Fill Nan with first Non Nan value above it

I have a dataframe let’s say

df = pd.DataFrame({'first':['fillwithit',None,None,'restbyit', None, None], 'test':[1,2,3,4,5,6]})
>>>In [10]: df
Out[10]: 
        first  test
0  fillwithit     1
1        None     2
2        None     3
3    restbyit     4
4        None     5
5        None     6

What I want is to fill all the None, such that None will look above it, find the first Non-None value, and fill it by it.

For example, None at index 1 and 2 will be filled via fillwithit and 4 & 5 via restbyit.

Output should be like

In [12]: df
Out[12]: 
        first  test
0  fillwithit     1
1  fillwithit     2
2  fillwithit     3
3    restbyit     4
4    restbyit     5
5    restbyit     6

I am thinking of a manual for looped version, but it seems inefficient. Do you have any better solution?

>Solution :

just put it here for reference:

import pandas as pd
df = pd.DataFrame({'first':['fillwithit',None,None,'restbyit', None, None], 'test':[1,2,3,4,5,6]})
df = df.ffill()

output:

        first  test
0  fillwithit     1
1  fillwithit     2
2  fillwithit     3
3    restbyit     4
4    restbyit     5
5    restbyit     6