Why can I not update customfield via jira-python library (REST API)?

[Didier Riedinger]

Using jira-python library, I want to update a <user picker> customfield.
My client is connected to jira instance as administrator and the instruction below give me

jira_session.fields() {u'name': u'Assign User to "Update/Re-Update" (MT)', u'searchable': True, u'navigable': True, u'custom': True, u'orderable': True, u'id': u'customfield_10617', u'schema': {u'customId': 10617, u'type': u'user', u'custom': u'com.atlassian.jira.plugin.system.customfieldtypes:userpicker'}}...

I enter the instruction below based on the documentation http://jira-python.readthedocs.org/en/latest/#jira.resources.Resource.update

test_issue = jira_session.issue(issue_key) test_issue.update(fields={'customfield_10600': {'name': 'lambda'}})

Traceback (most recent call last):
    test_issue.update(fields={'customfield_10600': {'name': 'lambda'}}) 
  File "C:\Python27\lib\site-packages\jira\resources.py", line 302, in update
    super(Issue, self).update(async=async, jira=jira, **data)
  File "C:\Python27\lib\site-packages\jira\resources.py", line 174, in update
    raise_on_error(r)
  File "C:\Python27\lib\site-packages\jira\exceptions.py", line 47, in raise_on_error
    raise JIRAError(r.status_code, error, r.url)
jira.exceptions.JIRAError: HTTP 400: "Field 'customfield_10600' cannot be set. It is not on the appropriate screen, or unknown." http://XXX/rest/api/2/issue/12562

I am able to update fields like <assignee>, <description>,... but why did this not work for the user picker custom field?
Did I made an error in the argument given in the update() method?

Thanks in advance.

[Nic Brough -Adaptavist- · Accepted solution]

The error message is telling you the problem - you haven't got the field on the issue edit screen.