QGIS geoprocessing scripts, default values for field names

Question

In QGIS geoprocessing scripts, default values must be provided for input numbers and strings, according to http://docs.qgis.org/2.8/en/docs/user_manual/processing/console.html#creating-scripts-and-running-them-from-the-toolbox like:

##mynumer=number 3.1415329

Since I often have to process layers with all the same field names (unfortunately not assurable in any case), I would like to pass a default value for a field name to the geoprocessing script, following a logic like 'take the default field name if it exists in the layer referred to, otherwise let the user select one'.

Is this possible somehow, perhaps with a bit of python?

Answer 1

Not sure how to modify the interface of such scripts but you could use an example script like the following where the Enter_field parameter has a default string of ID and if a field name matches this, it prints this field name otherwise it will print the one that is selected in the Or_select_field parameter:

##Example=name
##Input_layer=vector
##Enter_field_name=string ID
##Or_select_field=field Input_layer

layer = processing.getObject(Input_layer) 
field_names = [ field.name() for field in layer.pendingFields() ]

if Enter_field_name in field_names:
    print Enter_field_name
else:
    print Or_select_field

You would replace the print statements and use your code for both if, else statements while changing the Enter_field and Or_select_field parameters accordingly.

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Side note:

If that number is supposed to be pi, should it be 3.1415926? :)