 2.14?

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