implement the PintIndex (#163)

* add a `PintMetaIndex` that for now can only `sel`

* add a function to compare indexers

* expect indexer dicts for strip_indexer_units

* move the indexer comparison function to the utils

* change extract_indexer_units to expect a dict

* fix a few calls to extract_indexer_units

* one more call

* implement `create_variables` and `from_variables`

Co-authored-by: Benoit Bovy <[email protected]>

* use the new index to attach units to dimension coordinates

* pass the dictionary of indexers instead iterating manually

* use `Coordinates._construct_direct`

* delegate `isel` to the wrapped index and wrap the result

* add a inline `repr` for the index

* stubs for the remaining methods

* rename the index class to `PintIndex`

* add a utility method to wrap the output of the wrapped index's methods

* implement `equals`

* implement `roll`, `rename`, and `__getitem__` by forwarding

* start adding tests

* add tests for `create_variables`

* add tests for `sel`

* add tests for `isel`

* improve the tests for `sel`

* add tests for `equals`

* add tests for `roll`

* add tests for `rename`

* add tests for `__getitem__`

* add tests for `_repr_inline_`

* configure coverage, just in case

* use `_replace` instead of manually constructing the new index

* explicitly check that the pint index gets created

* also verify that non-quantity variables don't become `PintIndex`ed

* don't use `.pint.sel`

* fix `PintIndex.from_variables` and properly test it

* quantify the test data

This allows us to test with quantified indexes.

* explicity quantify the input of the `interp_like` tests

* also strip the units of `other`

* change expectations in the conversion tests

* refactor `attach_units_dataset`

* get `convert_units` to accept indexes

* strip indexes as well

* change the `.pint.to` tests to not include indexes

These are already covered by the conversion tests, so no need to repeat.

* extract the units of `other` in `.pint.interp_like`

* quantify the input and expected data in the `reindex` tests

* remove the left-over explicit quantification in the `interp` tests

* get `.pint.reindex` to work by explicitly converting, stripping, and then reattaching

* quantify the input and expected objects in the `reindex_like` tests

* get `reindex_like` to work with indexes

* quantify expected only if we expect to make use of it

* quantify input and expected objects in the `sel` and `loc` tests

* get `.pint.sel` and `.pint.loc` to work with the indexes

In theory, we could also use `sel` and `loc` directly, but that would
not allow us to change the units of the result (or at least, as far as
I can tell).

* remove the warning about indexed coordinates

* preserve the order of the variables

* remove the remaining uses of `Coordinates._construct_direct`

* whats-new entry

* expose the index

---------

Co-authored-by: Benoit Bovy <[email protected]>

docs/examples/plotting.ipynb

@@ -108,7 +108,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "monthly_means.pint.sel(\n",
+    "monthly_means.sel(\n",
     "    lat=ureg.Quantity(4350, \"angular_minute\"),\n",
     "    lon=ureg.Quantity(12000, \"angular_minute\"),\n",
     ")"

docs/whats-new.rst

@@ -6,6 +6,8 @@ What's new
 ------------------
 - drop support for python 3.9 (:pull:`266`)
   By `Justus Magin <https://github.com/keewis>`_.
+- create a `PintIndex` to allow units on indexed coordinates (:pull:`163`, :issue:`162`)
+  By `Justus Magin <https://github.com/keewis>`_ and `Benoit Bovy <https://github.com/benbovy>`_.
 
 0.4 (23 Jun 2024)
 -----------------

pint_xarray/__init__.py

@@ -5,6 +5,7 @@
 from . import accessors, formatting, testing  # noqa: F401
 from .accessors import default_registry as unit_registry
 from .accessors import setup_registry
+from .index import PintIndex
 
 try:
     __version__ = version("pint-xarray")
@@ -21,4 +22,5 @@
     "testing",
     "unit_registry",
     "setup_registry",
+    "PintIndex",
 ]