model comparison fails on latest versions due to pandas indexing (at -> loc)

[falkmielke]

model comparison fails on my system when running the standard comparison example provided here:
https://arviz-devs.github.io/arviz/api/generated/arviz.compare.html

import arviz as az
data1 = az.load_arviz_data("non_centered_eight")
data2 = az.load_arviz_data("centered_eight")
compare_dict = {"non centered": data1, "centered": data2}
comparison = az.compare(compare_dict)

print (comparison)

(trace below)

Python 3.10.7
arviz.__version__ = 0.12.1
pandas.__version__ = 1.5.0

This can be mended by replacing
pandas.DataFrame.at[val] = (...) call in
stats.stats.py::compare(...) L306
with the more appropriate pandas.DataFrame.loc[val, :] = (...).

more appropriate because according to the pandas documentation, at is used to "Access a single value for a row/column label pair.". The code in compare() obviously accesses multiple values.

Full error trace:

Traceback (most recent call last):
  File "/usr/lib/python3.10/site-packages/pandas/core/indexes/base.py", line 3800, in get_loc
    return self._engine.get_loc(casted_key)
  File "pandas/_libs/index.pyx", line 138, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/index.pyx", line 144, in pandas._libs.index.IndexEngine.get_loc
TypeError: 'slice(None, None, None)' is an invalid key

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "test.py", line 6, in <module>
    comparison = az.compare(compare_dict)
  File "/usr/lib/python3.10/site-packages/arviz/stats/stats.py", line 306, in compare
    df_comp.at[val] = (
  File "/usr/lib/python3.10/site-packages/pandas/core/indexing.py", line 2438, in __setitem__
    return super().__setitem__(key, value)
  File "/usr/lib/python3.10/site-packages/pandas/core/indexing.py", line 2393, in __setitem__
    self.obj._set_value(*key, value=value, takeable=self._takeable)
  File "/usr/lib/python3.10/site-packages/pandas/core/frame.py", line 4208, in _set_value
    icol = self.columns.get_loc(col)
  File "/usr/lib/python3.10/site-packages/pandas/core/indexes/base.py", line 3807, in get_loc
    self._check_indexing_error(key)
  File "/usr/lib/python3.10/site-packages/pandas/core/indexes/base.py", line 5963, in _check_indexing_error
    raise InvalidIndexError(key)
pandas.errors.InvalidIndexError: slice(None, None, None)

Hope this helps! Thanks,
Falk

[ahartikainen]

Thanks for the report.

Issue was fixed in #2104