Fill in dataframe NULL cells with associated values-CodePudding

I need to fill in the Null values with values that are already populated with the ColA group

original df:

ColA    ColB     ColC
cat      20      blue
cat      28      yellow
fish     32       red
fish     NaN      NaN
fish     NaN      NaN
dog      3       black
dog      311      tan
dog      755     white
frog     NaN      NaN
frog     NaN      NaN
frog     400      green
frog     NaN      NaN

Leave alone the rows that have populated cells in each column, but where there are null values, I want to fill those cells with the associated value that the ColA group has populated, for example:

desired df:

ColA    ColB     ColC
cat      20      blue
cat      28      yellow
fish     32       red
fish     32       red
fish     32       red
dog      3       black
dog      311     tan
dog      755     white
frog     400     green
frog     400     green
frog     400     green
frog     400     green

Any ideas is appreciated. Thanks!

CodePudding user response：

Try goupby with forward fill (ffill) and back fill(bfill)

df[['ColB', 'ColC']] = df.groupby('ColA').ffill().bfill()

    ColA   ColB    ColC
0    cat   20.0    blue
1    cat   20.0  yellow
2   fish   32.0     red
3   fish   32.0     red
4   fish   32.0     red
5    dog    3.0   black
6    dog    3.0     tan
7    dog    3.0   white
8   frog  400.0   green
9   frog  400.0   green
10  frog  400.0   green
11  frog  400.0   green