CSVs: how to output missing data to make processing easier?
Both pandas and readr allow flexibility on the marker used to indicate missing values, allowing to specify values that should be treated as missing when reading csvs:


readr https://readr.tidyverse.org/reference/read_delim.html  
pandas https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.read_csv.html


given that, maybe you can handle missing data in your code rather than replacing values in the original csvs.

If you get to choose, maybe NA (no hyphens) is a sensible default as it's handled as missing value by both libs by default.