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

用NaN替换列子集中的空值：

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

Answer 2

You should replace the empty strings with np.NaN before:

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

Answer 3

Replace '' with np.nan first:

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