Changing a line in a text file

[Steve Holden]

kah wrote:
> Hi,
> 
> the example provided by Vishnu is quite close to what I want but it
> still required me to write all the data back to the file.
> 
> Is there any way where I can just write a particular line?
> 
If you are asking whether you can update a file in place, the answer is 
"yes" - look up "lseek" in the Python documentation (module os, under 
"File Descriptor Operations".

However, you asked about replacing one line with another of a different 
length: since this will mean changing the offsets of all subsequent 
bytes you have no way to do this other than writing out the whole 
content of the file following the modification. You would also have to 
ensure that you truncated the file to the correct length.

In general, although they don't make it obvious that they are doing so 
most programs that "change" files (text editors and the like) are really 
writing new copies.

regards
  Steve
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