fix freq/offset/indexer attrs in SPI (#1538)

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### Pull Request Checklist:
- [x] This PR addresses an already opened issue (for bug fixes /
features)
    - This PR fixes #xyz
- [x] Tests for the changes have been added (for bug fixes / features)
- [x] (If applicable) Documentation has been added / updated (for bug
fixes / features)
- [x] CHANGES.rst has been updated (with summary of main changes)
- [x] Link to issue (:issue:`number`) and pull request (:pull:`number`)
has been added

### What kind of change does this PR introduce?

* Fixes wrong attributes in SPI/SPEI functions, allow to save files in
netcdf files.

### Does this PR introduce a breaking change?
No

### Other information:

CHANGES.rst

@@ -8,7 +8,7 @@ Contributors to this version: Juliette Lavoie (:user:`juliettelavoie`), Pascal B
 
 Announcements
 ^^^^^^^^^^^^^
-* To circumvent issues stemming from changes to the frequency code convention in `pandas` v2.2, we have pinned `xarray` (<2023.11.0) and `pandas` (< 2.2) for this release. This change will be reverted in `xclim` v0.48.0 to support the newer versions (`xarray >= 2023.11.0` and `pandas >= 2.2`).
+* To circumvent issues stemming from changes to the frequency code convention in `pandas` v2.2, we have pinned `xarray` (< 2023.11.0) and `pandas` (< 2.2) for this release. This change will be reverted in `xclim` v0.48.0 to support the newer versions (`xarray>= 2023.11.0` and `pandas>= 2.2`).
 * `xclim` v0.47.0 will be the last release supporting Python3.8.
 
 New features and enhancements
@@ -18,9 +18,10 @@ New features and enhancements
 
 Bug fixes
 ^^^^^^^^^
-* Fixed a bug with ``n_escore=-1`` in ``xclim.sdba.adjustment.NpdfTransform``. (:issue:`1515`, :pull:`1515`).
-* In the documentation, fix the tooltips in the indicator search results. (:issue:`1524`, :pull:`1527`).
+* Fixed a bug with ``n_escore=-1`` in ``xclim.sdba.adjustment.NpdfTransform``. (:issue:`1515`, :pull:`1516`).
+* In the documentation, fixed the tooltips in the indicator search results. (:issue:`1524`, :pull:`1527`).
 * If chunked inputs are passed to indicators ``mean_radiant_temperature`` and ``potential_evapotranspiration``, sub-calculations of the solar angle will also use the same chunks, instead of a single one of the same size as the data. (:issue:`1536`, :pull:`1542`).
+* Fix wrong attributes in ``xclim.indices.standardized_precipitation_index``, ``xclim.indices.standardized_precipitation_evapotranspiration_index``. (:issue:`1537`, :pull:`1538`).
 
 Internal changes
 ^^^^^^^^^^^^^^^^

tests/test_indices.py

@@ -653,6 +653,7 @@ def test_standardized_index_modularity(self, open_dataset):
             dist=dist,
             method=method,
             offset="1 mm/d",
+            month=[2, 3],
         )
         spei1 = xci.standardized_precipitation_evapotranspiration_index(
             wb.sel(time=slice("1998", "2000")), params=params
@@ -667,6 +668,7 @@ def test_standardized_index_modularity(self, open_dataset):
             offset="1 mm/d",
             cal_start="1950",
             cal_end="1980",
+            month=[2, 3],
         ).sel(time=slice("1998", "2000"))
 
         # In the previous computation, the first {window-1} values are NaN because the rolling is performed on the period [1998,2000].

xclim/indices/_agro.py

@@ -1174,6 +1174,7 @@ def standardized_precipitation_index(
     ...     cal_end=cal_end,
     ... )  # Computing SPI-3 months using a gamma distribution for the fit
     >>> # Fitting parameters can also be obtained ...
+    >>> from xclim.indices.stats import standardized_index_fit_params
     >>> params = standardized_index_fit_params(
     ...     pr.sel(time=slice(cal_start, cal_end)),
     ...     freq="MS",
@@ -1189,7 +1190,12 @@ def standardized_precipitation_index(
     :cite:cts:`mckee_relationship_1993`
     """
     if params is not None and pr_cal is None:
-        freq, window = (params.attrs[s] for s in ["freq", "window"])
+        freq, window, indexer = (
+            params.attrs[s] for s in ["freq", "window", "time_indexer"]
+        )
+        # Unpack attrs to None and {} if needed
+        freq = None if freq == "" else freq
+        indexer = {} if indexer[0] == "" else {indexer[0]: indexer[1:]}
         if cal_start or cal_end:
             warnings.warn(
                 "Expected either `cal_{start|end}` or `params`, got both. The `params` input overrides other inputs."
@@ -1230,8 +1236,9 @@ def standardized_precipitation_index(
     if paramsd != template.sizes:
         params = params.broadcast_like(template)
 
-    spi = standardized_index(pr, params, **indexer)
+    spi = standardized_index(pr, params)
     spi.attrs = params.attrs
+    spi.attrs["freq"] = freq or xarray.infer_freq(spi.time)
     spi.attrs["units"] = ""
     return spi
 
@@ -1324,7 +1331,7 @@ def standardized_precipitation_evapotranspiration_index(
                 "Proceeding with the value given in `params`."
             )
         offset = params_offset
-    offset = 0 if offset is None else convert_units_to(offset, wb, context="hydro")
+    offset = 0 if offset == "" else convert_units_to(offset, wb, context="hydro")
     # Allowed distributions are constrained by the SPI function
     if dist in ["gamma", "fisk"] and offset <= 0:
         raise ValueError(

xclim/indices/stats.py

@@ -724,15 +724,20 @@ def standardized_index_fit_params(
     )
     params.attrs = {
         "calibration_period": cal_range,
-        "freq": freq,
+        "freq": freq or "",
         "window": window,
         "scipy_dist": dist,
         "method": method,
         "group": group,
-        "time_indexer": indexer,
         "units": "",
-        "offset": offset,
+        "offset": offset or "",
     }
+    if indexer != {}:
+        method, args = indexer.popitem()
+    else:
+        method, args = "", []
+    params.attrs["time_indexer"] = (method, *args)
+
     return params