BUG: Incorrect parsing of ISO 8601 durations strings in Timedelta constructor
- [x] Closes #51882
- [x] Closes #48122
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] ~~Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.~~
- [x] Added an entry in the latest `doc/source/whatsnew/v2.1.0.rst` file if fixing a bug or adding a new feature.

This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen. @mroeschke I would like to reopen this pull request, I have updated it to the main branch. Issues #51882 and #48122 are still not resolved. thanks for your PR

I do feel a bit nervous about this (as I do about the original implementation, tbh...), and think this might be a good case for a parametric test

would it be possible to add some reference implementation, such as [isoduration](https://github.com/bolsote/isoduration), and then to use [hypothesis](https://github.com/HypothesisWorks/hypothesis) to generate random tests in which you assert that the output matches that of the reference implementation? > would it be possible to add some reference implementation, such as [isoduration](https://github.com/bolsote/isoduration), and then to use [hypothesis](https://github.com/HypothesisWorks/hypothesis) to generate random tests in which you assert that the output matches that of the reference implementation?

@MarcoGorelli I agree that is a good idea.
I am struggling with isoduration because [it creates duration strings where the number parts can be in scientific notation ](https://github.com/bolsote/isoduration/blob/ae0bd61507aa04e032402566a2c7c01a0402929c/src/isoduration/formatter/formatting.py#L34), this does not seem to be part of ISO8601
 thanks - maybe there's something else we can use then? will take a look Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.