Pandas: Grouped DataFrame - divide values of a column by the value of a certain row within that column for each group

Question

I have a dataframe with groups. To normalize the values for each group I'd like to divide all values of each group by the value of a certain element within that group.

df = pd.DataFrame([['a','2018-02-03',42],
                   ['a','2018-02-04',22],
                   ['a','2018-02-05',10],
                   ['a','2018-02-06',32],
                   ['b','2018-02-03',10],
                   ['b','2018-02-04',8],
                   ['b','2018-02-05',2],
                   ['b','2018-02-06',12],
                   ['c','2018-02-03',20],
                   ['c','2018-02-04',30],
                   ['c','2018-02-05',5],
                   ['c','2018-02-06',15]])
df.columns = ['product','day','value']

I want to normalize column 'value' for each 'product' by 'value' of 'day' == '2018-02-05'

Expected Result:

    product     day         value
0   a           2018-02-03  4.2
1   a           2018-02-04  2.2
2   a           2018-02-05  1
3   a           2018-02-06  3.2
5   b           2018-02-03  5
6   b           2018-02-04  4
7   b           2018-02-05  1
8   b           2018-02-06  6
10  c           2018-02-03  4
11  c           2018-02-04  6
12  c           2018-02-05  1
13  c           2018-02-06  3

I tried df.groupby('product').transform().
To access the first value .transform('first') is possible. But I cannot find a way to access a certain value.

Annotation: Maybe this one can be solved without using .groupby() ?

Answer 1

Do like this:

df = pd.DataFrame([['a','2018-02-03',42],
                   ['a','2018-02-04',22],
                   ['a','2018-02-05',10],
                   ['a','2018-02-06',32],
                   ['b','2018-02-03',10],
                   ['b','2018-02-04',8],
                   ['b','2018-02-05',2],
                   ['b','2018-02-06',12],
                   ['c','2018-02-03',20],
                   ['c','2018-02-04',30],
                   ['c','2018-02-05',5],
                   ['c','2018-02-06',15]])
df.columns = ['product','day','value']

date = '2018-02-05'

# Set the index to ['product', 'day']
df.set_index(['product', 'day'], inplace=True)

# Helper Series - Values of date at index 'day'
s = df.xs(date, level=1)

# Divide df by helper Series and reset index
df = df.div(s, level=0).reset_index()
print(df)

   product         day  value
0        a  2018-02-03    4.2
1        a  2018-02-04    2.2
2        a  2018-02-05    1.0
3        a  2018-02-06    3.2
4        b  2018-02-03    5.0
5        b  2018-02-04    4.0
6        b  2018-02-05    1.0
7        b  2018-02-06    6.0
8        c  2018-02-03    4.0
9        c  2018-02-04    6.0
10       c  2018-02-05    1.0
11       c  2018-02-06    3.0