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

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* Any place where expressions are allowed, e.g. in Attribute Table as Select-Filter, in Field Calculator when creating/updatin fields.
 
* 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
 +
 +
How to use:
 
* See http://docs.qgis.org/testing/en/docs/user_manual/working_with_vector/expression.html?highlight=expression
 
* See http://docs.qgis.org/testing/en/docs/user_manual/working_with_vector/expression.html?highlight=expression
 +
*
  
 
Possible examples:
 
Possible examples:
−
# Function reading from other feature properties
+
# Function doing something "local".
−
# Functions aka predefined variables ("@layer_name"), "Singletons" like system environment variables (usage "env('USERNAME')") layer properties or number sequence (spcial Plugin needed?)  
+
# Function reading from other field(s).
−
# Function for remote webservice: geocoding (reading from the internet)
+
# Functions aka "Singleton" like system environment variables (usage "env('USERNAME')") layer properties or number sequence (spcial Plugin needed?).
−
# Function reading from other layers (1:1)
+
# Function for remote webservice: geocoding (reading from the internet).
 +
# Function reading from other layers (outlook, rather candidate for a plugin).
  
  

Version vom 9. März 2017, 15:22 Uhr

See also

Custom Python Expression Functions for QGIS


Using Expression Functions =

Where are Expression Functions used?

  • Any place where expressions are allowed, e.g. in Attribute Table as Select-Filter, in Field Calculator when creating/updatin fields.
  • Processing => Refactor fields

How to use:

Possible examples:

  1. Function doing something "local".
  2. Function reading from other field(s).
  3. Functions aka "Singleton" like system environment variables (usage "env('USERNAME')") layer properties or number sequence (spcial Plugin needed?).
  4. Function for remote webservice: geocoding (reading from the internet).
  5. Function reading from other layers (outlook, rather candidate for a plugin).


Writing Expression Functions

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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