Simple lexer in python

lexer.py

from collections import namedtuple
import re

# basic token container
Token = namedtuple("Token", ["tag", "val", "pos", "end"])

# regex scanner handler
t = lambda tag: lambda sc, val: Token(tag, val, sc.match.start(), sc.match.end())

def lex(text, rules, ignore_whitespace=True):
    handlers = [(reg, t(tag)) for (reg,tag) in rules.items()]
    if ignore_whitespace:
        handlers.append((r"\s+", None))
    toks, rem = re.Scanner(handlers).scan(text)
    return toks, rem

if __name__ == '__main__':
    string = "these are some words"
    rules = {r"[a-z]+": "WORD"}
    toks = lex(string, rules)
    for token in toks:
        print(token)