# pandas 0.22.0
    >>> df
       id    A
    0   1  2.0
    1   1  NaN
    2   2  3.0
    3   2  NaN
    >>>
    >>> df.groupby('id').transform('ffill')
         A
    0  2.0
    1  2.0
    2  3.0
    3  3.0
    >>>
    >>> df.groupby('id')['A'].transform('ffill')
    0    2.0
    1    2.0
    2    2.0
    3    2.0
    Name: A, dtype: float64