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18.28. Pre- and post-execution script hooks


This lesson shows how to use pre- and post-execution hooks, which allow to perform additional operations before and after actual processing.

Pre- and post-execution hooks are Processing scripts that run before and after actual data processing is performed. This can be used to automate tasks that should be performed whenever an algorithm is executed.

The syntax of the hooks is identical to the syntax of Processing scripts, see the corresponding chapter in the QGIS User Guide for more details.

In addition to all scripts features, in hooks you can use a special global variable named alg, which represents the algorithm that has just been (or is about to be) executed.

Here is an example post-execution script. By default, Processing stores analysis results in temporary files. This script will copy outputs to a specific directory, so they won’t be deleted after closing QGIS.

import os
import shutil
from processing.core.outputs import OutputVector, OutputRaster, OutputFile

MY_DIRECTORY = '/home/alex/outputs'

for output in alg.outputs:
    if isinstance(output, (OutputVector, OutputRaster, OutputFile)):
        dirname = os.path.split(output.value)[0]
        shutil.copytree(dirname, MY_DIRECTORY)

In the first two lines we import the required Python packages: os — for path manipulations, e.g. extracting file name, and shutil — for various filesystem operations like copying files. In the third line we import Processing outputs. This will be explained in more detail later in this lesson.

Then we define a MY_DIRECTORY constant, which is the path to the directory where we want to copy analysis results.

At the end of the script, we have the main hook code. In the loop we iterate over all algorithm outputs and check if this output is a file-based output and can be copied. If so, we determine top-level directory in which output files are located and then copy all files to our directory.

To activate this hook we neeed to open the Processing options, find the entry named Post-execution script file in the General group, and specify the filename of the hook script there. the specified hook will be executed after each Processing algorithm.

In a similar way, we can implement pre-execution hooks. For example, let’s create a hook to check input vectors for geometry errors.

from qgis.core import QgsGeometry, QgsFeatureRequest
from processing.core.parameters import ParameterVector

for param in alg.parameters:
    if isinstance(param, ParameterVector):
        layer = processing.getObject(param.value)
        for f in layer.getFeatures(QgsFeatureRequest().setSubsetOfAttributes([])):
            errors = f.geometry().validateGeometry()
            if len(errors) > 0:
                progress.setInfo('One of the input vectors contains invalid geometries!')

As in the previous example, first we import required QGIS and Processing packages.

Then we iterate over all the algorithm parameters and if a ParameterVector parameter is found, we get the corresponding vector layer object from it. We loop over all the features of the layer and check them for geometry errors. If at least one feature contains an invalid geometry, we print a warning message.

To activate this hook we need enter its filename in the Pre-execution script file option in the Processing configuration dialog. The hook will be executed before running any Processing algorithm.