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how to replace line on particular line in file[no need to write it back whole file again]


On 2018-10-11 13:57:57 +0200, Thomas Jollans wrote:
> There's no easy way to write to the middle of a file (though I think it
> can be done by mmap'ing a file)

#!/usr/bin/python3
import os

with open("foo.bin", "wb") as f:
    f.write(b'abcdefghijklmnopqrstuvwxyz')

with open("foo.bin", "rb") as f:
    print("-----")
    for i, c in enumerate(f.read()):
        print("%2d %2x" % (i, c))

with open("foo.bin", "r+b") as f:
    f.seek(10, os.SEEK_SET)
    f.write(b'000')
    f.seek(20, os.SEEK_SET)
    f.write(b'11')
    f.seek(30, os.SEEK_SET)
    f.write(b'2')

with open("foo.bin", "rb") as f:
    print("-----")
    for i, c in enumerate(f.read()):
        print("%2d %2x" % (i, c))

Mmap doesn't allow you to do anything you can't do with normal file I/O.
It's just sometimes more convenient and/or performant.

> -- and even if you manage to do that,
> you'd have to write back anything after the changed line if the length
> of that line changed by as little as a byte.

This is correct.

	hp

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   _  | Peter J. Holzer    | we build much bigger, better disasters now
|_|_) |                    | because we have much more sophisticated
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__/   | http://www.hjp.at/ | -- Ross Anderson <https://www.edge.org/>
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