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See also
 
See also
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* https://github.com/simran001/PyQGIS_workshop
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* Step-by-step guide for the workshop on [https://github.com/simran001/GeoPython-Workshop/blob/master/README.md Github.]
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* https://github.com/simran001/PyQGIS
 
* https://etherpad.coredump.ch/p/Python-Workshop
 
* https://etherpad.coredump.ch/p/Python-Workshop
  
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''Custom Python Expression Functions for QGIS''
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== Usage of Expression Functions ==
  
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Background - Where are Expression Functions used?
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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.
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* Any place
 
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* Processing => Refactor fields
 
  
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Collected use cases:
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This is a more systematic list:
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* Add new (calculated) field
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# "Server-side" / data-related:
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* Autoincrement / Sequence => QGIS 2.18 default Werte (QGIS 3 geplant)
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## "Select features using an expression" based on a field(Layer property).
 +
## "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)'.
 +
## "Layer > Properties > Tab Fields" in Button "Text Edit" in default- and in constraints field.
 +
## "Layer > Properties > Tab Style" in "Label with" field.
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## "Field Calculator" to defining new or virtual fields (Layer property).
 +
## "Conditional formatting" of a field in Attribute table (Create a new rule and set the condition to the output of a custom EFn).
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# Analysis-related:
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## "Statistics Panel" / "Statistical Summary" of a field.
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## "Atlas Generation" in the Print Composer. Project > Print Composer > Check 'generate an atlas' > Set EFn based values for page name, filter and output filename.
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## Processing
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# "Client-side" / style-related:
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## "Map Tip" tool (Layer property).
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## "Rule based labeling" (drop-down menu) in Layer Properties > Labels; click on '+' button and add a EFn based filter to a field.
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## Part of "Geometry generator" for geometry field in "Style > Symbol Layer Type"
  
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Possible examples:
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== Possible examples of custom Custom Expression Functions ==
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# Function reading from other feature properties
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# Function doing something "local" like from other field(s).
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# Functions aka predefined variables (@), "Singletons" like System Env (usage "env('USERNAME')") layer properties or number sequence (spcial Plugin needed?)  
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# Functions reading something like a "Singleton" like a system environment variable (usage "env('USERNAME')") layer properties or number sequence (spcial Plugin needed?).
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# Function for remote webservice: geocoding (reading from the internet)
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# Function for remote webservice: geocoding (reading from the internet).
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# Function reading from other layers (1:1)
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# 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
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* 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:
 
Plugins:
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* ExpressionPlus (for QGIS)
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* ExpressionPlus (for QGIS) Extra functions for the expression engine which didn't make it into QGIS 2.0: https://plugins.qgis.org/plugins/qgsexpressionsplus/
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* refFunctions
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* refFunctions: http://plugins.qgis.org/plugins/refFunctions/  
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* spatialJoin
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* spatialJoin (depends on refFunctions): http://plugins.qgis.org/plugins/spatialJoin/
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* AutoFields (obsolete)
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* AutoFields (obsolete?) - Automatic Field Calculator Plugin: http://plugins.qgis.org/plugins/AutoFields/  
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Writing:
 
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* How to install and share an Expression Function?
 
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** Save and Load in "Function Edtior" (saves to "%USERHOME%.qgis2\python\expressions" (where %USERHOME% under Windows is e.g. "C:\Users\sfkeller\").
 
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** Add Fn. to qgis2/python/startup.py
 
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** Add Fn. to any (own and loaded) QGIS plugin
 
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** Add Fn. to the "Expresion Plus Plugin"
 
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* Debugging:
 
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** use "print"
 
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** use pdb (see QGIS docs and giswiki)
 
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** (use Remote Debug Plugin)
 
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* FAQ
 
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** How to store a value in PyQGIS? Use QSettings (/.qgis) or QGISProject (.qgs)
 
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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/
 
−
 
 
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How to import a user defined expression functions on QGIS start up. => Add them to qgis2/python/startup.py
 
−
 
 
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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 
 
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http://gis.stackexchange.com/questions/212415/referencing-to-features-geometry-in-an-expression/212762#212762
 
−
 
 
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Automatic Field Calculator Plugin: http://plugins.qgis.org/plugins/AutoFields/  
 
−
 
 
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Expressions Plus Plugin - Extra functions for the expression engine which didn't make it into QGIS 2.0: https://plugins.qgis.org/plugins/qgsexpressionsplus/
 
  
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Resource Sharing Plugin (http://plugins.qgis.org/plugins/qgis_resource_sharing/)
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Open:
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E.g. https://github.com/mara91/qgis_styles.git
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* 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

Aktuelle Version vom 20. April 2018, 01:10 Uhr

See also

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:

Plugins:

Open:

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