Diskussion:Workshop QGIS Python GeoPython 2017

Function doing something "local" like from other field(s).
Functions reading something like a "Singleton" like a system environment variable (usage "env('USERNAME')") layer properties or number sequence (spcial Plugin needed?).
Function for remote webservice: geocoding (reading from the internet).
Out of scope: Function reading from other layers (rather candidate for a plugin).

After the GeoPython Workshop, Simran makes pull request to Expression Plus Plugin adding "env()" fn.

Notes

Collected (special) use cases:

Autoincrement / Sequence => QGIS 2.18 default Werte (QGIS 3 geplant)
Given polygon layer and a point one, calculate for each polygon feature, the number of points that it covers: https://lists.osgeo.org/pipermail/qgis-user/2016-October/038139.html http://gis.stackexchange.com/questions/212415/referencing-to-features-geometry-in-an-expression/212762#212762
Add attributes from a layer in the project or an external file (csv) to a layer (or the composer?) using insert expression? Create a custom expression python function to do this: http://nathanw.net/2012/11/10/user-defined-expression-functions-for-qgis/

Plugins:

ExpressionPlus (for QGIS) Extra functions for the expression engine which didn't make it into QGIS 2.0: https://plugins.qgis.org/plugins/qgsexpressionsplus/
refFunctions
spatialJoin
AutoFields (obsolete) - Automatic Field Calculator Plugin: http://plugins.qgis.org/plugins/AutoFields/

Open:

How to store a value in PyQGIS? Use QSettings (/.qgis) or QGISProject (.qgs)
Resource Sharing Plugin (http://plugins.qgis.org/plugins/qgis_resource_sharing/). e.g. https://github.com/mara91/qgis_styles.git