Convert a list of scopes to a space separated string.

def list_to_scope(scope):
    """Convert a list of scopes to a space separated string."""
    if isinstance(scope, unicode_type) or scope is None:
        return scope
    elif isinstance(scope, (set, tuple, list)):
        return " ".join([unicode_type(s) for s in scope])
    else:
        raise ValueError("Invalid scope (%s), must be string, tuple, set, or list." % scope)