Match words from one file to many files

match.py

import os

from nltk.corpus import stopwords
sw = set(stopwords.words('english'))

f1 = "<<>>"
directory = "<<>>"

with open(f1, 'r') as file1:
    words1 = file1.read().lower().split()

words1 = [word for word in words1 if word not in sw]

files = [f for f in os.listdir(directory) if os.path.isfile(os.path.join(directory, f))]

with open('outfile.csv', 'a') as output:
    output.write('{}|{}|{}|{}.\n'.format("word", "count_file_1", " count_file_2", "file_2"))

    for filename in files:
        f = os.path.join(directory, filename)

        with open(f, 'r') as file2:
            words2 = file2.read().lower().split()

        words = set(words1) & set(words2)

        for word in words:
            output.write('{}|{}|{}|{}.\n'.format(word, words1.count(word), words2.count(word), f))