To replace NaNs by preceding or next values in a Python Pandas DataFrame we can use the fillna method with the method argument set to 'ffill'.
For instance, we write
df = pd.DataFrame([[1, 2, 3], [4, None, None], [None, None, 9]])
df.fillna(method='ffill')
to call fillna on dataframe df with the method argument set to 'ffill' to fill NaNs with values before the next row.
We can also set method to 'bfill ' to fill NaNs with values after the next row.
