问题在于您正在将中的每一行与fileone中的同一行进行比较filetwo。只要一个文件中有多余的一行，您就会发现这些行再也不相等了。尝试这个：
with open('old.csv', 'r') as t1, open('new.csv', 'r') as t2:
fileone = t1.readlines()
filetwo = t2.readlines()
with open('update.csv', 'w') as outFile:
for line in filetwo:
if line not in fileone:
outFile.write(line)
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