用 Python 中上一行的内容覆盖 csv 文件中一行的内容

Question

I have a csv file that contains a timestamp and data for each stamp. It basically looks like this:

Timestamp,Column_1,Column_2,Column_3,Column_4,Column_5
2021-04-13 11:03:13+02:00,3,3,3,12,12
2021-04-13 11:03:14+02:00,999,999,999,999,999
2021-04-13 11:03:15+02:00,999,999,999,999,999
2021-04-13 11:03:17+02:00,3,48,3,3,3
2021-04-13 11:03:18+02:00,999,999,999,999,999

So now I want to run through the file and check if one of the values is "999" (since that is only a placeholder). If that is the case, I want the contents of the row above to overwrite the "999"'s. That should go through until it reaches a row where the contens aren't "999", since in that case it should save those values for the next time it finds a "999" row, to overwrite it with.

I already tried with the standard csv library but I just can't get anything to work.

Any help would be very much appreciated!

Answer 1

Read the file in pandas

df = pd.read_csv(<file_name>)

Then replace all entries having value as 999 with np.NaN

df = df.replace(999, np.NaN)

And then do the fillna with 'ffill'

df = df.fillna(method='ffill')