Diskussion:Workshop QGIS Python GeoPython 2017

See also

• https://github.com/simran001/PyQGIS_workshop
• https://etherpad.coredump.ch/p/Python-Workshop

Custom Python Expression Functions for QGIS

Background - Where are Expression Functions used?

• Any place where expressions are allowed, e.g. in Attribute Table as Select-Filter, in Field Calculator when creating/updatin fields.
• Processing => Refactor fields
• See http://docs.qgis.org/testing/en/docs/user_manual/working_with_vector/expression.html?highlight=expression

Possible examples:

1. Function reading from other feature properties
2. Functions aka predefined variables ("@layer_name"), "Singletons" like system environment variables (usage "env('USERNAME')") layer properties or number sequence (spcial Plugin needed?)
3. Function for remote webservice: geocoding (reading from the internet)
4. Function reading from other layers (1:1)

Writing

How to install and share an Expression Function?

• Save and (Re-)Load in "Function Edtior" (saves to "%USERHOME%.qgis2\python\expressions" (where %USERHOME% under Windows is e.g. "C:\Users\sfkeller\").
• Add Fn. to qgis2/python/startup.py
• Add Fn. to the "Expresion Plus Plugin"
• Add Fn. to any (own and loaded) QGIS plugin

Debugging:

• use "print"
• use pdb (see QGIS docs and giswiki)
• (use Remote Debug Plugin)

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