Diskussion:Workshop QGIS Python GeoPython 2017: Unterschied zwischen den Versionen

Version vom 9. März 2017, 14:55 Uhr

See also

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

Custom Python Expression Functions for QGIS

Possible examples:

• Function reading from other feature properties
• Function reading from other layers (1:1)
• Functions aka system variables and "Singletons" like layer properties or number sequence (spcial Plugin needed?)
• Function for remote webservice: geocoding (reading from the internet)

QGIS 2.18.x LTE

Where are Expression Functions used?

• Processing => Refactor fields

Use cases:

• z.B. env('USERNAME')
• Autoincrement / Sequence => QGIS 2.18 default Werte (QGIS 3 geplant)
• Add new (calculated) field

Plugins:

• ExpressionPlus (for QGIS)
• refFunctions
• spatialJoin
• AutoFields (obsolete)

Writing:

• How to install and share an Expression Function?
  • Save and 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 any (own and loaded) QGIS plugin
  • Add Fn. to the "Expresion Plus Plugin"
• Debugging:
  • use "print"
  • use pdb (see QGIS docs and giswiki)
  • (use Remote Debug Plugin)
• FAQ
  • How to store a value in PyQGIS? Use QSettings (/.qgis) or QGISProject (.qgs)

Notes:

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/

How to import a user defined expression functions on QGIS start up. => Add them to qgis2/python/startup.py

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

Automatic Field Calculator Plugin: http://plugins.qgis.org/plugins/AutoFields/

Expressions Plus Plugin - Extra functions for the expression engine which didn't make it into QGIS 2.0: https://plugins.qgis.org/plugins/qgsexpressionsplus/

Resource Sharing Plugin (http://plugins.qgis.org/plugins/qgis_resource_sharing/) E.g. https://github.com/mara91/qgis_styles.git