<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def assign_issue(issue_key,
                 assignee,
                 server=None,
                 username=None,
                 password=None):
    '''
    Assign the issue to an existing user. Return ``True`` when the issue has
    been properly assigned.

    issue_key
        The JIRA ID of the ticket to manipulate.

    assignee
        The name of the user to assign the ticket to.

    CLI Example:

        salt '*' jira.assign_issue NET-123 example_user
    '''
    jira_ = _get_jira(server=server,
                      username=username,
                      password=password)
    assigned = jira_.assign_issue(issue_key, assignee)
    return assigned