Pandas add increment index

I have the following Pandas DataFrame

Key Value 
A    10
A    20
B    30
B    40
C    50
A    60
A    70
A    70
B    80
A    90

And I need to create an index that auto increment only when the key repeats after sequence of different keys. So, I need this output:

Key Value Index
A    10     1
A    20     1
B    30     1
B    40     1
C    50     1
A    60     2
A    70     2
A    70     2
B    80     2
A    90     3

Thanks!

I try with the method groupby and cumcount() + 1 but it does not work.

>Solution :

Using an ordered Categorical and numpy.cumsum:

import numpy as np

s = pd.Categorical(df['Key'], ordered=True)
df['Index'] = np.cumsum(s<s.shift())+1

Output:

  Key  Value  Index
0   A     10      1
1   A     20      1
2   B     30      1
3   B     40      1
4   C     50      1
5   A     60      2
6   A     70      2
7   A     70      2
8   B     80      2
9   A     90      3