Pandas - Find difference based on two subsequent rows of Dataframe - pandas

Question

I have a Dataframe that captures date when ticket was raised by a customer that is captured in column labelled date. If the ref_column for the current cell is same as the following cell then I need to find difference of aging based on date column current cell and the following cell for the same cust_id. if the ref_column is to the same then I need to find difference of date and ref_date of the same row.
Given below is how my data is:
cust_id,date,ref_column,ref_date
101,15/01/19,abc,31/01/19
101,17/01/19,abc,31/01/19
101,19/01/19,xyz,31/01/19
102,15/01/19,abc,31/01/19
102,21/01/19,klm,31/01/19
102,25/01/19,xyz,31/01/19
103,15/01/19,xyz,31/01/19
Expected output:
cust_id,date,ref_column,ref_date,aging(in days)
101,15/01/19,abc,31/01/19,2
101,17/01/19,abc,31/01/19,14
101,19/01/19,xyz,31/01/19,0
102,15/01/19,abc,31/01/19,16
102,21/01/19,klm,31/01/19,10
102,25/01/19,xyz,31/01/19,0
103,15/01/19,xyz,31/01/19,0
Aging(in days) is 0 for the last entry for a given cust_id

Answer 1

Here's my approach:
# convert dates to datetime type
# ignore if already are
df['date'] = pd.to_datetime(df['date'])
df['ref_date'] = pd.to_datetime(df['ref_date'])
# customer group
groups = df.groupby('cust_id')
# where ref_column is the same with the next:
same_ = df['ref_column'].eq(groups['ref_column'].shift(-1))
# update these ones
df['aging'] = np.where(same_,
-groups['date'].diff(-1).dt.days, # same ref as next row
df['ref_date'].sub(df['date']).dt.days) # diff ref than next row
# update last elements in groups:
last_idx = groups['date'].idxmax()
df.loc[last_idx, 'aging'] = 0
Output:
cust_id date ref_column ref_date aging
0 101 2019-01-15 abc 2019-01-31 2.0
1 101 2019-01-17 abc 2019-01-31 14.0
2 101 2019-01-19 xyz 2019-01-31 0.0
3 102 2019-01-15 abc 2019-01-31 16.0
4 102 2019-01-21 klm 2019-01-31 10.0
5 102 2019-01-25 xyz 2019-01-31 0.0
6 103 2019-01-15 xyz 2019-01-31 0.0