file - Python: read a line and write back to that same line

Question

I am using python to make a template updater for html. I read a line and compare it with the template file to see if there are any changes that needs to be updated. Then I want to write any changes (if there are any) back to the same line I just read from.

Reading the file, my file pointer is positioned now on the next line after a readline(). Is there anyway I can write back to the same line without having to open two file handles for reading and writing?

Here is a code snippet of what I want to do:

cLine = fp.readline()
if cLine != templateLine:
   # Here is where I would like to write back to the line I read from
   # in cLine

Answer 1

Updating lines in place in text file - very difficult

Many questions in SO are trying to read the file and update it at once.

While this is technically possible, it is very difficult.

(text) files are not organized on disk by lines, but by bytes.

The problem is, that read number of bytes on old lines is very often different from new one, and this mess up the resulting file.

Update by creating a new file

While it sounds inefficient, it is the most effective way from programming point of view.

Just read from file on one side, write to another file on the other side, close the files and copy the content from newly created over the old one.

Or create the file in memory and finally do the writing over the old one after you close the old one.