How to divide each value in column by the maximum value of a subset of that column

Question

I am trying to divide each row in a column by the maximum of a sub-list in the column where the sub-list if the column filtered by a category variable

Is there a single line vector equation that creates col3? I have been trying to use groupby with transform(lambda x: x...) but can't seem to get the effect of maxif where it only takes the max of col2 where col1 = the rows with the same category as the row in col2 being divided.

Sample input code:

import pandas as pd

data = {'col1':['A', 'A', 'B', 'B'],
        'col2':[1, 2, 3, 4]}

df = pd.DataFrame(data)

df

Desired output:

col1	col2	col3	explanation
A	1	0.5	eg 1/2
A	2	1	eg 2/2
B	3	0.75	eg 3/4
B	4	1	eg 4/4

Answer 1

Sure:

>>> df['col2'] / df.groupby('col1')['col2'].transform(max)
0    0.50
1    1.00
2    0.75
3    1.00

You could then assign that result to a new column of your choice.