python one line file processing

[DeaD_EyE]

You can process the first time the whole file and use after the iteration of lines the method tell of of the file object, which tells you where you are (at which byte). You can convert the integer to a str and write it to a file. Next time the script looks for this file and if the file is present, it should load the content of the file, convert it back to an int and you use before you start iterating over the lines, you use seek(position) on the file object. Then you have the position, where your script finished last time.

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In [20]: with open('birds.txt') as fd: 
    ...:     for line in fd: 
    ...:         print(line.strip()) 
    ...:     print(fd.tell()) 
    ...: #fd.tell() <- file is already closed 
    ...:                                                                                                                                                                                    
2010-01-01 01:01:00.0000 left
2010-11-01 01:01:00.0000 right
2010-10-01 01:01:00.0000 right
91

So, if a program writes now to birds.txt, it starts as byte position 91.