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My program is keeping a log for the user. If the log ever gets bigger than a set amount, I want to delete the first 20% of lines.
From similar questions, I’ve seen suggestions to do read in the old file, and write out all the lines I want to keep into a new file. However, my files might be too large to be constantly reading them in, and using that method wouldn’t let me keep the same file name.
Can I delete lines from a file without reading in my old file?
The general method to achieve this for logfiles is ‘rotation’ – when the logfiles gets older or hits a certain size, you rename it and start writing a new one.
If you are using logging module, there is even a preconfigured one – RotatingFileHandler that does this automatically.
As for your question: you can only truncate from the back, not from the beginning. An approximate solution would be to seek() to 20% of the file, find first ‘\n’ and copy it out – but it will be slow and prone to race conditions. Go with logging and RotatingFileHandler.