Python datetime strptime with calendar week number

Question

According to resources, the method to transform a string that contains "year-week_number" (such as 2015-20) to a datetime object is the following:

from datetime import datetime
datetime.strptime("2015-20", "%Y-%W")

However, this does not give me the correct result. What I am getting is datetime.datetime(2015, 1, 1, 0, 0)

Moreover:

datetime.strftime(datetime.strptime("2015-20", "%Y-%W"). "%Y-%W")

actually returns "2015-00".

Is this just a bug or did I forget to do something?

Answer 1

From the documentation :

When used with the strptime() method, %U and %W are only used in calculations when the day of the week and the year are specified.

Given you are not specifying day of the week, the %W parameter is being ignored as per the documentation.

Add the weekday to whatever you want, I put Monday:

>>> datetime.strptime("2015-20-1", "%Y-%W-%w")
datetime.datetime(2015, 5, 18, 0, 0)

Answer 2

Point 6. of the documentation is what's important here:

When used with the strptime() method, %U and %W are only used in calculations when the day of the week and the year are specified.

---

from datetime import datetime
date = datetime.strptime("2016-12-0", "%Y-%m-%w")
print "Week:"m datetime.strftime(date, "%W")

>>>Week: 48

Also, be aware of the small difference between %W and %U .