用一年中的年和周计算的Python日期不正确

Question

We are having an issue when calculating the start of the first week in 2019.

The parameters are year and week_of_year . We are always staring the week at Monday.

The code to get the first day of a specific year:

from datetime import datetime

result = datetime.strptime('%04d-%02d-1' % (year, week_of_year), '%Y-%W-%w').date()

This works for most of the years, but not in the case when year = 2019 and week_of_year = 1 , Python computes this to the date 2019-01-02 and not the expected 2018-12-31 .

Our current solution to solve this problem is:

from datetime import datetime, timedelta

result = datetime.strptime('%04d-%02d-1' % (year, week_of_year), '%Y-%W-%w').date()

# Fixes issue with Python date computing returning the incorrect date
# This only occures for some years. E.g. 2019.
offset = result.isocalendar()[1] - datetime.strptime('%04d-01-01' % year, '%Y-%m-%d').date().isocalendar()[1]
if offset > 0:
    result = result - timedelta(days=7)

Any other ideas on how to fix this issue?

Answer 1

After consulting https://docs.python.org/3/library/datetime.html#strftime-and-strptime-behavior , I can tell that %W does not respect ISO-definition of the week counter, but simply counts from the first monday in a year. The days before are put into week count 0...

You can try %V together with %G :

result = datetime.strptime('%04d-%02d-1' % (year, 1), '%G-%V-%w').date()