Open the algorithm dialog of a custom QgsProcessingAlgorithm instance

Question

In a custom plugin I'm calling some custom QgsProcessingAlgorithm that are loaded with the plugin itself.

If I create an instance of these algorithms I need to prefill the parameters with some custom values and I think this can easily be done by passing a custom dictionary to the initAlgorithm method [0].

I'm setting some side variables needed by be plugin (that have not to be outputs), but again this can be done by setting some class variables (i.e. self.my_custom_id = 'zzz') and getting them when needed.

my_algorithm = MyCustomAlgorithm()
my_algorithm.initAlgorithm(
 'INPUT': 'custom_path'
)
# how to open and exec the algorithm dialog?

What I'm not able to do is to simply open/exec the algorithm dialog.

If I use processing.createAlgorithmDialog then I can exec the algorithm dialog but I cannot retrieve the side variables needed.

my_dialog = processing.createAlgorithmDialog(
 'my_custom_provider:my_custom_algorithm`,
 {
 'INPUT': 'custom_path'
 }
)
my_dialog.exec_()

I'm stuck in this loop. Somebody has already faced the same issue?

[0] https://qgis.org/pyqgis/master/core/QgsProcessingAlgorithm.html#qgis.core.QgsProcessingAlgorithm.initAlgorithm

Answer 1

In your second approach you could use the algorithm() method to retrieve the instance of your custom QgsProcessingAlgorithm and then access its class variables.

my_dialog = processing.createAlgorithmDialog(
 'my_custom_provider:my_custom_algorithm',
 {
 'INPUT': 'custom_path'
 }
)
# access your custom class variables
print(my_dialog.algorithm().my_custom_id) # prints 'zzz'
my_dialog.exec()

createAlgorithmDialog can also receive an instance of QgsProcessingAlgorithm as first argument to create a dialog. Therefore the following will also be a valid option:

my_algorithm = MyCustomAlgorithm()
my_dialog = processing.createAlgorithmDialog(
 my_algorithm,
 {
 'INPUT': 'custom_path'
 }
)
# access your custom class variables
print(my_algorithm.my_custom_id) # prints 'zzz'
# execute dialog
my_dialog.exec()

• matteo: thanks! No idea why but my QGIS (doesn't matter which version 3.22, 3.26, 3.27) is crashing if I make an instance of my algorithm and pass it to the createAlgorithmDialog method. Anyway, I can use the unique identifier. I also discovered that you have access to the resuls with my_dialog.results() after the execution.