Get the string values of keys in a dict-based schema.

    Non-string keys are ignored.

    Returns:
        Set of string keys of a schema which is in the form (eg):

            schema = Schema({Required("foo"): int,
                             Optional("bah"): basestring})

def schema_keys(schema):
    """Get the string values of keys in a dict-based schema.

    Non-string keys are ignored.

    Returns:
        Set of string keys of a schema which is in the form (eg):

            schema = Schema({Required("foo"): int,
                             Optional("bah"): basestring})
    """
    def _get_leaf(value):
        if isinstance(value, Schema):
            return _get_leaf(value._schema)
        return value

    keys = set()
    dict_ = schema._schema
    assert isinstance(dict_, dict)

    for key in dict_.iterkeys():
        key_ = _get_leaf(key)
        if isinstance(key_, basestring):
            keys.add(key_)

    return keys