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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 set_service_policy(host,
                       username,
                       password,
                       service_name,
                       service_policy,
                       protocol=None,
                       port=None,
                       host_names=None):
    '''
    Set the service name's policy for a given host or list of hosts.

    host
        The location of the host.

    username
        The username used to login to the host, such as ``root``.

    password
        The password used to login to the host.

    service_name
        The name of the service for which to set the policy. Supported service names are:
          - DCUI
          - TSM
          - SSH
          - lbtd
          - lsassd
          - lwiod
          - netlogond
          - ntpd
          - sfcbd-watchdog
          - snmpd
          - vprobed
          - vpxa
          - xorg

    service_policy
        The policy to set for the service. For example, 'automatic'.

    protocol
        Optionally set to alternate protocol if the host is not using the default
        protocol. Default protocol is ``https``.

    port
        Optionally set to alternate port if the host is not using the default
        port. Default port is ``443``.

    host_names
        List of ESXi host names. When the host, username, and password credentials
        are provided for a vCenter Server, the host_names argument is required to tell
        vCenter the hosts for which to set the service policy.

        If host_names is not provided, the service policy information will be retrieved
        for the ``host`` location instead. This is useful for when service instance
        connection information is used for a single ESXi host.

    CLI Example:

    .. code-block:: bash

        # Used for single ESXi host connection information
        salt '*' vsphere.set_service_policy my.esxi.host root bad-password 'ntpd' 'automatic'

        # Used for connecting to a vCenter Server
        salt '*' vsphere.set_service_policy my.vcenter.location root bad-password 'ntpd' 'automatic' \
        host_names='[esxi-1.host.com, esxi-2.host.com]'
    '''
    service_instance = salt.utils.vmware.get_service_instance(host=host,
                                                              username=username,
                                                              password=password,
                                                              protocol=protocol,
                                                              port=port)
    host_names = _check_hosts(service_instance, host, host_names)
    valid_services = ['DCUI', 'TSM', 'SSH', 'ssh', 'lbtd', 'lsassd', 'lwiod', 'netlogond',
                      'ntpd', 'sfcbd-watchdog', 'snmpd', 'vprobed', 'vpxa', 'xorg']
    ret = {}

    for host_name in host_names:
        # Check if the service_name provided is a valid one.
        # If we don't have a valid service, return. The service will be invalid for all hosts.
        if service_name not in valid_services:
            ret.update({host_name: {'Error': '{0} is not a valid service name.'.format(service_name)}})
            return ret

        host_ref = _get_host_ref(service_instance, host, host_name=host_name)
        service_manager = _get_service_manager(host_ref)
        services = host_ref.configManager.serviceSystem.serviceInfo.service

        # Services are stored in a general list - we need loop through the list and find
        # service key that matches our service name.
        for service in services:
            service_key = None

            # Find the service key based on the given service_name
            if service.key == service_name:
                service_key = service.key
            elif service_name == 'ssh' or service_name == 'SSH':
                if service.key == 'TSM-SSH':
                    service_key = 'TSM-SSH'

            # If we have a service_key, we've found a match. Update the policy.
            if service_key:
                try:
                    service_manager.UpdateServicePolicy(id=service_key, policy=service_policy)
                except vim.fault.NotFound:
                    msg = 'The service name \'{0}\' was not found.'.format(service_name)
                    log.debug(msg)
                    ret.update({host_name: {'Error': msg}})
                    continue
                # Some services are restricted by the vSphere License Level.
                except vim.fault.HostConfigFault as err:
                    msg = '\'vsphere.set_service_policy\' failed for host {0}: {1}'.format(host_name, err)
                    log.debug(msg)
                    ret.update({host_name: {'Error': msg}})
                    continue

                ret.update({host_name: True})

            # If we made it this far, something else has gone wrong.
            if ret.get(host_name) is None:
                msg = 'Could not find service \'{0}\' for host \'{1}\'.'.format(service_name, host_name)
                log.debug(msg)
                ret.update({host_name: {'Error': msg}})

    return ret