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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 list_hosts_by_cluster(kwargs=None, call=None):
    '''
    List hosts for each cluster; or hosts for a specified cluster in
    this VMware environment

    To list hosts for each cluster:

    CLI Example:

    .. code-block:: bash

        salt-cloud -f list_hosts_by_cluster my-vmware-config

    To list hosts for a specified cluster:

    CLI Example:

    .. code-block:: bash

        salt-cloud -f list_hosts_by_cluster my-vmware-config cluster="clusterName"
    '''
    if call != 'function':
        raise SaltCloudSystemExit(
            'The list_hosts_by_cluster function must be called with '
            '-f or --function.'
        )

    ret = {}
    cluster_name = kwargs.get('cluster') if kwargs and 'cluster' in kwargs else None
    cluster_properties = ["name"]

    cluster_list = salt.utils.vmware.get_mors_with_properties(_get_si(),
                                                              vim.ClusterComputeResource,
                                                              cluster_properties)

    for cluster in cluster_list:
        ret[cluster['name']] = []
        for host in cluster['object'].host:
            if isinstance(host, vim.HostSystem):
                ret[cluster['name']].append(host.name)
        if cluster_name and cluster_name == cluster['name']:
            return {'Hosts by Cluster': {cluster_name: ret[cluster_name]}}

    return {'Hosts by Cluster': ret}