PYQGIS 3: Change layout scalebar item to a round scale (e.g. 1:5000)

Question

I'm loading a layout template to a .qgs project and would like to set up the scale bar so that it has a round numeric scale (e.g. 1:5'000, or 1:25'000 but not 1:12'500). How do I do that?

Here is my piece of code:

import os
import processing
from qgis.core import *
from qgis.gui import *
import qgis.utils
from qgis.PyQt import QtGui
from qgis.PyQt.QtXml import QDomDocument

project = QgsProject.instance()
    project.setCrs( QgsCoordinateReferenceSystem(2056, QgsCoordinateReferenceSystem.EpsgCrsId) )
    project.setFileName(qgis_project_name)

project.write()

# load layout template
composition = QgsPrintLayout(project)
document = QDomDocument()

template_file = open('C:\\my_QGIS\\my_qgis_projects\\ramms_map\\RAMMS_Karte.qpt')
template_content = template_file.read()
template_file.close()
document.setContent(template_content)

composition.loadFromTemplate(document, QgsReadWriteContext()) 
project.layoutManager().addLayout(composition)

layout = project.layoutManager().layoutByName("RAMMS_Karte_layout_de")

# access the items contained in the layout
sel = layout.itemById("scale") # that's the id of the scalebar item
sel.??? --> here set the numeric number 1: xxxx

# my scalebar is connected to a map ("map1")


EDIT:

# here is the correct piece of code missing

layout = project.layoutManager().layoutByName("RAMMS_Karte_layout_de")

map1 = layout.itemById("map1")

scale_nr = map1.scale() # takes the scale of map1
if (scale_nr % 1000.0 == 0.0) == False: # if not divisible by 1000, it rounds the scale value to the next 1000
    scale_nr = int(round(scale_nr, -3))
map1.setScale(scale_nr) # sets the new scale

Answer 1

In this case, you have to change the map, no the scale, you can use this:

map = layout.itemById("principal map")
map.setScale(12500)

if you don't have a itemByID, the other option is this:

layout = project.layoutManager().layoutByName("RAMMS_Karte_layout_de")
page = layout.pageCollection()

for i in items:   
    if isinstance(i, QgsLayoutItemMap):
        i.setScale(12500)

Answer 2

For corroborating what kind of result your code produces, I created a qpt file (for this template https://raw.githubusercontent.com/qgis/QGIS/master/tests/testdata/template.qpt) and I loaded it in QGIS 3.8 with following code:

project = QgsProject.instance()
composition = QgsPrintLayout(project)
document = QDomDocument()

template_file = open('/home/zeito/Templates/template.qpt')
template_content = template_file.read()
template_file.close()

document.setContent(template_content)

composition.loadFromTemplate(document, QgsReadWriteContext()) 
project.layoutManager().addLayout(composition)

layout = project.layoutManager().layoutByName("Composer 1")

sel = layout.itemById("scale")

print(type(sel))

where type printed for sel object was 'NoneType'. So, it doesn't exist any item named "scale" into QgsPrintLayout object (layout).

If you print layout.items() you have four types of objects:

>>>items = layout.items()
>>>items
[<qgis._core.QgsLayoutItemLabel object at 0x7f6a8c4ce0d8>, <qgis._core.QgsLayoutFrame object at 0x7f6a8c4ce288>, <PyQt5.QtWidgets.QGraphicsRectItem object at 0x7f6a8c4ce168>, <qgis._core.QgsLayoutItemPage object at 0x7f6afc0ae798>]

QgsLayoutItemLabel object (items[0]) have 'scale' and 'setScale' methods where I guess that you can set numeric scale 1:12,500.

In my case, default scale is 1.0:

>>>items[0]
<qgis._core.QgsLayoutItemLabel object at 0x7f6a8c4ce0d8>
>>>items[0].scale()
1.0

I hope that it helps.