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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_user_list(host_name, client_name, client_pass):
 """ Pulls the list of users in a client. Inputs: - host_name: A string containing the address of the machine where the PServer instance is hosted. - client_name: The PServer client name. - client_pass: The PServer client's password. Output: - user_id_list: A python list of user ids. """

    # Construct request.
    request = construct_request(model_type="pers",
                                client_name=client_name,
                                client_pass=client_pass,
                                command="getusrs",
                                values="whr=*")

    # Make request.
    request_result = send_request(host_name, request)

    # Extract a python list from xml object.
    user_id_list = list()
    append_user_id = user_id_list.append

    if request_result is not None:
        user_list_xml = request_result.text
        tree = etree.parse(StringIO(user_list_xml))
        root = tree.getroot()

        xml_rows = root.findall("./result/row/usr")
        for xml_row in xml_rows:
            append_user_id(xml_row.text)

    return user_id_list