ENH: assert_* has very superficial description if CategorialIndex are different

[topper-123]

Code Sample, a copy-pastable example if possible

>>> c1 = pd.CategoricalIndex(['a', 'b'])
>>> c2 = pd.CategoricalIndex(['c', 'd'])
>>> s1 = pd.Series([1,2], index=c1)
>>> s2 = pd.Series([1,2], index=c2)
>>> pd.testing.assert_series_equal(s1, s2)
AssertionError: Series.index are different

Attribute "dtype" are different
[left]:  category
[right]: category

Note [left] and [right] output the same, so this help message isn't helpful.

Problem description

You can't see from the description where the differences in the CategoricalIndexes are. The reason is that Categorical return "category" for str(c1) and to see details, you need repr(c1).

Expected Output

The solution could be in the function util.testing.py::raise_assert_detail to replace

msg = """{obj} are different

{message}
[left]:  {left}
[right]: {right}""".format(obj=obj, message=message, left=left, right=right)

with the repr-formattes strings:

msg = """{obj} are different

{message}
[left]:  {left!r}
[right]: {right!r}""".format(obj=obj, message=message, left=left, right=right)

Note the (!r). This will give the full repr output which you'll almost always want anyway.

Alternatively, the decision to let str(c1) output "category" could be changed. That would be a breaking change, though, and would require proper deprecation warning.

I could submit a PR if the solution with !r format option is acceptable.

[TomAugspurger]

Alternatively, the decision to let str(c1) output "category" could be changed. That would be a breaking change, though, and would require proper deprecation warning.

In the CategoricalDtype refactor, we decided to have str(categoricaldtype) be 'category', and reprbe the full one.

A PR changing to use !r would be good I think.

[topper-123]

Ok, I'll look into that.

Is there any chance this gets in v0.21.1? This output string comes up in quite a lot of places in my code.

[jreback]

@topper-123 sure this could go in 0.21.1. The criteria is bug fixes, or small not-too-invasive enhancements.