Interchange Protocol: unused nan_as_null keyword?

[jorisvandenbossche]

The DataFrame Interchange Protocol has a nan_as_null keyword in __dataframe__ that can be specified by the consumer, i.e. the person/library calling this method. The docstring explains its goal:

dataframe-api/protocol/dataframe_protocol.py

Lines 400 to 403 in d10a096

	``nan_as_null`` is a keyword intended for the consumer to tell the
	producer to overwrite null values in the data with ``NaN``.
	It is intended for cases where the consumer does not support the bit
	mask or byte mask that is the producer's native representation.

However, at the moment I think none of the existing implementation we are aware of actually supports this keyword: pandas, vaex, modin, cudf all simply ignore it (and cudf actually uses a wrong default), and pyarrow as well but will raise an error if you pass a non-default value (i.e. True).
The following disclaimer in the docstring seems to have been copied around as some version of this appears in all the listed projects:

        `nan_as_null` currently has no effect; once support for nullable extension
        dtypes is added, this value should be propagated to columns.

I am not fully sure what this note is meant for (I suppose it originates from the pandas implementation where for nullable extension types this keyword could be implemented? Since the default numpy dtypes already use NaN as null, the keyword wouldn't have any effect for that. But for other libraries that explanation doesn't necessarily hold).
And also, if nobody implemented it, do we actually need it?

[kkraus14]

I would vote to remove this. This is redundant to describe_null on the column level with much more rigid capabilities.

[rgommers]

Seems okay to remove indeed. The original motivation was that Arrow uses bit rather than byte masks, and those aren't supported by NumPy. So the nan-to-null mapping has to be done somewhere, and it's easier to do on the producer than the consumer side. However, Joris says Pandas already has the code to handle this on the consumer side. So, since no one currently uses nan_as_null, it seems safe to remove it.

[rgommers]

gh-228, which deprecates the nan_as_null keyword, is now merged. As discussed in gh-226, we should ensure it's removed from all the consumers before removing it from the signature of __dataframe__. So let's leave this issue open, and use it as the reference to point to for PRs where the keyword is removed from consumers which call the __dataframe__ method.