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

Version vom 27. April 2017, 16:33 Uhr

See also

• Step-by-step guide for the workshop on Github.
• https://github.com/simran001/PyQGIS
• https://etherpad.coredump.ch/p/Python-Workshop

User Defined or Custom Expression Functions (CEFs) can be used in ...

1. "Layer > Properties > Tab Fields" in Button "Text Edit" in Default- and in Constraints-Field.
2. "Layer > Properties > Tab Style" in "Label with".
3. "Select features using an expression".
4. "Field Calculator".
5. "Statistical Summary" Panel.
6. (other places?)

Possible examples

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

Optional (but highly demanded) use case: A user wants to create a layer (table) which as a primary key (id) which behaves like 'autoincrement' when user edit a new feature. So user does not have to care about unique, sequential values for the PK, like autoincrement in MySQL). In QGIS 3 this is available when user chooses "UNIQUE" as constraint. In QGIS 2.18 (or before) there is a clever custom expression function needed. => Solution?

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: http://plugins.qgis.org/plugins/refFunctions/
• spatialJoin (depends on refFunctions): http://plugins.qgis.org/plugins/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