Can I update Row-level security via Rest API?

If you mean can you add users to an RLS role you defined in the pbix file, then the answer is yes,

You can get a list of the roles in a specified report with a call like

http://localhost/reports/api/v2.0/PowerBIReports(1fe72f5d-5cce-43c2-905b-fe699ed9a46a)/DataModelRoles

You would use the /DataModelRoleAssignments endpoint

eg.

http://localhost/reports/api/v2.0/PowerBIReports(1fe72f5d-5cce-43c2-905b-fe699ed9a46a)/DataModelRoleAssignments

You would do a GET request to get the all the current users in your RLS roles, then you would post back a new array with any new users added (or remove users who should no longer have access). Note that this array will replace the entire list of users.

The following would add user1 and user2 to a specific role.

[
    {
        "GroupUserName": "user1",
        "DataModelRoles": [
            "ef5680de-2218-4c40-b308-831ce3d108b4"
        ]
    },
    {
        "GroupUserName": "user2",
        "DataModelRoles": [
            "ef5680de-2218-4c40-b308-831ce3d108b4"
        ]
    }
]

Power BI Report Server uses Windows Authentication by default. I know in PowerShell you can just use the -UseDefaultCredentials switch of the Invoke-RestMethod cmdlet to send the windows credentials. You will have to search for something like "python rest api windows authentication" in google or bing for information on how to do this in Python - it probably depends on what rest module you are using.

Thanks a lot.

I found the python way. I'd like to share the pkg and code with everyone.