Compares two files and puts their common lines in a file called output.txt

comparetextfile.py

import sys

def common_item(lista, listb, mode=0):
    items = []
    for x in lista:
        for y in listb:
            if x==y:
                if not mode:
                    return True
                if mode:
                    items += [x]
    if not mode:
        return False
    if mode:
        return items

f = sys.argv[1]
f2 = sys.argv[2]

testfileobj = open(f)
testfileobj2 = open(f2)

testfilelines = testfileobj.readlines()
testfile2lines = testfileobj2.readlines()
print("Scanning file...")
commonlines = common_item(testfilelines, testfile2lines, 1)
print("Found {} lines, creating output file...".format(len(commonlines)))
output = open("output.txt", "w")
for i in commonlines:
    output.write(i)
output.close()
print("Done.")