Object data types are no longer inferred from data frames

[joelostblom]

Support for inferring the data type from object columns in data frames used to work, but is not working anymore:

import altair as alt
import pandas as pd

alt.Chart(
    pd.DataFrame({'not_ints': pd.Series([1, 2, 3], dtype='object')})
).mark_circle().encode(x='not_ints')

NotImplementedError: Non-string object dtypes are not supported yet

@jonmmease I think this is a regression from #3114 that added support for the dataframe interchange as it used to work fine in Altair 5 (see screenshot below). Do you think it is possible to add a workaround similar to what you have done for datetime data until there is official pyarrow support?

Altair 5.0:

[jonmmease]

Yeah, this is from #3114. I don't think the workaround datetime workaround is going to work here because the error message doesn't include enough information to determine the correct type to infer.

I think we could catch NotImplementedErrors exceptions raised from this line:

https://github.com/altair-viz/altair/blob/17e8d07fee3c3d891c9667a14a2a6b3df3d3b581/altair/utils/core.py#L589

And then try again with the pandas implementation (if the datarame is a pandas DataFrame).

I'll take this. Do you think warrants an immediate 5.1.1?

[mattijn]

Or maybe using the infer_objects() function?
I can imagine doing a 5.1.1 for this

[joelostblom]

I'll take this. Do you think warrants an immediate 5.1.1?

Great, thank you! I think I quick 5.1.1 would be helpful if you both have time for that, but not critical if time is tight. I agree with @mattijn that infer_objects could be a good option for just restoring the regressed behavior (pandas df support). In terms of also supporting inference in other dataframes, I guess we can either wait for it to be added in pyarrow and be content with raising an error for now (lowest effort and totally ok with me) or try to infer ourselves by inspecting the objects (maybe with a logic inspired from pd.inferobject? Could be brittle and higher maintenance though). What do you think?

[jonmmease]

I'll get this fixed shortly. I'll tag you for review @mattijn, and then you can make the 5.1.1 release at your convenience.