Repository: pandas-dev/pandas (python)

Issue:
BUG: interchange protocol with nullable datatypes a non-null validity provides nonsense results
### Pandas version checks

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

- [X] I have confirmed this bug exists on the [main branch](https://pandas.pydata.org/docs/dev/getting_started/install.html#installing-the-development-version-of-pandas) of pandas.


### Reproducible Example

```python
import pyarrow.interchange as pai
import pandas as pd
import polars as pl

data = pd.DataFrame(
    {
        "b1": pd.Series([1, 2, None], dtype='Int64'),
    }
)
print(pai.from_dataframe(data))
print(pl.from_dataframe(data))
```


### Issue Description

Result:
```
pyarrow.Table
b1: int64
----
b1: [[4607182418800017408,4611686018427387904,9221120237041090560]]
shape: (3, 1)
┌─────────────────────┐
│ b1                  │
│ ---                 │
│ i64                 │
╞═════════════════════╡
│ 4607182418800017408 │
│ 4611686018427387904 │
│ 9221120237041090560 │
└─────────────────────┘
```

### Expected Behavior

`[1, 2, None]`

### Installed Versions

<details>

INSTALLED VERSIONS
------------------
commit                : 92a52e231534de236c4e878008a4365b4b1da291
python                : 3.10.12.final.0
python-bits           : 64
OS                    : Linux
OS-release            : 5.15.133.1-microsoft-standard-WSL2
Version               : #1 SMP Thu Oct 5 21:02:42 UTC 2023
machine               : x86_64
processor             : x86_64
byteorder             : little
LC_ALL                : None
LANG                  : C.UTF-8
LOCALE                : en_US.UTF-8

pandas                : 3.0.0.dev0+355.g92a52e2315
numpy                 : 1.26.4
pytz                  : 2024.1
dateutil              : 2.8.2
setuptools            : 59.6.0
pip                   : 24.0
Cython                : 3.0.8
pytest                : 8.0.0
hypothesis            : 6.98.6
sphinx                : 7.2.6
blosc                 : None
feather               : None
xlsxwriter            : 3.1.9
lxml.etree            : 5.1.0
html5lib              : 1.1
pymysql               : 1.4.6
psycopg2              : 2.9.9
jinja2                : 3.1.3
IPython               : 8.21.0
pandas_datareader     : None
adbc-driver-postgresql: None
adbc-driver-sqlite    : None
bs4                   : 4.12.3
bottleneck            : 1.3.7
fastparquet           : 2024.2.0
fsspec                : 2024.2.0
gcsfs                 : 2024.2.0
matplotlib            : 3.8.3
numba                 : 0.59.0
numexpr               : 2.9.0
odfpy                 : None
openpyxl              : 3.1.2
pyarrow               : 15.0.0
pyreadstat            : 1.2.6
python-calamine       : None
pyxlsb                : 1.0.10
s3fs                  : 2024.2.0
scipy                 : 1.12.0
sqlalchemy            : 2.0.27
tables                : 3.9.2
tabulate              : 0.9.0
xarray                : 2024.1.1
xlrd                  : 2.0.1
zstandard             : 0.22.0
tzdata                : 2024.1
qtpy                  : 2.4.1
pyqt5                 : None

</details>


Pull request:
BUG: interchange protocol with nullable datatypes a non-null validity
- [ ] closes #57643 (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

This deals with pandas nullable dtypes

pyarrow nullables ones will require changes to the PandasBuffer class, so I'll handle those separately

Patch:
diff --git a/doc/source/user_guide/scale.rst b/doc/source/user_guide/scale.rst
index b262de5d71..080f8484ce 100644
--- a/doc/source/user_guide/scale.rst
+++ b/doc/source/user_guide/scale.rst
@@ -374,5 +374,33 @@ datasets.
 
 You see more dask examples at https://examples.dask.org.
 
+Use Modin
+---------
+
+Modin_ is a scalable dataframe library, which aims to be a drop-in replacement API for pandas and
+provides the ability to scale pandas workflows across nodes and CPUs available. It is also able
+to work with larger than memory datasets. To start working with Modin you just need
+to replace a single line of code, namely, the import statement.
+
+.. code-block:: ipython
+
+   # import pandas as pd
+   import modin.pandas as pd
+
+After you have changed the import statement, you can proceed using the well-known pandas API
+to scale computation. Modin distributes computation across nodes and CPUs available utilizing
+an execution engine it runs on. At the time of Modin 0.27.0 the following execution engines are supported
+in Modin: Ray_, Dask_, `MPI through unidist`_, HDK_. The partitioning schema of a Modin DataFrame partitions it
+along both columns and rows because it gives Modin flexibility and scalability in both the number of columns and
+the number of rows.
+
+For more information refer to `Modin's documentation`_ or the `Modin's tutorials`_.
+
+.. _Modin: https://github.com/modin-project/modin
+.. _`Modin's documentation`: https://modin.readthedocs.io/en/latest
+.. _`Modin's tutorials`: https://github.com/modin-project/modin/tree/master/examples/tutorial/jupyter/execution
+.. _Ray: https://github.com/ray-project/ray
 .. _Dask: https://dask.org
+.. _`MPI through unidist`: https://github.com/modin-project/unidist
+.. _HDK: https://github.com/intel-ai/hdk
 .. _dask.dataframe: https://docs.dask.org/en/latest/dataframe.html
diff --git a/doc/source/whatsnew/v2.2.2.rst b/doc/source/whatsnew/v2.2.2.rst
index 058f7aebcd..96f210ce6b 100644
--- a/doc/source/whatsnew/v2.2.2.rst
+++ b/doc/source/whatsnew/v2.2.2.rst
@@ -13,6 +13,7 @@ including other versions of pandas.
 
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
+- :meth:`DataFrame.__dataframe__` was producing incorrect data buffers when the a column's type was a pandas nullable on with missing values (:issue:`56702`)
 -
 
 .. ---------------------------------------------------------------------------
diff --git a/pandas/core/interchange/column.py b/pandas/core/interchange/column.py
index 7effc42d5b..bf20f0b543 100644
--- a/pandas/core/interchange/column.py
+++ b/pandas/core/interchange/column.py
@@ -190,6 +190,10 @@ class PandasColumn(Column):
 
     @property
     def describe_null(self):
+        if isinstance(self._col.dtype, BaseMaskedDtype):
+            column_null_dtype = ColumnNullType.USE_BYTEMASK
+            null_value = 1
+            return column_null_dtype, null_value
         kind = self.dtype[0]
         try:
             null, value = _NULL_DESCRIPTION[kind]
@@ -298,7 +302,13 @@ class PandasColumn(Column):
             DtypeKind.FLOAT,
             DtypeKind.BOOL,
         ):
-            np_arr = self._col.to_numpy()
+            arr = self._col.array
+            if isinstance(self._col.dtype, BaseMaskedDtype):
+                np_arr = arr._data  # type: ignore[attr-defined]
+            elif isinstance(self._col.dtype, ArrowDtype):
+                raise NotImplementedError("ArrowDtype not handled yet")
+            else:
+                np_arr = arr._ndarray  # type: ignore[attr-defined]
             buffer = PandasBuffer(np_arr, allow_copy=self._allow_copy)
             dtype = self.dtype
         elif self.dtype[0] == DtypeKind.CATEGORICAL:
@@ -341,6 +351,12 @@ class PandasColumn(Column):
         """
         null, invalid = self.describe_null
 
+        if isinstance(self._col.dtype, BaseMaskedDtype):
+            mask = self._col.array._mask  # type: ignore[attr-defined]
+            buffer = PandasBuffer(mask)
+            dtype = (DtypeKind.BOOL, 8, ArrowCTypes.BOOL, Endianness.NATIVE)
+            return buffer, dtype
+
         if self.dtype[0] == DtypeKind.STRING:
             # For now, use byte array as the mask.
             # TODO: maybe store as bit array to save space?..


Test patch:
diff --git a/pandas/tests/interchange/test_impl.py b/pandas/tests/interchange/test_impl.py
index e4fa6e4451..94b2da894a 100644
--- a/pandas/tests/interchange/test_impl.py
+++ b/pandas/tests/interchange/test_impl.py
@@ -8,7 +8,6 @@ from pandas.compat import (
     is_ci_environment,
     is_platform_windows,
 )
-import pandas.util._test_decorators as td
 
 import pandas as pd
 import pandas._testing as tm
@@ -417,17 +416,50 @@ def test_non_str_names_w_duplicates():
         pd.api.interchange.from_dataframe(dfi, allow_copy=False)
 
 
-@pytest.mark.parametrize(
-    "dtype", ["Int8", pytest.param("Int8[pyarrow]", marks=td.skip_if_no("pyarrow"))]
-)
-def test_nullable_integers(dtype: str) -> None:
+def test_nullable_integers() -> None:
     # https://github.com/pandas-dev/pandas/issues/55069
-    df = pd.DataFrame({"a": [1]}, dtype=dtype)
+    df = pd.DataFrame({"a": [1]}, dtype="Int8")
     expected = pd.DataFrame({"a": [1]}, dtype="int8")
     result = pd.api.interchange.from_dataframe(df.__dataframe__())
     tm.assert_frame_equal(result, expected)
 
 
+@pytest.mark.xfail(reason="https://github.com/pandas-dev/pandas/issues/57664")
+def test_nullable_integers_pyarrow() -> None:
+    # https://github.com/pandas-dev/pandas/issues/55069
+    df = pd.DataFrame({"a": [1]}, dtype="Int8[pyarrow]")
+    expected = pd.DataFrame({"a": [1]}, dtype="int8")
+    result = pd.api.interchange.from_dataframe(df.__dataframe__())
+    tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    ("data", "dtype", "expected_dtype"),
+    [
+        ([1, 2, None], "Int64", "int64"),
+        (
+            [1, 2, None],
+            "UInt64",
+            "uint64",
+        ),
+        ([1.0, 2.25, None], "Float32", "float32"),
+    ],
+)
+def test_pandas_nullable_w_missing_values(
+    data: list, dtype: str, expected_dtype: str
+) -> None:
+    # https://github.com/pandas-dev/pandas/issues/57643
+    pytest.importorskip("pyarrow", "11.0.0")
+    import pyarrow.interchange as pai
+
+    df = pd.DataFrame({"a": data}, dtype=dtype)
+    result = pai.from_dataframe(df.__dataframe__())["a"]
+    assert result.type == expected_dtype
+    assert result[0].as_py() == data[0]
+    assert result[1].as_py() == data[1]
+    assert result[2].as_py() is None
+
+
 def test_empty_dataframe():
     # https://github.com/pandas-dev/pandas/issues/56700
     df = pd.DataFrame({"a": []}, dtype="int8")