compare two text files

Hi Experts,
I am new to Python. The query is to Compare the 4th filed of two text files if the difference is greater than or less than 15%, then we need to print the filenames.

The two text files are attached and the code i tried is also attached.

arr1 = []
arr2 = []


for line1 in open("testReport_flame3_0202.txt"):
    linesa= line1.strip().split(",")
    arr1.append(linesa[4])
for line2 in open("testReport_flame3_0209.txt"):
    linesb= line2.strip().split(",")
    #print linesb[4]
    arr2.append(linesb[4])

lenth = len(open("testReport_flame3_0202.txt").readlines())

for i in range(1,lenth):
   if((((arr1[i] - arr2[i])/arr1[i])*100) > 15):   # failing at this step
       print i

# after this i am not getting how to fetch the respective filename

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testReport-flame3-0202.txt
testReport-flame3-0209.txt