Save-to-file code not quite working completely

[dn]

On 09/08/2020 08:51, Steve wrote:
> 
> I have a file containing specifications. My .py program can read and
> manipulate the data but I cannot seem to get it to update the original file
> properly.
> 
> The code is simple and works except that the original line remains in the
> file after the new updated one has been added.
...
> That second line shown was the original line for MSN. The replacement line
> should have replaced the original line not just get added to the file.  It
> should have been replaced. What is in line 2, should have "1 MSN" at the
> beginning but that somehow disappeared.
> 
> So close, so simple...
> How do I fix this?


To be a logomach, let's talk about "update":-
May I advise that a 'good practice' would be to create a new file, and 
thus be able to (also) maintain the old version as a 'backup'.
(also: advantage when debugging/testing logic!)

The pattern is to read the 'old' file, line-by-line. If there is a 
key-match, ignore the read record, writing the new data  in its stead. 
If there is not a match, copy the input-data into an output record.

The major addition will be the need to rename the 'old file', ensuring 
logic to handle any "clash" (based upon whatever security 
requirements/numbers of generations - which it would appear don't 
currently exist).

The above assumes that the program will either only ever update a single 
record per execution-run; or that the logic collects a list of changes 
to the file which will be saved to the file, as above.


Further, databases have been designed for this sort of partial update 
scenario. If/when updates become too frequent, or the file becomes 
extensive, recommend you look at using an RDBMS instead of using this 
'flat-file' approach...
-- 
Regards =dn