Processing dialogs don't export data-defined overrides in qgis_process and JSON syntax

[florisvdh]

What is the bug or the crash?

Several processing algorithms provide a data-defined override button next to a parameter, so that an attribute field or an expression can be inserted instead of a fixed value. Examples are the numerical DISTANCE parameter of native:buffer, and the boolean ALL_PARTS parameter of native:centroids.

The processing dialogs (tool dialog and history dialog) provide a means to export the chosen parameters as qgis_process command syntax or as a JSON string ('Advanced' button, or right clicking a history item). Both options are used in geospatial scripting outside of QGIS and Python, e.g. in bash or R.

However, the value of a data-overridden parameter is currently not captured in the exported string; instead it is empty (null in the case of JSON). E.g. we get below result – look at the DISTANCE value:

qgis_process run native:buffer --ellipsoid=EPSG:7030 --INPUT='/usr/share/qgis/resources/data/world_map.gpkg|layername=countries' --DISTANCE= --SEGMENTS=5 --END_CAP_STYLE=0 --JOIN_STYLE=0 --MITER_LIMIT=2 --DISSOLVE=false --OUTPUT=TEMPORARY_OUTPUT

and

{
  "ellipsoid": "EPSG:7030",
  "inputs": {
    "DISSOLVE": false,
    "DISTANCE": null,
    "END_CAP_STYLE": 0,
    "INPUT": "/usr/share/qgis/resources/data/world_map.gpkg|layername=countries",
    "JOIN_STYLE": 0,
    "MITER_LIMIT": 2.0,
    "OUTPUT": "TEMPORARY_OUTPUT",
    "SEGMENTS": 5
  }
}

Steps to reproduce the issue

1. Make a QGIS project with at least 1 layer. E.g. a polygon layer.
2. From the toolbox, choose an algorithm that provides a parameter with the data-defined override widget, e.g. native:buffer (to control a numerical value) or native:centroids (to control a boolean value).
3. Apply a data-defined override by clicking the corresponding button next to the parameter and select an appropriate attribute field or provide an appropriate expression – that expression may even just be a fixed value.
4. In the processing dialog, click 'Advanced' and choose either 'Copy as qgis_process command' or 'Copy as JSON'.
5. Inspect the content of your clipboard: the value of the parameter is empty (null in the case of JSON).

Versions

QGIS version	3.26.3-Buenos Aires	QGIS code revision	65e4edf
Qt version	5.12.8
Python version	3.8.10
GDAL/OGR version	3.4.3
PROJ version	8.2.0
EPSG Registry database version	v10.038 (2021-10-21)
GEOS version	3.10.2-CAPI-1.16.0
SQLite version	3.31.1
PDAL version	2.2.0
PostgreSQL client version	12.12 (Ubuntu 12.12-0ubuntu0.20.04.1)
SpatiaLite version	5.0.1
QWT version	6.1.4
QScintilla2 version	2.11.2
OS version	Linux Mint 20
Active Python plugins
geopunt4Qgis	2.2.4
ViewshedAnalysis	1.7
cartography_tools	1.2.1
quick_map_services	0.19.27
grassprovider	2.12.99
processing	2.12.99
sagaprovider	2.12.99
db_manager	0.1.20
MetaSearch	0.3.6
otbprovider	2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

No response