How to get the week number starting from monday of given month in python?

I was trying to calculate the week number starting from first Monday of October. Is there any functions in pandas or datetime to do the calculation efficiently?

MWE

import pandas as pd
from datetime import date,datetime

df = pd.DataFrame({'date': pd.date_range('2021-10-01','2022-11-01',freq='1D')})
df['day_name'] = df['date'].dt.day_name()

df2 = df.head()
df2['Fiscal_Week'] = [52,52,52,1,1] # 2021-10-4 is monday, so for oct4, week is 1
# How to do it programatically for any year?

df2
    date        day_name Fiscal_Week
0   2021-10-01  Friday   52
1   2021-10-02  Saturday 52
2   2021-10-03  Sunday   52
3   2021-10-04  Monday   1
4   2021-10-05  Tuesday  1

Answer

Shift dates by the number of days to new year, they use standard Monday week number formatting (%W):

df['Fiscal_Week'] = (
    df['date'] + pd.DateOffset(days=91)
).dt.strftime('%W').astype(int).replace({0:None}).fillna(method='ffill')

• The offset in days can be calculated manually (I assume the fiscal year start is fixed)
• The replace part is needed because leftovers from the previous year are considered week 0. The previous week might be 52 or 53, so replacing with NA and then fill forward