Adding a column thats result of difference in consecutive rows in pandas

You could use diff and pass -1 as the periods argument:

>>> df = pd.DataFrame({"A": [9, 4, 2, 1], "B": [12, 7, 5, 4]})
>>> df["dA"] = df["A"].diff(-1)
>>> df
   A   B  dA
0  9  12   5
1  4   7   2
2  2   5   1
3  1   4 NaN

[4 rows x 3 columns]

Use shift.

df['dA'] = df['A'] - df['A'].shift(-1)