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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 avail_sizes(kwargs=None, call=None):
    '''
    Return a list of the instance sizes that are on the provider.

    CLI Examples:

    .. code-block:: bash

        salt-cloud --list-sizes my-qingcloud
        salt-cloud -f avail_sizes my-qingcloud zone=pek2
    '''
    if call == 'action':
        raise SaltCloudSystemExit(
            'The avail_sizes function must be called with '
            '-f or --function, or with the --list-sizes option'
        )

    zone = _get_specified_zone(kwargs, get_configured_provider())

    result = {}
    for size_key in QINGCLOUD_SIZES[zone]:
        result[size_key] = {}
        for attribute_key in QINGCLOUD_SIZES[zone][size_key]:
            result[size_key][attribute_key] = QINGCLOUD_SIZES[zone][size_key][attribute_key]

    return result