While many processing algorithms supported data-defined parameter values, these were not possible to specify when using the qgis_process tool (or when representing algorithm settings as JSON and qgis_process commands), introducing limitations for utilizing QGIS within command line processing operations.
A new syntax is now provided to allow the specification of data-defined parameter values when running qgis_process operations, and copying the algorithm settings from QGIS will now correctly handle data-defined values.
Command line arguments are specified with the expression: prefix within an encapsulated string, whilst the JSON definition is implemented using an expression object with a data_defined type definition. Note that special characters within the expression will need to use the appropriate escape characters relevant to the context in which they are used.
Considering the expression @variable*"field_name"*to_int('2')/100 for the following examples, the command-line syntax would be:
--PARAMETER='expression:@variable*"field_name"*to_int('\''2'\'')/100'
Whilst the JSON representation would be:
"inputs": {
"PARAMETER": {
"expression": "@variable*\"field_name\"*to_int('2')/100",
"type": "data_defined"
},
"OUTPUT": "TEMPORARY_OUTPUT"
}
This feature was funded by the Research Institute for Nature and Forest, Flemish Govt
This feature was developed by Nyall Dawson