python .tell() giving unexpected position in file?

Question

I'm trying to work on a function that will change a variable in the script depending on user-input. I started to use the .tell() built-in to pinpoint the variable to be compared but, the position that it writes to is off by at least 10 bytes?

#! /usr/bin/env python
import re

class file_input:
    def __init__(self):
        count = 0
        change = raw_input('Input? ')
        with open('/home/Downloads/FILES/adobe.py','a+') as f:
            for line in f.readlines():
                if re.findall('script_data', line):
                    count += 1
                    ## i put in a count to ignore the first 'script_data' mentioned in the __init__ method ##
                    if change != line[13:] and count == 2:
                        ## if the user-input is not the same, re-write that variable ##
                        pos = f.tell()
                        f.seek(pos)
                        ## i checked the position here and its not where i would think it would be ##
                        print pos
                        print 'data not matched up, changing now...'
                        f.write(change)
                        print line[13:]
        f.close()



if __name__ == '__main__':
    file_input()


script_data = 'this is going to be some data...'

When I go to check the file though script_data variable will still be there even if the input data was different and the new data will be a line below.

Answer 1

It is unsafe to assume anything about the file position during readlines(). The implementation might read the whole file, it might use a read-ahead buffer etc., which cause tell to return unexpected positions.

I propose you do the following:

1. read all the lines from the file (lines = f.readlines())
2. change the lines variable
3. re-write the file