pandas dataframe groupby apply multi columns and get count

Question

I have a excel like this:

year	a	b
2021	12	23
2021	31	0
2021	15	21
2021	14	0
2022	32	0
2022	24	15
2022	28	29
2022	33	0

I wanna get count of condition: a>=30 and b==0 group by year the final output like this:

2021 1

2022 2

I wanna use pandas dataframe to implement this, can anyone help? I'm quite new to python

Answer 1

For count matched rows chain both conditions by & for bitwise AND and aggregate sum, Trues are processing like 1 and False like 0:

df1 = ((df.a>=30) & (df.b==0)).astype(int)
           .groupby(df['year']).sum().reset_index(name='count')
print (df1)
   year  count
0  2021      1
1  2022      2

Similar idea with helper column:

df1 = (df.assign(count = ((df.a>=30) & (df.b==0)).astype(int))
         .groupby('year', as_index=False)['count']
         .sum())