returns the lowest available integer in a set of dict keys

def _next_lowest_integer(group_keys):
    """
    returns the lowest available integer in a set of dict keys
    """
    try: #TODO Replace with max default value when dropping compatibility with Python < 3.4
        largest_int= max([ int(val) for val in group_keys if _is_int(val)])
    except:
        largest_int= 0
    return largest_int + 1