Filtering None values from dictionary

def to_dict_simple(d):
    """Returns the model properties as a dict without None values."""
    clean = {}

    for k, v in list(six.iteritems(d)):
        if isinstance(v, list):
                clean[k] = list(map(lambda x: to_dict_simple(x) if isinstance(x, dict) else x, v))
        elif isinstance(v, dict):
            nested = to_dict_simple(v)
            if len(list(six.iterkeys(nested))) > 0:
                clean[k] = nested
        elif v is not None:
            clean[k] = v
    return clean