[QGIS-Developer] set data defined override expression with python?

[Raymond Nijssen]

Hi devs!

I'd like my python plugin to set an expression for overriding a property 
and I have no clue how to do so. I expect there must be some generic way 
to set expressions.

Specifically, I want to set the expression for the "Image source" of a 
QgsLayoutItemPicture to:

'/path/to/pictures/' || attribute(@atlas_feature, 'ID') || '.svg'

Hope anyone can explain.

Thanks!
Raymond