if nan in column replave by value in another column pandas code example

Example 1: pandas replace null values with values from another column

#Python
#Col 1 = where you want the values replaced
#Col 2 = where you want to take the values from
df.["Col 1"].fillna(df.["Col 2"], inplace=True)

Example 2: pandas if nan, then the row above

df.fillna(method='ffill')