Blank Values in Excel File From CSV (not just rows)
You can also do this entirely in excel:
Select column (or whatever range you're working with), then go to Edit>Go To  (Ctrl+G) and click Special. 
Check Blanks & click OK.
This will select only the empty cells within the list. 
Now type the = key, then up arrow and ctrl-enter.
This will put a formula in every blank cell to equal the cell above it.  You could then copy ^ paste values only to get rid of the formulas.
I'm currently opening CSV files in Excel with multiple columns, where values will only appear if a number changes. For example, the data may ACTUALLY be: 90,90,90,90,91. But it will only appear as 90,,,,91. I'd really like the values in between to be filled with 90s. Is there anyway python could help with this? I really appreciate your help!