How to create a column with the value of a column from the previous row?

Question

I have a dataset and I would like to create a new column where the value of this new column is equal to the value of the previous row.

What I have:

col_a	col_b
x	189
y	33
z	0
k	837
H	33

What I am looking for:

col_a	col_b	new_col
x	189	189
y	33	189
z	0	33
k	837	0
H	86	837

It is something very similar to what df = df.fillna(method='ffill') does, but it is not working for what I need.

Answer 1

use shift in creating a col_c and then ffill to update the missing values from col_b

df['col_c'] = df['col_b'].shift(1)
df.fillna(method='ffill', axis=1) 

    col_a   col_b   col_c
0       x   189     189
1       y   33      189
2       z   0        33
3       k   837       0
4       H   33      837