How to repeat a value in a column based on another

I have the following dataframe:

import pandas as pd

df = pd.DataFrame({
    'a': [1, 1, 2, 2],
    'b': [None, 'w', None, 'z']
})

a	b
1	None
1	‘w’
2	None
2	‘z’

And I want to repeat the values that are not None in column ‘b’, but based on the value in column ‘a’.

At the end I would have this dataframe:

a	b
1	‘w’
1	‘w’
2	‘z’
2	‘z’

Answer

The logic is not fully clear on how you would like to generalize, but you could bfill/ffill per group:

df['b'] = df.groupby('a')['b'].apply(lambda x: x.bfill().ffill())

output:

   a  b
0  1  w
1  1  w
2  2  z
3  2  z