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

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Background - Where are Expression Functions used?
Background - Where are Expression Functions used?
* Any place  
* Any place where expressions are allowed, e.g. in Attribute Table as Select-Filter, in Field Calculator when creating/updatin fields.
* Processing => Refactor fields
* Processing => Refactor fields
 
* See http://docs.qgis.org/testing/en/docs/user_manual/working_with_vector/expression.html?highlight=expression
Collected use cases:
* Add new (calculated) field
* Autoincrement / Sequence => QGIS 2.18 default Werte (QGIS 3 geplant)


Possible examples:
Possible examples:
# Function reading from other feature properties
# Function reading from other feature properties
# Functions aka predefined variables (@), "Singletons" like System Env (usage "env('USERNAME')") layer properties or number sequence (spcial Plugin needed?)  
# Functions aka predefined variables ("@layer_name"), "Singletons" like system environment variables (usage "env('USERNAME')") layer properties or number sequence (spcial Plugin needed?)  
# Function for remote webservice: geocoding (reading from the internet)
# Function for remote webservice: geocoding (reading from the internet)
# Function reading from other layers (1:1)
# Function reading from other layers (1:1)


=== Writing ===
How to install and share an Expression Function?
* Save and (Re-)Load in "Function Edtior" (saves to "%USERHOME%.qgis2\python\expressions" (where %USERHOME% under Windows is e.g. "C:\Users\sfkeller\").
* Add Fn. to qgis2/python/startup.py
* Add Fn. to the "Expresion Plus Plugin"
* Add Fn. to any (own and loaded) QGIS plugin
Debugging:
* use "print"
* use pdb (see QGIS docs and giswiki)
* (use Remote Debug Plugin)
== 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:
Plugins:
* ExpressionPlus (for QGIS)
* 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
* refFunctions
* spatialJoin
* spatialJoin
* AutoFields (obsolete)
* AutoFields (obsolete) - Automatic Field Calculator Plugin: http://plugins.qgis.org/plugins/AutoFields/  
 
Writing:
* How to install and share an Expression Function?
** Save and (Re-)Load in "Function Edtior" (saves to "%USERHOME%.qgis2\python\expressions" (where %USERHOME% under Windows is e.g. "C:\Users\sfkeller\").
** Add Fn. to qgis2/python/startup.py
** Add Fn. to any (own and loaded) QGIS plugin
** Add Fn. to the "Expresion Plus Plugin"
* Debugging:
** use "print"
** use pdb (see QGIS docs and giswiki)
** (use Remote Debug Plugin)
* FAQ
** How to store a value in PyQGIS? Use QSettings (/.qgis) or QGISProject (.qgs)
 
 
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:
Open:
http://nathanw.net/2012/11/10/user-defined-expression-functions-for-qgis/
* How to store a value in PyQGIS? Use QSettings (/.qgis) or QGISProject (.qgs)


How to import a user defined expression functions on QGIS start up. => Add them to qgis2/python/startup.py


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


Automatic Field Calculator Plugin: http://plugins.qgis.org/plugins/AutoFields/


Expressions Plus Plugin - Extra functions for the expression engine which didn't make it into QGIS 2.0: https://plugins.qgis.org/plugins/qgsexpressionsplus/


Resource Sharing Plugin (http://plugins.qgis.org/plugins/qgis_resource_sharing/)
* Resource Sharing Plugin (http://plugins.qgis.org/plugins/qgis_resource_sharing/). e.g. https://github.com/mara91/qgis_styles.git
E.g. https://github.com/mara91/qgis_styles.git

Version vom 9. März 2017, 13:17 Uhr

See also

Custom Python Expression Functions for QGIS

Background - Where are Expression Functions used?

Possible examples:

  1. Function reading from other feature properties
  2. Functions aka predefined variables ("@layer_name"), "Singletons" like system environment variables (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 (1:1)

Writing

How to install and share an Expression Function?

  • Save and (Re-)Load in "Function Edtior" (saves to "%USERHOME%.qgis2\python\expressions" (where %USERHOME% under Windows is e.g. "C:\Users\sfkeller\").
  • Add Fn. to qgis2/python/startup.py
  • Add Fn. to the "Expresion Plus Plugin"
  • Add Fn. to any (own and loaded) QGIS plugin

Debugging:

  • use "print"
  • use pdb (see QGIS docs and giswiki)
  • (use Remote Debug Plugin)


Notes

Collected (special) use cases:

Plugins:

Open:

  • How to store a value in PyQGIS? Use QSettings (/.qgis) or QGISProject (.qgs)



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