replace a value with nan using condition based on another column code example

Example 1: replace "-" for nan in dataframe

df.replace(np.nan,0)

Example 2: pandas replace nan with value above

>>> df = pd.DataFrame([[1, 2, 3], [4, None, None], [None, None, 9]])
>>> df.fillna(method='ffill')
   0  1  2
0  1  2  3
1  4  2  3
2  4  2  9