How to change a project variable using Python?

Question

I'd like to change the value of a user-defined project variable (can be manually edited in Project Settings | Variables) using the Python console. I tracked down the setVariable() function in the QgsExpressionContextScope class but haven't succeeded in actually changing the variable in the project settings. My code so far:

iface.mapCanvas().mapSettings().expressionContext().scope(0).setVariable('myvar',1)

I guess I'm getting lost in the different expression contexts ...

Answer 1

Look at QgsExpressionContextUtils (https://qgis.org/api/classQgsExpressionContextUtils.html). The method you need is QgsExpressionContextUtils.setProjectVariable, e.g.

QgsExpressionContextUtils.setProjectVariable('myvar','hello world')

• ismailsunni: What about deleting a variable? I didn't find method for it in QgsExpressionContextUtils
• ndawson: There's no high level API to do this. Possibly you could get away with NULLING the variable (setting it to None), but if not, you need to use QgsExpressionContextUtils.setProjectVariables({} ) and reset the whole lot. You'd first need to check QgsExpressionContextUtils.projectScope() and create a dict of all the variables you want to keep. It's far from ideal - but would also be a trivial addition to the API if you wanted to get involved in QGIS development and send through a pull request on github...
• ismailsunni: Thanks @ndawson, it works with your work around to reset with setProjectVariables(preserved_variables) I have checked the C++ code, I hope I can contribute for the functionality.
• letmaik: QgsExpressionContextUtils.removeProjectVariable exists now!
• TimD: There is a new required parameter since QGIS 3.0. See an updated answer here: gis.stackexchange.com/questions/222494/…

Answer 2

In QGIS3, the API needs to pass QgsProject as the first argument.

It is: setProjectVariable(project: QgsProject, name: str, value: Any)

QgsExpressionContextUtils.setProjectVariable(QgsProject.instance(), 'myvar','hello world')