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I can change user defined project variables by QgsExpressionContextUtils.setProjectVariable('myvar','hello world')

But how can I access/read this variable in a python script?

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    I cannot try this but documentation suggests that there should be a QgsExpressionContextUtils.projectScope method through which you can access that information.
    – Christoph
    Dec 24, 2016 at 8:23

2 Answers 2

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Since QGIS 3.0 you need also to specify the project when setting and reading a variable:

project = QgsProject.instance()
QgsExpressionContextUtils.setProjectVariable(project,'myvar','Hello World!')
QgsExpressionContextUtils.projectScope(project).variable('myvar')
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If you set a project variable using:

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

You can use the following to read the value of this variable:

QgsExpressionContextUtils.projectScope().variable('myvar')
>>> u'hello world'

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