DEPR: Index.get_loc with method

[jbrockmendel]

• closes #xxxx
• tests added / passed
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• whatsnew entry

We only use it one place internally; this will allow us to simplify get_loc, which will marginally improve perf.

[shoyer]

We can probably work around it, but just as a heads up, this is also used externally in Xarray in a pretty extensively used code path (#5721).

[lukemanley]

Is it correct that the tolerance argument will also need to be removed as part of this? I think the tolerence is only relevant to inexact methods. The tolerance argument is not currently marked as deprecated, but maybe its implicit given the relationship?

[jbrockmendel]

Is it correct that the tolerance argument will also need to be removed as part of this? I

yes.

[jbrockmendel]

trying to enforce this I found that for DatetimeIndex cases the alternative get_indexer([key], ...) doesn't work because get_loc does string parsing that get_indexer doesn't.

I'd be OK with either a) requiring users to do the parsing themselves before calling get_indexer or b) reverting this deprecation.