[QGIS-Developer] Expression Functions: Where are they used in QGIS Desktop? Which ones won't work "outside"?

[Stefan Keller]

Hi,

Tomorrow I'll be giving a Workshop about "Easy Programming QGIS with
Python for custom Expression Functions (EFns)" [1].

Question 1: Below you'll find an initial list I compiled where EFns
are used in QGIS Desktop. => Did I miss an occurrence?

Question 2: I tried to make a categorization for several reasons. One
reason for this is to realize that a EFn may not work outside QGIS
Desktop. Style-related EFns for example won't work currently in
qgis2web or QGISCloud. => Any comments?

:Stefan

[1] https://2017.geopython.net/#schedule

List where EFns are used in QGIS (probably incomplete):

"Server-side" / data-related:
* "Select features using an expression" (Layer property).
* "Filtering features" while loading from source. Switch to the form
view in the attribute table (button on bottom right corner) > click on
'Show All Features' (bottom left) and select 'Advanced Filter
(Expression)'.
* "Layer > Properties > Tab Fields" in Button "Text Edit" in Default-
and in Constraints-Field.
* "Layer > Properties > Tab Style" in "Label with".
* "Field Calculator" to defining new or virtual fields (Layer property).
* "Conditional formatting" in the attribute table (Create a new rule
and set the condition to the output of a custom EFn).
Analysis-related:
* "Statistical Summary" Panel (Layer property).
* "Atlas Generation" in the Print Composer. Project > Print Composer >
Check 'generate an atlas' > Set CEF based values for page name, filter
and output filename.
"Client-side" / style-related:
* "Map Tip" tool (Layer property).
* "Rule based labeling". Layer Properties > Labels > Select Rule based
labeling from the drop-down menu > Click on the '+' button and add a
CEF based filter (... button).
* Part of "Geometry generator" in "Style > Symbol Layer Type"