Fitted paramers reloaded from NC lead to errors in standardized index, and setting multiple indexers leads to inconsistent behaviour

[Hem-W]

Setup Information

• Xclim version: 0.51.0
• Python version: 3.11.8
• Operating System: Ubuntu 20.04

Description

The time indexer in the xc.indices.stats.standardized_index_fit_params was not correctly stored in the attributes, leading to errors in different cases:

1. When there is no indexer, the indexer in the xc.indices.stats.standardized_index_fit_params was changed after to_netcdf and reloaded from xr.open_dataset(), leading to an error when running standardized_precipitation_index(pr, params=params);
2. When the value of indexer is a non-array single value, there will be an error when encoding the attribute params.attrs["time_indexer"] in stats.standardized_index_fit_params;
3. When there are multiple indexers, only one indexer is saved, leading to inconsistent behaviour when running standardized_precipitation_index(pr, params=params).

Steps To Reproduce

Load dataset:

import xarray as xr
import xclim.indices as xci
from xclim.testing import open_dataset

ds = open_dataset("sdba/CanESM2_1950-2100.nc").isel(location=1)
pr = ds.pr.sel(time=slice("1998", "2000"))
pr_cal = ds.pr.sel(time=slice("1950", "1980"))
params = xci.stats.standardized_index_fit_params(
    pr_cal, freq="MS", window=1, dist="gamma", method="APP", fitkwargs={"floc": 0}, zero_inflated=True)

For the first issue,

params.to_dataset(name="params").to_netcdf("params.nc")
params = xr.open_dataset("params.nc").params

spi = xci.standardized_precipitation_index(pr, params=params)  # this line leads to the error

For the second issue,

params = xci.stats.standardized_index_fit_params(
    pr_cal, freq="MS", window=1, dist="gamma", method="APP", fitkwargs={"floc": 0}, zero_inflated=True, month=2)
# executable only when an array is assigned to `month`

For the third issue,

params = xci.stats.standardized_index_fit_params(
    pr_cal, freq="MS", window=1, dist="gamma", method="APP", fitkwargs={"floc": 0}, zero_inflated=True, drop=True, month=[2, 3])  # `drop=True` is not saved in params.attrs["time_indexer"]`
xci.standardized_precipitation_index(pr, params=params)  # the months other than 2 & 3 were not dropped

Additional context

The error logs are attached here.
For the first issue,

File ~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:882, in standardized_index(da, freq, window, dist, method, zero_inflated, fitkwargs, cal_start, cal_end, params, **indexer)
    [880](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:880) # Unpack attrs to None and {} if needed
    [881](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:881) freq = None if freq == "" else freq
--> [882](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:882) indexer = {} if indexer[0] == "" else {indexer[0]: indexer[1:]}
    [883](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:883) if cal_start or cal_end:
    [884](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:884)     warnings.warn(
    [885](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:885)         "Expected either `cal_{start|end}` or `params`, got both. The `params` input overrides other inputs."
    [886](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:886)         "If `cal_start`, `cal_end`, `freq`, `window`, and/or `dist` were given as input, they will be ignored."
    [887](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:887)     )

IndexError: string index out of range

For the second issue,

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[3], [line 1](vscode-notebook-cell:?execution_count=3&line=1)
----> [1](vscode-notebook-cell:?execution_count=3&line=1) params = xci.stats.standardized_index_fit_params(
      [2](vscode-notebook-cell:?execution_count=3&line=2)     pr_cal, freq="MS", window=1, dist="gamma", method="APP", fitkwargs={"floc": 0}, zero_inflated=True, month=2)

File ~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:801, in standardized_index_fit_params(da, freq, window, dist, method, zero_inflated, fitkwargs, offset, **indexer)
    [791](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:791) params.attrs = {
    [792](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:792)     "calibration_period": cal_range,
    [793](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:793)     "freq": freq or "",
   (...)
    [798](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:798)     "units": "",
    [799](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:799) }
    [800](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:800) method, args = ("", []) if indexer == {} else indexer.popitem()
--> [801](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:801) params.attrs["time_indexer"] = (method, *args)
    [802](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:802) if offset:
    [803](~/.conda/envs/syncfuture/lib/python3.11/site-packages/xclim/indices/stats.py:803)     params.attrs["offset"] = offset

For the third issue, the attributes in the params are like

and the obtained SPI would be like
.

Contribution

• I would be willing/able to open a Pull Request to address this bug.

Code of Conduct

• I agree to follow this project's Code of Conduct

[aulemahal]

Thanks for the issue @Hem-W ! Indeed, we only tested this mechanic by loading the params in-memory, and forgot about all these issues when going through netCDF!