Write new data into the middle of a file

Question

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How do I modify a text file in Python?

It's easy to write a new file, append to it, etc. But how can I write to the middle of a file?

I have an html report that is generated off of an automated process, and rather than rewrite the whole report, I want to just insert new data where it's new. Perhaps I want to look for a comment and append there. So, add new data after the line 

http://stackoverflow.com/questions/125703/how-do-i-modify-a-text-file-in-python — Jacob, Jul 11 '11 at 12:57
You don't need to rewrite the entire file, but you _will_ need to rewrite the entire part after your inserted piece — John La Rooy, Jul 11 '11 at 13:24

score 6 · Answer 1 · answered Jul 11 '11 at 12:57

Yes.

Read the file in
Modify what you need
Overwrite the file with the modified version

If the file is too big to fit into memory, read and write it line by line (or block by block, or whatever fits) into some temporary file, and then copy it instead of the original file.

score 0 · Answer 2 · answered Jul 11 '11 at 13:06

0

try fileObject.seek(offset[, whence])

answered Jul 11 '11 at 13:06

Max

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When opening a file for appending `seek` won't help for writing, only for reading: new contents will be written to the end of the file regardless. – Tom Oct 04 '13 at 17:02

score 0 · Answer 3 · answered Jul 11 '11 at 14:13

If your edit would change the size of the file, then you will need to rewrite everything after the edit (as Eli suggests). If you have a large amount of blank space somewhere in the file, then you should be able to just re-write new data into that blank space without affecting the later parts of the file (and you would use file.seek() for this, as Max suggests).

So your idea (leaving a comment telling the program where to insert data) should work as long as the comment is followed by enough blank space to contain any new data you might put there.

Write new data into the middle of a file

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