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

Examples where Expression Functions (EFns) can be used in QGIS are: symbology, labeling, calculating fields, defining virtual fields, selecting and filtering data, layouts, atlas serial printing and reports, map tips, custom python expressions, parameters for processing, and many more.

This is a more systematic list:

1. "Server-side" / data-related:
   1. "Select features using an expression" based on a field(Layer property).
   2. "Filtering features" while loading from source. See form view of 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" field.
   5. "Field Calculator" to defining new or virtual fields (Layer property).
   6. "Conditional formatting" of a field in Attribute table (Create a new rule and set the condition to the output of a custom EFn).
2. Analysis-related:
   1. "Statistics Panel" / "Statistical Summary" of a field.
   2. "Atlas Generation" in the Print Composer. Project > Print Composer > Check 'generate an atlas' > Set EFn based values for page name, filter and output filename.
   3. Processing
3. "Client-side" / style-related:
   1. "Map Tip" tool (Layer property).
   2. "Rule based labeling" (drop-down menu) in Layer Properties > Labels; click on '+' button and add a EFn based filter to a field.
   3. Part of "Geometry generator" for geometry field in "Style > Symbol Layer Type"

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. Function reading from other layers (out of scope for a workshop rather, ev. also a processing or plugin candidate).

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