Preserve dtypes when get() is called with an index

[frankenjoe]

Closes #324

Solves issue described in #324 and adds a test for it.

[codecov]

Codecov Report

Merging #346 (4459232) into main (3901a51) will not change coverage.
The diff coverage is 100.0%.

Impacted Files	Coverage Δ
audformat/core/table.py	100.0% <100.0%> (ø)

[hagenw]

Interestingly, this does not solve #324 completely:

import audformat


db = audformat.Database('db')

db.schemes['spk'] = audformat.Scheme('str', labels=['a', 'b'])
index = audformat.filewise_index(['f1'])
db['files'] = audformat.Table(index)
db['files']['spk'] = audformat.Column(scheme_id='spk')
db['files']['spk'].set(['a'])

db.schemes['label'] = audformat.Scheme('int')
index = audformat.segmented_index(['f1', 'f1'], [0, 1], [1, 2])
db['segments'] = audformat.Table(index)
db['segments']['label'] = audformat.Column(scheme_id='label')
db['segments']['label'].set([0, 1])

and then:

>>> df = db['files'].get()
>>> df.spk.cat.categories
Index(['a', 'b'], dtype='object')
>>> df.loc['f1', 'spk'] = 'c'
...
TypeError: Cannot setitem on a Categorical with a new category (c), set the categories first
>>> df
     spk
file    
f1     a

but

>>> df = db['files'].get(db['segments'].index)
>>> df.spk.cat.categories
Index(['a', 'b'], dtype='object')
>>> df.loc['f1', 'spk'] = 'c'
>>> df
                                     spk
file start           end                
f1   0 days 00:00:00 0 days 00:00:01   c
     0 days 00:00:01 0 days 00:00:02   c
>>> df.spk.cat.categories
...
AttributeError: Can only use .cat accessor with a 'category' dtype

But maybe the difference in behavior is now attributed to the fact that the first is a filewise index whereas the second a segmented index?

[frankenjoe]

But maybe the difference in behavior is now attributed to the fact that the first is a filewise index whereas the second a segmented index?

Maybe a bug in pandas as in both cases the dtypes seem to be right:

df = db['files'].get()
df.spk.cat.categories

Index(['a', 'b'], dtype='object')

df = db['files'].get(db['segments'].index)
df.spk.cat.categories

Index(['a', 'b'], dtype='object')

So I guess there is not much we can do about it.

[hagenw]

I would then propose that we remove "Closes #324" here and update the issue after this is merged.
Afterwards we can then have a look if this is reported somewhere in pandas already.

[frankenjoe]

Ok, but then we should at least remove the 'bug' label from #324 as I don't see we can do anything else but make sure the categorical dtype is kept.

[hagenw]

OK, let's leave the "Close #324" here as CategoricalDtype is now indeed preserved. I will open a new issue after this here is merged.