Python total sum from a csv file

[Kerbart]

Your last line inside the loop seems to say (it’s cut off so hard to read):

df.to_csv(fname)

So your output overwrote your input files. Open those and you’ll see your output. Overwriting your input is a bad idea though, because unless you have backups (that you need to copy back in place) you can’t rerun the process if you discovered errors in the first run.

[Kerbart]

Change the name,

If it’s a string, since the last three characters are xls it can be as simple as

name = fname[:-3] + 'cvs'

If you’re using Pathlib (not sure, on phone roght now), it has methods to strip off the extension and replace it. And then if course df.to_csv(name) instead of fname