5

I would like to create and populate a virtual field in a QGIS vector layer attribute table using a python script. I think the easiest way to do this is to create a custom expression function, and, then use this function to populate the virtual field. How do I instantiate a custom expression function from within a python script? What I have tried with Python 3 from a Python IDE is:

    from qgis.core import *

@qgsfunction(args=0, group='Custom', usesGeometry=False)
def a_function():
    return "'Test'"

QgsExpression.registerFunction(a_function)
print(QgsExpression(a_function()).evaluate())

This returns a TypeError:

print(QgsExpression(a_function()).evaluate())
TypeError: 'QgsPyExpressionFunction' object is not callable

2 Answers 2

4

The following works, but, I don't know why:

from qgis.core import *
@qgsfunction(args='auto', group='Custom', usesGeometry=False)
def a_function(feature, parent):
    return 'Test'
print(QgsExpression('a_function()').evaluate())
1
  • Custom functions need to always take a feature and parent argument, which provide access to the feature's attributes, geometry, etc. and the expression which is being evaluated (see howtoinqgis.wordpress.com/2017/05/20/… ). Then QgsExpression parses and evaluates the expression 'after' it is invoked; it accepts a string to parse and evaluate. Your original invocation without the quotes around a_function() tried to evaluate its value rather than parsing it. (The @qgs decorator also modifies the definition)
    – Houska
    Feb 13, 2020 at 15:34
-1

Try this

from qgis.core import *

@qgsfunction(args=0, group='Custom', usesGeometry=False)
def a_function():

    QgsExpression.registerFunction(a_function)
    print(QgsExpression(a_function()).evaluate())
    return "'Test'"
1
  • I would like to call the function in an expression. In my example, the output of print(QgsExpression(a_function()).evaluate()) should be the string 'Test'. . In your provided answer the statement print(QgsExpression(a_function()).evaluate()) does not return anything. Jun 3, 2019 at 10:30

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