Empty misc table can lead to errors

[hagenw]

One can initialize a misc table with None as index:

>>> import audformat
>>> table = audformat.MiscTable(None)

But this let's some of its method fail:

>>> table.copy()
...
ValueError: Got index with levels [None], but names must be non-empty and unique.

>>> table.extend_index(pd.Index([0, 1, 2], name="idx"))
...
ValueError: Got index with levels [None], but names must be non-empty and unique.

>>> table.extend_index(pd.Index([0, 1, 2], name="idx"), inplace=True)
ValueError: Cannot extend table if input index and table index are not alike.
Expected index:
        idx    int64
but yours is:
        None    int64

You can extend the table with:

>>> table.extend_index(pd.Index([0, 1, 2]), inplace=True)
>>> table.index
Index([0, 1, 2], dtype='Int64')

But now you have created a table without a name for the index and empty levels:

>>> table.levels

If you would have tried this when creating the table, it would have failed with

>>> table = audformat.MiscTable(pd.Index([0, 1, 2]))
...
ValueError: Got index with levels [None], but names must be non-empty and unique.

[hagenw]

I think I raised this error, as the above code works with a normal table:

>>> import audformat
>>> table = audformat.Table(None)
>>> table.copy()
{type: filewise}
>>> table.extend_index(audformat.filewise_index(["f1", "f2", "f3"]))
{type: filewise}
>>> table.index
Index([], dtype='string', name='file')

But we do not mention in the specification explicitly that we allow for an empty index. In the docstring we do also not mention None as possible value for index. So I will change the label here from bug to question.