Why Does Converting Week Numbers to Dates in Python Fail with '2013-W26'?

Converting Week Numbers to Dates

You've encountered an issue while attempting to convert a week number to a date using the following code:

import datetime
d = "2013-W26"
r = datetime.datetime.strptime(d, "%Y-W%W")
print(r)

When executing this code, you expected to see "2013-01-01 00:00:00", but instead, you encountered an error. Let's explore what's causing this and how to resolve it.

Addressing the Issue

To accurately convert a week number to a date, you require additional information beyond the week number itself, namely, the day of the week. Adding a default day of the week to your code will address this issue. Here's the modified code:

import datetime
d = "2013-W26"
r = datetime.datetime.strptime(d + '-1', "%Y-W%W-%w")
print(r)

Explanation of the Modification

The ' -1' and '-%w' pattern additions are crucial in this modification. The '-1' tells the parser to pick Monday in that particular week, and the '-%w' adjusts the day of the week relative to Monday. For instance, using '-%w' would return Sunday if the supplied week number corresponds to Monday.

Behavior of %W and Footnote 4

In the Python documentation's strftime() and strptime() behavior section, footnote 4 provides valuable insight: "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."

Additional Notes

If you're dealing with an ISO week date, you can utilize %G-W%V-%u instead. These directives require Python 3.6 or later.

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