BUG: Fix parsing of ISO8601 durations

[mgmarino]

This PR fixes the following behavior:

• Adds "W" as a valid weeks designator
• Fixes parsing of durations with empty periods (e.g. "PT10M")
• Fixes partial parsing of durations (e.g. "P1DT1H")

Fixes #29773.
Fixes #36204.

• closes #xxxx
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[jreback]

do we handle negatives of these? (IIRC that's on one of the issues)

[mgmarino]

I didn't change the behavior of negativity, and the question is how to handle it. One of the only references I can find regarding the expected behavior is this comment, which also seems to indicate it's not very well defined in the ISO 8601 standard. How would you feel about pulling that out into a separate issue?

[jreback]

yes pls create a separate issue. only thing for now is do we raise if a negative is passed in? (i think we should unless / until we decide on how to handle it), though isn't the result just negative of the Timedelta itself which is well defined.

[mgmarino]

Ok, will do.

No, nothing is raised when it is negative, and the behavior is, admittedly, quite odd, i.e.

"P-6DT0H50M3.010010012S" parses as Timedelta( days=-6, minutes=50, seconds=3, milliseconds=10, microseconds=10, nanoseconds=12, )
, and the negative is only allowed right after the P descriptor. A negative in any other position will raise an error. As far as I can tell, there are two possibilities to go here:

• only support an overall "-" e.g. "-P6DT1H" = Timedelta('-7 days +23:00:00') and/or
• support positive/negative on each, e.g. "P7DT-1H3M" = Timedelta('6 days 23:03:00')

[mgmarino]

Issue now here: #37172. I would prefer to change/update the negativity behavior in a separate PR if that's ok.

[jreback]

yep that's great. well scoped PRs are good. one thing at a time. and thanks for working on this.

[jreback]

thanks @mgmarino very nice!