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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 ...

underdark
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2 Answers2

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

QgsExpressionContextUtils.setProjectVariable('myvar','hello world')
underdark
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ndawson
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  • What about deleting a variable? I didn't find method for it in QgsExpressionContextUtils – ismailsunni Jul 24 '17 at 15:39
  • 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... – ndawson Jul 24 '17 at 22:58
  • 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. – ismailsunni Jul 25 '17 at 02:06
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    QgsExpressionContextUtils.removeProjectVariable exists now! – letmaik Jan 20 '18 at 23:21
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    There is a new required parameter since QGIS 3.0. See an updated answer here: https://gis.stackexchange.com/questions/222494/how-to-read-user-defined-project-variables-using-python – TimD Jun 26 '20 at 03:57
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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')

Zac
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