DEPR: Deprecate non-ISO date string formats in DatetimeIndex.loc

[raghavendranath]

• closes BUG: .loc with DateTimeIndex allows day first string for stop in slice, but not start #58302
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Before:

ser.loc["1/10/2024":"1/14/2024"]      # Returns Jan 10-14
ser.loc["1/10/2024":"14/1/2024"]      # Also returns Jan 10-14
ser.loc["10/1/2024":"14/1/2024"]      # Returns empty

After:

ser.loc["1/10/2024":"1/14/2024"]      # Still returns Jan 10-14 + FutureWarning
ser.loc["2024-01-10":"2024-01-14"]    # Use ISO format instead

There's no way to know if "1/10/2024" is being parsed as MM/DD or DD/MM until you run it. This deprecation pushes users toward ISO format to avoid the confusion.

[raghavendranath]

Hi @WillAyd @MarcoGorelli @mroeschke The PR is ready for review whenever you have time. Thanks

[WillAyd]

Credit to @stefmolin for the original idea, but I don't think we should try and roll our own regex here if we can avoid it.

I see the standard library provides date.fromisostring, although that is documented to not work with "Reduced Precision" dates:

https://docs.python.org/3/library/datetime.html

Even still I wonder if we can't use that first and only fallback when it fails

[raghavendranath]

Thanks @stefmolin @WillAyd!. Switched to using date.fromisoformat() like you suggested. Added a regex fallback to handle the reduced precision dates (YYYY and YYYY-MM) that fromisoformat doesn't support .

[WillAyd]

Why does this test need to filter warnings? It seems unrelated to the change?

[raghavendranath]

When we group by keys='2nd' (the date column - from parameterized tests), it triggers the deprecation warning internally during the groupby operation. Without it, those test cases fail in CI. So, added the filterwarnings.

[WillAyd]

Does this test still exhibit the expected behavior if you change the test to use ISO strings?

[raghavendranath]

It works as expected. Updated the test to use ISO format strings instead and removed the filterwarnings.

[WillAyd]

You can change the test data here to use ISO strings, unless it explicitly tests non-ISO for a reason

[raghavendranath]

Converting all the dates to ISO format in this test.

[raghavendranath]

@WillAyd Fixed all your comments. Ready for review whenever you get a chance.