Add a summary document for the dataframe interchange protocol

Summarizes the various discussions about and goals/non-goals
and requirements for the `__dataframe__` data interchange
protocol.

The intended audience for this document is Consortium members
and dataframe library maintainers who may want to support this
protocol. @datapythonista will add a companion document that's
a more gentle introduction/tutorial in a "from zero to a protocol"
style.

The aim is to keep updating this till we have captured all
the requirements and answered all the FAQs, so we can actually
design the protocol after and verify it meets all our requirements.

Closes gh-29

Author: rgommers

protocol/dataframe_protocol_summary.md

@@ -0,0 +1,193 @@
+# `__dataframe__` protocol - summary
+
+_We've had a lot of discussion in a couple of GitHub issues and in meetings.
+This description attempts to summarize that, and extract the essential design
+requirements/principles and functionality it needs to support._
+
+## Purpose of `__dataframe__`
+
+The purpose of `__dataframe__` is to be a _data interchange_ protocol. I.e., a way to convert one type of dataframe into another type (for example, convert a Koalas dataframe into a Pandas dataframe, or a cuDF dataframe into a Vaex dataframe). 
+
+Currently (Sep'20) there is no way to do this in an implementation-independent way.
+
+The main use case this protocol intends to enable is to make it possible to
+write code that can accept any type of dataframe instead of being tied to a
+single type of dataframe. To illustrate that:
+
+```python
+def somefunc(df, ...):
+    """`df` can be any dataframe supporting the protocol, rather than (say)
+    only a pandas.DataFrame"""
+    # could also be `cudf.from_dataframe(df)`, or `vaex.from_dataframe(df)`
+    df = pd.from_dataframe(df)
+    # From now on, use Pandas dataframe internally
+```
+
+### Non-goals
+
+Providing a _complete standardized dataframe API_ is not a goal of the
+`__dataframe__` protocol. Instead, this is a goal of the full dataframe API
+standard, which the Consortium for Python Data API Standards aims to provide
+in the future. When that full API standard is implemented by dataframe
+libraries, the example above can change to:
+
+```python
+def get_df_module(df):
+    """Utility function to support programming against a dataframe API"""
+    if hasattr(df, '__dataframe_namespace__'):
+       # Retrieve the namespace 
+       pdx = df.__dataframe_namespace__()  
+    else:
+        # Here we can raise an exception if we only want to support compliant dataframes,
+        # or convert to our default choice of dataframe if we want to accept (e.g.) dicts
+        pdx = pd
+        df = pd.DataFrame(df)
+
+    return pdx, df
+
+
+def somefunc(df, ...):
+    """`df` can be any dataframe conforming to the dataframe API standard"""
+    pdx, df = get_df_module(df)
+    # From now on, use `df` methods and `pdx` functions/objects
+```
+
+### Constraints
+
+An important constraint on the `__dataframe__` protocol is that it should not
+make achieving the goal of the complete standardized dataframe API more
+difficult to achieve.
+
+There is a small concern here. Say that a library adopts `__dataframe__` first,
+and it goes from supporting only Pandas to officially supporting other
+dataframes like `modin.pandas.DataFrame`. At that point, changing to
+supporting the full dataframe API standard as a next step _implies a
+backwards compatibility break_ for users that now start relying on Modin
+dataframe support. E.g., the second transition will change from returning a
+Pandas dataframe from `somefunc(df_modin)` to returning a Modin dataframe
+later. It must be made very clear to libraries accepting `__dataframe__` that
+this is a consequence, and that that should be acceptable to them.
+
+
+### Progression / timeline
+
+- **Current status**: most dataframe-consuming libraries work _only_ with
+  Pandas, and rely on many Pandas-specific functions, methods and behavior.
+- **Status after `__dataframe__`**: with minor code changes (as in first
+  example above), libraries can start supporting all conforming dataframes,
+  convert them to Pandas dataframes, and still rely on the same
+  Pandas-specific functions, methods and behavior.
+- **Status after standard dataframe API adoption**: libraries can start
+  supporting all conforming dataframes _without converting to Pandas or
+  relying on its implementation details_. At this point, it's possible to
+  "program to an interface" rather than to a specific library like Pandas.
+
+
+## Protocol design requirements
+
+1. Must be a standard API that is unambiguously specified, and not rely on
+   implementation details of any particular dataframe library.
+2. Must treat dataframes as a collection of columns (which are 1-D arrays
+   with a dtype and missing data support).
+3. Must include device support
+4. Must avoid device transfers by default (e.g. copy data from GPU to CPU),
+   and provide an explicit way to force such transfers (e.g. a `force=` or
+   `copy=` keyword that the caller can set to `True`).
+5. Must be zero-copy if possible.
+6. Must be able to support "virtual columns" (e.g., a library like Vaex which
+   may not have data in memory because it uses lazy evaluation).
+7. Must support missing values (`NA`) for all supported dtypes.
+8. Must supports string and categorical dtypes
+   (_TBD: not discussed a lot, is this a hard requirement?_)
+
+We'll also list some things that were discussed but are not requirements:
+
+1. Object dtype does not need to be supported (_TBD: this is what Joris said,
+   but doesn't Pandas use object dtype to represent strings?_).
+2. Heterogeneous/structured dtypes within a single column does not need to be
+   supported.
+   _Rationale: not used a lot, additional design complexity not justified._
+
+
+## Frequently asked questions
+
+### Can the Arrow C Data Interface be used for this?
+
+What we are aiming for is quite similar to the Arrow C Data Interface (see
+the [rationale for the Arrow C Data Interface](https://arrow.apache.org/docs/format/CDataInterface.html#rationale)),
+except `__dataframe__` is a Python-level rather than C-level interface.
+
+The limitations seem to be:
+- No device support (@kkraus14 will bring this up on the Arrow dev mailing list)
+- Specific to columnar data (_at least, this is what its docs say_).
+  TODO: are there any concerns for, e.g., Koalas or Ibis.
+
+Note that categoricals are supported, Arrow uses the phrasing
+"dictionary-encoded types" for categorical.
+
+The Arrow C Data Interface says specifically it was inspired by [Python's
+buffer protocol](https://docs.python.org/3/c-api/buffer.html), which is also
+a C-only and CPU-only interface. See `__array_interface__` below for a
+Python-level equivalent of the buffer protocol.
+
+
+### Is `__dataframe__` analogous to `__array__` or `__array_interface__`?
+
+Yes, it is fairly analogous to `__array_interface__`. There will be some
+differences though, for example `__array_interface__` doesn't know about
+devices, and it's a `dict` with a pointer to memory so there's an assumption
+that the data lives in CPU memory (which may not be true, e.g. in the case of
+cuDF or Vaex).
+
+It is _not_ analogous to `__array__`, which is NumPy-specific. `__array__` is a
+method attached to array/tensor-like objects, and calling it is requesting
+the object it's attached to to turn itself into a NumPy array. Hence, the
+library that implements `__array__` must depend on NumPy, and call a NumPy
+`ndarray` constructor itself from within `__array__`.
+
+
+### What is wrong with `.to_numpy?` and `.to_arrow()`? 
+
+Such methods ask the object it is attached to to turn itself into a NumPy or
+Arrow array. Which means each library must have at least an optional
+dependency on NumPy and on Arrow if it implements those methods. This leads
+to unnecessary coupling between libraries, and hence is a suboptimal choice -
+we'd like to avoid this if we can.
+
+Instead, it should be dataframe consumers that rely on NumPy or Arrow, since
+they are the ones that need such a particular format. So, it can call the
+constructor it needs. For example, `x = np.asarray(df['colname'])` (where
+`df` supports `__dataframe__`).
+
+
+### Does an interface describing memory work for virtual columns?
+
+Vaex is an example of a library that can have "virtual columns" (see @maartenbreddels
+[comment here](https://github.com/data-apis/dataframe-api/issues/29#issuecomment-686373569)).
+If the protocol includes a description of data layout in memory, does that
+work for such a virtual column?
+
+Yes. Virtual columns need to be materialized in memory before they can be
+turned into a column for a different type of dataframe - that will be true
+for every discussed form of the protocol; whether there's a `to_arrow()` or
+something else does not matter. Vaex can choose _how_ to materialize (e.g.,
+to an Arrow array, a NumPy array, or a raw memory buffer) - as long as the
+returned description of memory layout is valid, all those options can later
+be turned into the desired column format without a data copy, so the
+implementation choice here really doesn't matter much.
+
+_Note: the above statement on materialization assumes that there are many
+forms a virtual column can be implemented, and that those are all
+custom/different and that at this point it makes little sense to standardize
+that. For example, one could do this with a simple string DSL (`'col_C =
+col_A + col_B'`, with a fancier C++-style lazy evaluation, with a
+computational graph approach like Dask uses, etc.)._
+
+
+## Possible direction for implementation
+
+The `cuDFDataFrame`, `cuDFColumn` and `cuDFBuffer` sketched out by @kkraus14
+[here](https://github.com/data-apis/dataframe-api/issues/29#issuecomment-685123386)
+seems to be in the right direction.
+
+TODO: work this out after making sure we're all on the same page regarding requirements.