Last Day previous Month - python-3.x

Question

I have this dataframe
import pandas as pd
df = pd.DataFrame({'Found':['A','A','A','A','A','B','B','B','B'],
'Date':['14/10/2021','19/10/2021','29/10/2021','30/09/2021','20/09/2021','20/10/2021','29/10/2021','15/10/2021','10/09/2021'],
'LastDayMonth':['29/10/2021','29/10/2021','29/10/2021','30/09/2021','30/09/2021','29/10/2021','29/10/2021','29/10/2021','30/09/2021'],
'Mark':[1,2,3,4,3,1,2,3,2]
})
print(df)
Found Date LastDayMonth Mark
0 A 14/10/2021 29/10/2021 1
1 A 19/10/2021 29/10/2021 2
2 A 29/10/2021 29/10/2021 3
3 A 30/09/2021 30/09/2021 4
4 A 20/09/2021 30/09/2021 3
5 B 20/10/2021 29/10/2021 1
6 B 29/10/2021 29/10/2021 2
7 B 15/10/2021 29/10/2021 3
8 B 10/09/2021 30/09/2021 2
based on this dataframe I need to create a new column that is the "Mark" of the last day of the month to form this new column.
that is, I need the value of the 'Mark' column of the last day of the month of each Found
how i did
mark_last_day = df.loc[df.apply(lambda x: x['Date']==x['LastDayMonth'], 1)]
df.merge(mark_last_day[['Found', 'LastDayMonth', 'Mark']],
how='left',
on=['Found', 'LastDayMonth'],
suffixes=('', '_LastDayMonth'))
# Output
Found Date LastDayMonth Mark Mark_LastDayMonth
0 A 14/10/2021 29/10/2021 1 3
1 A 19/10/2021 29/10/2021 2 3
2 A 29/10/2021 29/10/2021 3 3
3 A 30/09/2021 30/09/2021 4 4
4 A 20/09/2021 30/09/2021 3 4
5 B 20/10/2021 29/10/2021 1 2
6 B 29/10/2021 29/10/2021 2 2
7 B 15/10/2021 29/10/2021 3 2
So far so good but I'm having trouble creating a new column with the Mark_LastDayMonth of the previous month or I need the last day of the current month and the previous month
how do i do it
Ex.
Found Date LastDayMonth Mark Mark_LastDayMonth Mark_LastDayPrevious_Month
0 A 14/10/2021 29/10/2021 1 3 4
1 A 19/10/2021 29/10/2021 2 3 4
2 A 29/10/2021 29/10/2021 3 3 4
3 A 30/09/2021 30/09/2021 4 4 x
4 A 20/09/2021 30/09/2021 3 4 x
5 B 20/10/2021 29/10/2021 1 2 1
6 B 29/10/2021 29/10/2021 2 2 1
7 B 15/10/2021 29/10/2021 3 2 1
8 B 10/09/2021 30/09/2021 1 1 x

Answer 1

Here is a function to get the last day of the previous month
import datetime
def get_prev_month(date_str):
format_str = '%d/%m/%Y'
datetime_obj = datetime.datetime.strptime(date_str, format_str)
first_day_of_this_month = datetime_obj.replace(day=1)
last_day_of_prev_month = first_day_of_this_month - datetime.timedelta(days=1)
return last_day_of_prev_month.strftime("%d/%m/%Y")
Here is a function to get the mark of any "date" and "found" from your mark_last_day variable
def get_mark_of(date_str, found):
same_date = last_day_mark.Date==date_str
same_found = last_day_mark.Found == found
return last_day_mark.where(same_date & same_found).dropna().Mark
If you want to add the LastDayPrevMonth column You don't need to do so unless you want it
df["LastDayPrevMonth"] = df.LastDayMonth.apply(lambda x: get_prev_month(x))
And at last the creating the column Mark_LastDayPrevMonth, and setting 0 if there exist no that previous month in the dataset.
df["Mark_LastDayPrevMonth"] = df.apply(lambda x: get_mark_of(get_prev_month(x["LastDayMonth"]), x["Found"]), axis=1).fillna(0).astype(int)

Answer 2

Use the date offset MonthEnd
from pandas.tseries.offsets import MonthEnd
df['LastDayPreviousMonth'] = df['Date'] - MonthEnd()
>>> df[['Date', 'LastDayPreviousMonth']]
Date LastDayPreviousMonth
0 2021-10-14 2021-09-30
1 2021-10-19 2021-09-30
2 2021-10-29 2021-09-30
3 2021-09-30 2021-08-31
4 2021-09-20 2021-08-31
5 2021-10-20 2021-09-30
6 2021-10-29 2021-09-30
7 2021-10-15 2021-09-30
Then do a similarly merge as you did for 'LastDayMonth'.
Does this help you complete the solution?
Note: I'm assuming 'Date' and 'LastDayPreviousMonth' are datetime-like. If they aren't you need to convert them first using
df[['Date', 'LastDayMonth']] = df[['Date', 'LastDayMonth']].apply(pd.to_datetime)