获取上个月的第一天 - 月份必须在 1..12

Question

I tried this and I am getting an error month must be in 1..12 :

today_date = date.today()
# ERROR - month value becomes -1 here instead of 12
previous_month_4_start_date = today_date.replace(month=today_date.month - 4, day=1)

I can write some boilerplate code to fix this but looking for an efficient approach where if month is -1 or -2 or -3 or -4 then it should be replaced to 12 or 11 or 10 or 9 respectively.

Please help

EDIT

Thanks for helping with the month, how to take care of the year part since it will also change from 2020 to 2019 ? Please help

Answer 1

You can use modulo arithmetic for this problem, but first you have to convert the month number into a value which is valid modulo 12 (ie 0-11 ) by subtracting one from it. You can then subtract n months from that value, take the modulus and convert it back to a month number by adding 1 again:

today_date = date.today()

previous_month_4_start_date = today_date.replace(month=(today_date.month - 1 - 4) % 12 + 1, day=1)
print(previous_month_4_start_date)

Output (as of 2020-04-14 ):

2020-12-01

This doesn't deal with the fact that the year changes, to do that you need to compare the current month with the number being subtracted and if it is more than the current month, also subtract one from the year:

previous_month_4_start_date = today_date.replace(year=today_date.year-1 if today_date.month <= 4 else today_date.year,
                                                 month=(today_date.month - 1 - 4) % 12 + 1, 
                                                 day=1)

Output:

2019-12-01

But

That's the hard way to do it. The simple solution is to set the day to 1 and then use relativedelta to subtract 4 months:

from datetime import date
from dateutil.relativedelta import relativedelta

today_date = date.today()

previous_month_4_start_date = today_date.replace(day = 1) - relativedelta(months = 4)
print(previous_month_4_start_date)

Output:

2019-12-01

Answer 2

The modulus operator is your best bet here. It does exactly what you need. You need to subtract 5 instead of 4 because the number needs to flow into negatives for the code to work:

previous_month_4_start_date = today_date.replace(month=(today_date.month - 5) % 12 + 1, day=1)
# Returns: 12

Answer 3

#Try this to get the previous month's first day    

from datetime import datetime,date
today_date = date.today()
if today_date.month == 1:
    previous_month_4_start_date = today_date.replace(month=(12), day=1)
else:
    previous_month_4_start_date = today_date.replace(month=(today_date.month-1), day=1)
day_name= ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday','Sunday']
print(day_name[previous_month_4_start_date.weekday()])
print(previous_month_4_start_date)