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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 download(*packages, **kwargs):
    '''
    Download packages to the local disk.

    refresh
        force a refresh if set to True.
        If set to False (default) it depends on zypper if a refresh is
        executed.

    root
        operate on a different root directory.

    CLI example:

    .. code-block:: bash

        salt '*' pkg.download httpd
        salt '*' pkg.download httpd postfix
    '''
    if not packages:
        raise SaltInvocationError('No packages specified')

    root = kwargs.get('root', None)

    refresh = kwargs.get('refresh', False)
    if refresh:
        refresh_db(root)

    pkg_ret = {}
    for dld_result in __zypper__(root=root).xml.call('download', *packages).getElementsByTagName("download-result"):
        repo = dld_result.getElementsByTagName("repository")[0]
        path = dld_result.getElementsByTagName("localfile")[0].getAttribute("path")
        pkg_info = {
            'repository-name': repo.getAttribute('name'),
            'repository-alias': repo.getAttribute('alias'),
            'path': path,
        }
        key = _get_first_aggregate_text(
            dld_result.getElementsByTagName('name')
        )
        if __salt__['lowpkg.checksum'](pkg_info['path'], root=root):
            pkg_ret[key] = pkg_info

    if pkg_ret:
        failed = [pkg for pkg in packages if pkg not in pkg_ret]
        if failed:
            pkg_ret['_error'] = ('The following package(s) failed to download: {0}'.format(', '.join(failed)))
        return pkg_ret

    raise CommandExecutionError(
        'Unable to download packages: {0}'.format(', '.join(packages))
    )