Default Layer in the Inputfield in QGIS processing script

Question

I am writing a script and I would like to let the user choose the layer but i would like to default a layer (when it is open in the project) in the input field and if the user changes it he can choose another one from the list. I can specify a the drop down list or a specific layer but then the dropdown option is no longer available. Can I specify a default layer like I can do it with numbers?

This gives me the dropdown list with my layer options from the list

alg_params = {
 'INPUT': parameters['Input_name'],
 'INPUT_FIELDS': [''],
 'OVERLAY': parameters['Overlay_name'],
 'OVERLAY_FIELDS': [''],
 'OVERLAY_FIELDS_PREFIX': '',
 'OUTPUT': parameters['result_name']

In this I choose the layer, but its not a default, it is hard set and my dropdown list disappears.

alg_params = {
 'INPUT': 'Input_name_d9798079_c0d4_4e60_a3f7_ce2ca427d004',
 'INPUT_FIELDS': [''],
 'OVERLAY': parameters['Overlay_name'],
 'OVERLAY_FIELDS': [''],
 'OVERLAY_FIELDS_PREFIX': '',
 'OUTPUT': parameters['result_name']

Answer 1

What you want can be done when you add the parameters to your processing script class in the initAlgorithm function

...
 def initAlgorithm(self, config=None):
 """
 Here we define the inputs and output of the algorithm, along
 with some other properties.
 """
 # We add the input vector features source. It can have any kind of
 # geometry.
 self.addParameter(
 QgsProcessingParameterFeatureSource(
 self.INPUT,
 self.tr('Input layer'),
 [QgsProcessing.TypeVectorAnyGeometry],
 defaultValue="the layer name in legend or the layer.shp file path" # HERE
 )
 )
 ...

• miraculix: perfekt, it worked out. Thanks a lot!
• Kalak: Happy to help, accept the answer if it worked, thank you
• miraculix: Sure, good point.
• miraculix: Hi Louis, do you happen to know how to define the path to a layer in a geopackage? It not working with just the path I tried it with QgsProject.instance().readPath but I can't get it. In another Step i load a layer from the GeoPackage by the name with this: C:/Users/.../../Geopacke.gpkg|layername=Potenzial_WEA
• Kalak: The path seems good, are you trying to load it through qgsvectorlayer() or the processing algorithms input?