title: How to add (append) user in multiuser picker field using jira-python
body: <p>I have been trying to append user to the existing multi-user picker field without replacing users in it.</p>
  <p>Ex:
  The Jira JQL finds the user rkapoor in the <em>Testers</em> field, then append rkapoor1 to the <em>Testers</em> field</p>
  <p>Note: The Testers field might contain other users along with rkapoor (just wanted append, not to replace other users)</p>
  <pre><code>for issue in jira.search_issues('project = &quot;ABC&quot; and issuetype = Epic and status not in (Closed) and Testers in (rkapoor)', maxResults=100):
  testuser = jira.user('rkapoor1')
  testlist = issue.fields.customfield_10600
  testlist.append(testuser)
  issue.update(fields={&quot;customfield_10600&quot;: [testlist]})
  </code></pre>
  <p>Console error:</p>
  <blockquote>
  <p>Traceback (most recent call last): File &quot;Jiratest1.py&quot;, line 32, in
   issue.update(fields={&quot;customfield_10600&quot;: [testlist]}) File
  &quot;C:\Python\Python38-32\lib\site-packages\jira\resources.py&quot;, line 485,
  in update super(Issue, self).update(async_=async_, jira=jira,
  notify=notify, fields=data) File
  &quot;C:\Python\Python38-32\lib\site-packages\jira\resources.py&quot;, line 225,
  in update data = json.dumps(data) File
  &quot;C:\Python\Python38-32\lib\json_<em>init</em>_.py&quot;, line 231, in dumps``
  return _default_encoder.encode(obj) File
  &quot;C:\Python\Python38-32\lib\json\encoder.py&quot;, line 199, in encode
  chunks = self.iterencode(o, _one_shot=True) File
  &quot;C:\Python\Python38-32\lib\json\encoder.py&quot;, line 257, in iterencode
  return _iterencode(o, 0) File
  &quot;C:\Python\Python38-32\lib\json\encoder.py&quot;, line 179, in default
  raise TypeError(f'Object of type {o.<strong>class</strong>.<strong>name</strong>} ' TypeError:
  Object of type User is not JSON serializable</p>
  </blockquote>
  <p>Any help would be appreciated.</p>