Diskussion:Workshop QGIS Python GeoPython 2017

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. "Map Tip" tool
7. "Conditional formatting" in the attribute table (Create a new rule and set the condition to the output of a CEF).
8. "Filtering features". Switch to the form view in the attribute table (button on bottom right corner) > click on 'Show All Features' (bottom left) and select 'Advanced Filter (Expression)'.
9. "Atlas Generation" in the Print Composer. Project > Print Composer > Check 'generate an atlas' > Set CEF based values for page name, filter and output filename.
10. "Rule based labeling". Layer Properties > Labels > Select Rule based labeling from the drop-down menu > Click on the '+' button and add a CEF based filter (... button).
11. (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