PyQGIS - Changing scalebar width from an existing layout

Question

I have a project on QGIS 3.16 with several layouts. I would like to change the scalebar width according to the value of a Python variable, either a fixed sized of 1000 or 500 units (for instance).

First, I tried to do it without the Python variable, by using the value of 1000 units. I think the following code I use try to create a new scalebar, instead of calling the existing one. Here, the name of the actual layout is 'cumul_eoliennes':

(once the QGIS projet is opened)

project = QgsProject.instance() 
projectLayoutManager = project.layoutManager()
composeur_eol = 'cumul_eoliennes'
layout_eol= projectLayoutManager.layoutByName(composeur_eol)
scalebar=QgsLayoutItemScaleBar(layout_eol)

scalebar.setHeight(1)
 # scalebar.height() gives the default value of 3 mm
scalebar.setFixedSize(1000)

The last code gives :

TypeError: QgsLayoutItemScaleBar.setFixedSize(): argument 1 has unexpected type 'int

In conclusion, I meet two problems:

• it seems I am not able to call the actual scalebar of a layout
• I don't know how to change the "FixedSize" of a scalebar

[EDIT about the FixedSize] I am looking for the scalebar fixed size function like below:

Answer 1

This line in your code: scalebar=QgsLayoutItemScaleBar(layout_eol) is creating a new QgsLayoutItemScaleBar object. This is not the way to access an existing scale bar object. In order to do that you can do something like:

scale_bar = [i for i in layout_eol.items() if isinstance(i, QgsLayoutItemScaleBar)][0]

Which works fine if you only have one scale bar in your layout. Alternatively, you could use the method itemById().

For your second problem, the setFixedSize() method takes a QgsLayoutSize() argument. The QgsLayoutSize() class constructor takes width and height arguments as well as a units argument which defaults to QgsUnitTypes.LayoutMillimeters.

Try the following:

project = QgsProject.instance() 
projectLayoutManager = project.layoutManager()
composeur_eol = 'cumul_eoliennes'
layout_eol= projectLayoutManager.layoutByName(composeur_eol)
scale_bar = [i for i in layout_eol.items() if isinstance(i, QgsLayoutItemScaleBar)][0]
#print(scale_bar)
scale_bar.setFixedSize(QgsLayoutSize(100, 20))