Undo a file readline() operation so file-pointer is back in original state

MikeN_36680 0
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I'm browsing through a Python file pointer of a text file in read-only mode using file.readline() looking for a special line. Once I find that line I want to pass the file pointer to a method that is expecting the file pointer to be at the START of that readline (not right after it.)

How do I essentially undo one file.readline() operation on a file pointer?

readlines readline file-io python
5 Answers

If you don't know the last line because you didn't visit it you can read backwards until you see a newline character:

with open(logfile, 'r') as f:
    # go to EOF
    f.seek(0, os.SEEK_END)
    nlines = f.tell()
    i=0
    while True:
        f.seek(nlines-i)
        char = f.read(1)
        if char=='\n':
            break
        i+=1
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