I've already created a branch locally, but I have no rights to push a new branch, sadly.
Could you please allow me access?

@ferhat-aram Python-gitlab is just a wrapper around the GitLab API. I can't help you with your permission issues.

Yeah I know that, but the Wrapper itself does this and not the GitLab API -> The GitLab API works completley fine here. I already tested it several times via curl and as I already said, I fixed it locally by changing the handling of the should_remove_source_branch and merge_when_pipeline_succeeds as query_data instead of post_data

So the problem here relates to the wrapper itself and not the actual API.

This problem isn't something new. I've got to do that, after it was broken for several months. I guess the GitLab API changed at one time and the wrapper wasn't updated accordingly to those API changes.

Ah yes. You can fork the project and create a merge request: https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/creating-a-pull-request

What's weird is that this works for me as is, currently. I have pipelines with scripts using this from today where I've had no issues. Example (I have no idea why GitLab provides both remove_source_branch on create and should_remove_source_branch on accept, but I use both just in case):

    mr = downstream.mergerequests.create({'source_branch': source_branch,
                                          'target_branch': target_branch,
                                          'assignee_id': assignee,
                                          'title': title,
                                          'description': description,
                                          'remove_source_branch': 'true'
                                          })

    time.sleep(10) # wait for sidekiq
    print('[Info] Applying MR merge settings..')
    mr.merge(should_remove_source_branch=True,
             merge_when_pipeline_succeeds=True)

So I'm wondering - are you getting this consistently on a clean open MR (previous API calls can affect that)? can you post the code and the logs? You can get 405 for a bunch of reasons (Work in Progress, Closed, Pipeline Pending Completion, or Failed while requiring Success). I've also seen this when I try to apply the merge too quickly after creation (hence the hardcoded wait above). I'll check if my existing project config or the remove_source_branch on creation means it's just a fluke that this works for me.

The request generally doesn't work for me with post data. I tried it several times. What GitLab version do you use?

I just tried this on gitlab.com (13.1.0-pre) on an open MR with an in-progress pipeline, and this works (merge is set for after pipeline succeeds and branch is set for deletion, and the MR is eventually merged).

curl -s -X PUT -H "Private-Token: $GL_TOKEN" \
    -H "Content-type: application/json" \
    -d '{"merge_when_pipeline_succeeds": "true", "should_remove_source_branch": "true"}' \
    https://gitlab.com/api/v4/projects/$PROJECT_ID/merge_requests/$MR_ID/merge

I also tried the curl call without the data and got a 405 response (with the project configured to require pipelines to succeed before merge). That's why I was wondering if your API calls were hitting one of the conditions that trigger 405 as described above. But maybe this fixes some other quirks, like reverse proxies that mess with request bodies or something.

Since the PR was merged I guess this can be closed :)