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

Using Expression Functions

Where are Expression Functions used?

• Any place where expressions are allowed, e.g. in Attribute Table as Select-Filter, Label-Properties, in Field Calculator when creating/updating fields.
• Processing => Refactor fields

How to use:

• See http://docs.qgis.org/testing/en/docs/user_manual/working_with_vector/expression.html?highlight=expression
• and this workshop

Possible examples:

1. Function doing something "local".
2. Function reading from other field(s).
3. Functions aka "Singleton" like system environment variables (usage "env('USERNAME')") layer properties or number sequence (spcial Plugin needed?).
4. Function for remote webservice: geocoding (reading from the internet).
5. Function reading from other layers (outlook, rather candidate for a plugin).

Writing Expression Functions

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