Counting rows since condition

Question

I have a column in Pandas which contains booleans and want to count thr rows since the last True value, something like this:

a           b
False       0
True        0
False       1
False       2 
False       3  
True        0
False       1
True        0

I could do it via a loop but it seems there must be a better way

Answer 1

a = ~df['a']
b = a.cumsum()
c = b-b.where(~a).ffill().fillna(1).astype(int)
print (c)
0    0
1    0
2    1
3    2
4    3
5    0
6    1
7    0
Name: a, dtype: int32