[BUG] pd.api.interchange.from_dataframe fails with simple cuDF dataframe

[MarcoGorelli]

Describe the bug
pd.api.interchange.from_dataframe fails with simple cuDF dataframe

Steps/Code to reproduce bug

import cudf
import pandas as pd
df = cudf.DataFrame({'a': [1,2,3], 'b': [4,5,6]})
pd.api.interchange.from_dataframe(df)

this crashes the session https://colab.research.google.com/drive/1QXtKPcKQONi1g8WY9lI6FPZFhik_VVYg?usp=sharing

Expected behavior
it should convert to pandas dataframe with the same data

Environment overview (please complete the following information)

• Environment location: [Bare-metal, Docker, Cloud(specify cloud provider)]
• Method of cuDF install: [conda, Docker, or from source]
  • If method of install is [Docker], provide docker pull & docker run commands used

Environment details
Please run and paste the output of the cudf/print_env.sh script here, to gather any other relevant environment details

colab notebook

Additional context
Add any other context about the problem here.

[brandon-b-miller]

Thanks for reporting! I'm looking into this.

[MarcoGorelli]

Thanks!

Note that the PyCapsule Interface works perfectly well here, and outputs

pyarrow.Table
a: int64
b: int64
----
a: [[1,2,3]]
b: [[4,5,6]]

[brandon-b-miller]

The technical reason this is happening is because pandas is trying to construct dataframe columns around buffers that correspond to GPU data, triggering a segfault. That's what you get when you access the __dataframe__ property - device columns.

The real reason it's happening though is that there's not really a standard spec for whose responsibility it is to move the data to the current memory space (CPU in this case). There will need to be a little wider discussion on what to do more broadly in situations like these.

Hopefully you are able to use to_pandas() as a workaround here for now.

cc @mroeschke @vyasr @quasiben

[wence-]

I do not think this is a cudf bug. cudf delivers an object that obeys the dataframe protocol. However, as noted, the protocol is silent on whose responsibility it is to do cross-memory-region copies. Pandas should probably inspect the __dlpack_device__() enum tag for the location of the data and fail sensibly if the memory in the interchange protocol isn't on CPU.

I note that in the pandas implementation it deliberately constructs numpy arrays from the raw pointers (rather than going through dlpack, which at the time was not supported). dlpack is now supported in numpy, and if you use that instead you at least get a useful error message from numpy: RuntimeError: Unsupported device in DLTensor.

@MarcoGorelli: I don't think we can fix this here without deciding to eagerly copy everything to host (which we really don't want to do). Is this a bug report because you really want this to work, or to point out that the interchange protocol doesn't handle an obvious usecase?

[MarcoGorelli]

Thanks for your response!

I reported this because I was expecting it to work, and was surprised that it didn't when I tested out Plotly with cudF

At least in plotly/plotly.py#4244 it looks like people were expecting that pd.api.interchange.from_dataframe would enable support for cuDF