Calculate Percent of Groupby Variable to Sum Column

Question

I'm not finding a similar example to understand this in python. I have a dataset that looks like this:

ID    Capacity
A     50
A     50
A     50
B     30
B     30
B     30
C    100
C    100
C    100

I need to find the percent of each ID for the sum of the "Capacity" column. So, the answer looks like this:

ID    Capacity   Percent_Capacity
A     50         0.2777
A     50         0.2777
A     50         0.2777
B     30         0.1666
B     30         0.1666
B     30         0.1666
C    100         0.5555
C    100         0.5555
C    100         0.5555

Thank you - still learning python.

Answer 1

total=df.groupby('ID')['Capacity'].first().sum()
df['percent_capacity'] = df['Capacity']/total
df

    ID  Capacity    percent_capacity
0   A         50    0.277778
1   A         50    0.277778
2   A         50    0.277778
3   B         30    0.166667
4   B         30    0.166667
5   B         30    0.166667
6   C        100    0.555556
7   C        100    0.555556
8   C        100    0.555556