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

Usage of Expression Functions

This is a list of places where Expression Functions (EFns) can be used in QGIS:

1. "Server-side" / data-related:
   1. "Select features using an expression" (Layer property).
   2. "Filtering features" while loading from source. 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)'.
   3. "Layer > Properties > Tab Fields" in Button "Text Edit" in Default- and in Constraints-Field.
   4. "Layer > Properties > Tab Style" in "Label with".
   5. "Field Calculator" to defining new or virtual fields (Layer property).
   6. "Conditional formatting" in the attribute table (Create a new rule and set the condition to the output of a custom EFn).
2. Analysis-related:
   1. "Statistical Summary" Panel (Layer property).
   2. "Atlas Generation" in the Print Composer. Project > Print Composer > Check 'generate an atlas' > Set CEF based values for page name, filter and output filename.
3. "Client-side" / style-related:
   1. "Map Tip" tool (Layer property).
   2. "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).
   3. Part of "Geometry generator" in "Style > Symbol Layer Type"

(Add other places to the list above if you find more).

Possible examples of custom Custom Expression Functions

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