Register custom python function in QGIS Server 3.10

Question

I have some custom python functions I have defined in QGIS which I use in print composer. I tried copying the python folder into the QGIS Server path but they do not get propagated. I have seen the following http://osgeo-org.1560.x6.nabble.com/QGIS-Developer-Custom-python-expressions-in-QGIS-Server-td5362875.html but the answer seems vague to me. Any hints / documentation or that I could read on this anyone out there with an example that I could look at.

Answer 1

I have tried quickly by writing a quick QGIS Server plugin and it worked. I have followed the link you gave more or less.

1. Make a folder called ServerExpression for instance.

2. In this folder, add a quick metadata.txt:

    [general]
    name=Server expression
    description=Expose Python expressions on QGIS Server
    about=Expose Python expressions on QGIS Server
    version=1.0
    qgisMinimumVersion=3.4
    author=Etienne Trimaille
    email=etrimaille@3liz.com
    server=True
   

3. In this folder, add a __init__.py:

    from qgis.core import QgsMessageLog, Qgis, QgsExpression
    from qgis.utils import qgsfunction
   @qgsfunction(
        args='auto', group='Your group', usesGeometry=False, referencedColumns=[], helpText='Define the help string here')
    def your_expression(params, feature, parent):
        # UPDATE the qgsfunction above
        # ADD HERE THE EXPRESSION CODE THAT YOU WROTE IN QGIS.
        return params.upper()
   class ServerExpressionPlugin:
        def init(self):
            QgsMessageLog.logMessage('Loading expressions', 'ServerExpression', Qgis.Info)
            QgsExpression.registerFunction(your_expression)
   def serverClassFactory(serverIface):
        _ = serverIface
        return ServerExpressionPlugin()
   

4. Transfer this folder in the plugin directory on the server, reload QGIS Server if needed

5. If in your layout, you have a label with

    [%your_expression('hello')%]
   

You will have a PDF with HELLO.

To make things easy, I have added all the code in the __init__.py. Of course, you can make your Python files listing your expressions, to make it easier to copy paste between Desktop and Server.