6. Processing Algorithms Testing

6.1. Algorithm tests


The original version of these instructions is available at https://github.com/qgis/QGIS/blob/release-3_16/python/plugins/processing/tests/README.md

QGIS provides several algorithms under the Processing framework. You can extend this list with algorithms of your own and, like any new feature, adding tests is required.

To test algorithms you can add entries into testdata/qgis_algorithm_tests.yaml or testdata/gdal_algorithm_tests.yaml as appropriate.

This file is structured with yaml syntax.

A basic test appears under the toplevel key tests and looks like this:

- name: centroid
  algorithm: qgis:polygoncentroids
    - type: vector
      name: polys.gml
      type: vector
      name: expected/polys_centroid.gml

6.1.1. How To

To add a new test please follow these steps:

  1. Run the algorithm you want to test in QGIS from the processing toolbox. If the result is a vector layer prefer GML, with its XSD, as output for its support of mixed geometry types and good readability. Redirect output to python/plugins/processing/tests/testdata/expected. For input layers prefer to use what’s already there in the folder testdata. If you need extra data, put it into testdata/custom.

  2. When you have run the algorithm, go to Processing ► History and find the algorithm which you have just run.

  3. Right click the algorithm and click Create Test. A new window will open with a text definition.

  4. Open the file python/plugins/processing/tests/testdata/algorithm_tests.yaml, copy the text definition there.

The first string from the command goes to the key algorithm, the subsequent ones to params and the last one(s) to results.

The above translates to

- name: densify
  algorithm: qgis:densifygeometriesgivenaninterval
    - type: vector
      name: polys.gml
    - 2 # Interval
      type: vector
      name: expected/polys_densify.gml

It is also possible to create tests for Processing scripts. Scripts should be placed in the scripts subdirectory in the test data directory python/plugins/processing/tests/testdata/. The script file name should match the script algorithm name.

6.1.2. Parameters and results

Trivial type parameters

Parameters and results are specified as lists or dictionaries:

  NAME: A processing test


  - 2
  - string
  - another param

Layer type parameters

You will often need to specify layers as parameters. To specify a layer you will need to specify:

  • the type, ie vector or raster

  • a name, with a relative path like expected/polys_centroid.gml

This is what it looks like in action:

  PAR: 2
  STR: string
    type: vector
    name: polys.gml
  OTHER: another param

File type parameters

If you need an external file for the algorithm test, you need to specify the ‚file‘ type and the (relative) path to the file in its ‚name‘:

  PAR: 2
  STR: string
    type: file
    name: custom/grass7/extfile.txt
  OTHER: another param


Results are specified very similarly.

Basic vector files

It couldn’t be more trivial

 name: expected/qgis_intersection.gml
 type: vector

Add the expected GML and XSD files in the folder.

Vector with tolerance

Sometimes different platforms create slightly different results which are still acceptable. In this case (but only then) you may also use additional properties to define how a layer is compared.

To deal with a certain tolerance for output values you can specify a compare property for an output. The compare property can contain sub-properties for fields. This contains information about how precisely a certain field is compared (precision) or a field can even entirely be skip``ed. There is a special field name ``__all__ which will apply a certain tolerance to all fields. There is another property geometry which also accepts a precision which is applied to each vertex.

 type: vector
 name: expected/abcd.gml
       precision: 5 # compare to a precision of .00001 on all fields
     A: skip # skip field A
     precision: 5 # compare coordinates with a precision of 5 digits
Raster files

Raster files are compared with a hash checksum. This is calculated when you create a test from the processing history.

 type: rasterhash
 hash: f1fedeb6782f9389cf43590d4c85ada9155ab61fef6dc285aaeb54d6

You can compare the content of an output file to an expected result reference file

 name: expected/basic_statistics_string.html
 type: file

Or you can use one or more regular expressions that will be matched against the file content

 name: layer_info.html
 type: regex
   - 'Extent: \(-1.000000, -3.000000\) - \(11.000000, 5.000000\)'
   - 'Geometry: Line String'
   - 'Feature Count: 6'

You can compare the content of an output directory with an expected result reference directory

 name: expected/tiles_xyz/test_1
 type: directory

6.1.3. Algorithm Context

There are a few more definitions that can modify the context of the algorithm - these can be specified at the top level of test:

  • project - will load a specified QGIS project file before running the algorithm. If not specified, the algorithm will run with an empty project

  • project_crs - overrides the default project CRS - e.g. EPSG:27700

  • ellipsoid - overrides the default project ellipsoid used for measurements, e.g. GRS80

6.1.4. Running tests locally

ctest -V -R ProcessingQgisAlgorithmsTest

or one of the following values listed in the CMakelists.txt