Why do Python's datetime.strftime('%w') and datetime.weekday() use different indexes for the days of the week?

Question

In Python, showing the day of the week as an integer using datetime.strftime() shows a different result than using datetime.weekday().

>>> import datetime
>>> now = datetime.datetime.now()
>>> now.strftime('%A')
'Sunday'
>>> now.strftime('%w') # Day of the week as an integer.
'0'
>>> now.weekday() # Day of the week as an integer, a different way.
6

With strftime(), the string format %w has Sunday as the first day of the week. With weekday(), it's Monday instead.

What's the history of why these two are different?

Answer 1

Originally, the ISO 8601 standard used 1 .. 7 to represent Monday through Sunday. For convenience, later on the interpretation 0=Sunday was permitted.

If you want to use something that is more consistent, try using isoweekday

The 0=Monday standard is the European convention. I guess that's no surprise :P

Answer 2

Python's strftime function emulates that in the c library. Thus, the motivation that %w returns 0 for a Sunday comes entirely from that.

In contrast, the method date.weekday() returns a 6 for Sunday as it seeks to match the behaviour of the much older time module. Within that module times are generally represented by a struct_time and within this, struct_time.tm_day uses a 6 to represent a Sunday.

The correct question then becomes ... why does time.struct_time represent a Sunday as a 6, when the C library's tm struct uses a 0 ??

And the answer is ... because it just does. This behaviour has existed ever since Guido first checked in gmtime and localtime functions in 1993.

And Guido cannot be wrong ... so you best ask him.