>>> import gitlab
>>> gitlab.__version__
'1.3.0'
>>> gl = gitlab.Gitlab('https://dev.domain.tld', 'xxx')
>>> p = gl.projects.get(30)
>>> i = p.issues.get(9364)
>>> i.save()
>>> i.milestone = 1812
>>> i.save()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/gitlab/mixins.py", line 312, in save
    server_data = self.manager.update(obj_id, updated_data, **kwargs)
  File "/usr/lib/python2.7/dist-packages/gitlab/exceptions.py", line 253, in wrapped_f
    raise error(e.error_message, e.response_code, e.response_body)
gitlab.exceptions.GitlabUpdateError: 400: {"error":"title, description, assignee_ids, assignee_id, milestone_id, discussion_locked, labels, created_at, due_date, confidential, state_event are missing, at least one parameter must be provided"}

>>> m = p.milestones.get(1812)
>>> m
<ProjectMilestone id:1812>
>>> i.milestone = m
>>> i.save()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/gitlab/mixins.py", line 312, in save
    server_data = self.manager.update(obj_id, updated_data, **kwargs)
  File "/usr/lib/python2.7/dist-packages/gitlab/exceptions.py", line 251, in wrapped_f
    return f(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/gitlab/mixins.py", line 227, in update
    return self.gitlab.http_put(path, post_data=data, **kwargs)
  File "/usr/lib/python2.7/dist-packages/gitlab/__init__.py", line 827, in http_put
    post_data=post_data, **kwargs)
  File "/usr/lib/python2.7/dist-packages/gitlab/__init__.py", line 685, in http_request
    prepped = self.session.prepare_request(req)
  File "/usr/lib/python2.7/dist-packages/requests/sessions.py", line 407, in prepare_request
    hooks=merge_hooks(request.hooks, self.hooks),
  File "/usr/lib/python2.7/dist-packages/requests/models.py", line 305, in prepare
    self.prepare_body(data, files, json)
  File "/usr/lib/python2.7/dist-packages/requests/models.py", line 445, in prepare_body
    body = complexjson.dumps(json)
  File "/usr/lib/python2.7/json/__init__.py", line 244, in dumps
    return _default_encoder.encode(obj)
  File "/usr/lib/python2.7/json/encoder.py", line 207, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/lib/python2.7/json/encoder.py", line 270, in iterencode
    return _iterencode(o, 0)
  File "/usr/lib/python2.7/json/encoder.py", line 184, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: <ProjectMilestone id:1812> is not JSON serializable