python - ffill not filling data in pandas dataframe

Question

I have a dataframe like this :

  A  B  C  E D
---------------
0 a  r  g    g
1            x
2 x  f  f    r
3            t
3            y

I am trying for forward filling using ffill. It is not working

 cols = df.columns[:4].tolist()
 df[cols] = df[cols].ffill()

I also tried :

df[cols] = df[cols].fillna(method='ffill')

But it is not getting filled. Is it the empty columns in data causing this issue?

Data is mocked. Exact data is different (contains strings,numbers and empty columns)

desired o/p:

  A  B  C  E D
---------------
0 a  r  g    g
1 a  r  g    x
2 x  f  f    r
3 x  f  f    t
3 x  f  f    y

Answer 1

Replace empty values in subset of columns by NaN:

df[cols] = df[cols].replace('', np.nan).ffill()