Chain method grouping and calculating differences in a Pandas DataFrame

I have a Pandas DataFrame with the following structure:

import pandas as pd

data = {
    'glob_order': [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11],
    'trans': ['A', 'A', 'A', 'A', 'B', 'B', 'B', 'C', 'C', 'C', 'C'],
    'chain': [1, 1, 2, 2, 1, 1, 2, 1, 1, 2, 2],
    'date': ['1/08/2023', '2/08/2023', '3/08/2023', '4/08/2023', '5/08/2023', '6/08/2023', '7/08/2023', '8/08/2023', '9/08/2023', '10/08/2023', '11/08/2023']
}

df = pd.DataFrame(data)

# Convert 'date' column to datetime
df['date'] = pd.to_datetime(df['date'], format='%d/%m/%Y')

print(df)

I want to perform two operations:

Group the DataFrame by the 'trans' and 'chain' columns and select the very first row from each group.
Create a new column named 'delta' that represents the difference in days between the current date and the previous date within each group.

I tried the following code:

(df
.groupby(['trans', 'chain'])
.first()
.assign(
 delta=lambda x: (x['date'] - x['date'].shift(1)).dt.total_seconds() / (60*60*24),
 ).reset_index()
 )

However, the output I'm getting is not as expected. It seems to insert NaNs in the first delta calculation for each individual group, which is not what I want

trans	chain	glob_order	date	delta
A	1	1	2023-08-01	NaN
A	2	3	2023-08-03	2.0
B	1	5	2023-08-05	2.0
B	2	7	2023-08-07	2.0
C	1	8	2023-08-08	1.0
C	2	10	2023-08-10	2.0

I'd like to understand why this is happening and what I need to do to get the desired output.

This is my desired output

trans	chain	glob_order	date	delta
A	1	1	2023-08-01	NaN
A	2	3	2023-08-03	2.0
B	1	5	2023-08-05	Nan
B	2	7	2023-08-07	2.0
C	1	8	2023-08-08	Nan
C	2	10	2023-08-10	2.0

I am seeking a solution using a method similar to chaining for clarity and readability since I'm very new to Python.

Solution

If I understand you correctly, you want:

as first step group by "trans"/"chain", get first row
as second step group by "trans" and get difference between days

I don't think simple chaining is possible, but you can use := operator (but I recommend, for readability, to split it two separate commands):

df = (df := df.groupby(["trans", "chain"], as_index=False).first()).assign(
    delta=df.groupby("trans")["date"].diff().dt.days
)
print(df)

Prints:

  trans  chain  glob_order       date  delta
0     A      1           1 2023-08-01    NaN
1     A      2           3 2023-08-03    2.0
2     B      1           5 2023-08-05    NaN
3     B      2           7 2023-08-07    2.0
4     C      1           8 2023-08-08    NaN
5     C      2          10 2023-08-10    2.0