Question

I have this simple code which is really just to help me understand how Python I/O works:

inFile = open("inFile.txt",'r')
outFile = open("outFile.txt",'w')
lines = inFile.readlines()

first = True
for line in lines:
    if first == True:
        outFile.write(line)  #always print the header
        first = False
        continue
    nums = line.split()
    outFile.write(nums[3] + "\n") #print the 4th column of each row
outFile.close()

My input file is something like this:

#header
34.2 3.42 64.56 54.43 3.45
4.53 65.6 5.743 34.52 56.4
4.53 90.8 53.45 134.5 4.58
5.76 53.9 89.43 54.33 3.45

The output prints out into the file just as it should but I also get the error:

    outFile.write(nums[3] + "\n")
IndexError: list index out of range 

I’m assuming this is because it has continued to read the next line although there is no longer any data?

Answer 1

Others have already answered your question. Here is a better way to “always print out the file header”, avoiding testing for first at every iteration:

with open('inFile.txt', 'r') as inFile, open('outFile.txt', 'w') as outFile:
    outFile.write(inFile.readline()) #always print the header
    for line in inFile:
        nums = line.split()
        if len(nums) >= 4: #Checks to make sure a fourth column exists.
            outFile.write(nums[3] + "\n") #print the 4th column of each row

A couple things are going on here:

with open('inFile.txt', 'r') as inFile, open('outFile.txt', 'w') as outFile:

The with expression is a convenient way to open files because it automatically closes the files even if an exception occurs and the with block exits early.

Note: In Python 2.6, you will need to use two with statements, as support for multiple contexts was not added until 2.7. e.g:

with open(somefile, 'r') as f:
    with open(someotherfile, 'w') as g:
        #code here.

---

outFile.write(inFile.readline()) #always print the header

The file object is an iterator that gets consumed. When readline() is called, the buffer position advances forwards and the first line is returned.

---

for line in inFile:

As mentioned before, the file object is an iterator, so you can use it directly in a for loop.