问错误：'file‘对象没有'lower’属性

EN

Traceback (most recent call last):
File "stop2.py", line 17, in <module>
print preprocess(sentence)
File "stop2.py", line 10, in preprocess
sentence = sentence.lower()
AttributeError: 'file' object has no attribute 'lower'

# -*- coding: utf-8 -*-
from __future__ import division, unicode_literals
import string
import nltk
from nltk.tokenize import RegexpTokenizer
from nltk.corpus import stopwords
import re

def preprocess(sentence):
    sentence = sentence.lower()
    tokenizer = RegexpTokenizer(r'\w')
    tokens = tokenizer.tokenize(sentence)
    filtered_words = [w for w in tokens if not w in stopwords.words('english')]
    return " ".join(filtered_words)

sentence = open('pathtofile')
print preprocess(sentence)