def clean_shellname(self):
    """Validate that shellname is available."""
    shellname = self.cleaned_data['shellname']
    if not utils.shellname_available(shellname):
        raise forms.ValidationError(_('Shell username is already in use.'))

    # Check that it isn't banned by some abusefilter type rule
    user = utils.check_username_create(shellname)
    if user['ok'] is False:
        raise forms.ValidationError(user['error'])

    return shellname

def check_username_create(name):
    """Check to see if a given name would be allowed as a username.

    Returns True if the username would be allowed. Returns either False or a
    reason specifier if the username is not allowed.
    Returns a dict with these keys:
    - ok : Can a new user be created with this name (True/False)
    - name : Canonicalized version of the given name
    - error : Error message if ok is False; None otherwise
    """
    # Make sure to use the anon client here because on-wiki rights can affect
    # the result of the cancreate check.
    mwapi = mediawiki.Client.anon_client()
    user = mwapi.query_users_cancreate(name)[0]
    # Example response:
    # [{'missing': True, 'name': 'Puppet',
    # 'cancreate': False, 'cancreateerror': [{'message':
    # 'titleblacklist-forbidden-new-account', 'params': ['
    # ^(User:)?puppet$ <newaccountonly>', 'Puppet'], 'type': 'error'}]}]
    ret = {
        'ok': False,
        'name': user['name'],
        'error': None,
    }
    if user['missing'] and user['cancreate']:
        ret['ok'] = True
    elif 'userid' in user:
        ret['error'] = _('%(name)s is already in use.') % ret
    elif 'cancreateerror' in user:
        try:
            ret['error'] = mwapi.get_message(
                    user['cancreateerror'][0]['message'],
                    *user['cancreateerror'][0]['params'],
                    lang=translation.get_language().split('-')[0])
        except Exception:
            logger.exception('Failed to get expanded message for %s', user)
            ret['error'] = user['cancreateerror'][0]['message']
    return ret

def query_users_cancreate(self, *users):
    """Check to see if the given usernames could be created or not.

    Note: if this Client is authenticated to the target wiki, the result
    that you get from this request may or may not be the same result that
    an anonymous user would get.
    """
    result = self.site.api(
        'query', formatversion=2,
        list='users',
        usprop='cancreate', ususers='|'.join(users),
    )

    logger.debug(result)
    # Example result:
    # {'query': {'users': [{'missing': True, 'name': 'Puppet',
    # 'cancreate': False, 'cancreateerror': [{'message':
    # 'titleblacklist-forbidden-new-account', 'params': ['
    # ^(User:)?puppet$ <newaccountonly>', 'Puppet'], 'type': 'error'}]}],
    # 'userinfo': {'anon': True, 'messages': False, 'name':
    # '137.164.12.107', 'id': 0}}, 'batchcomplete': True}
    # TODO: error handling
    return result['query']['users']

{
    "batchcomplete": true,
    "query": {
        "users": [
            {
                "name": "Root",
                "missing": true,
                "cancreate": false,
                "cancreateerror": [
                    {
                        "message": "titleblacklist-forbidden-new-account",
                        "params": [
                            " ^(User:)?root$ <newaccountonly>",
                            "Root"
                        ],
                        "code": "titleblacklist-forbidden",
                        "type": "error"
                    }
                ]
            }
        ]
    }