BUG: Interchange object data buffer has the wrong dtype / from_datafr…

…ame incorrect (#55227)

* use buffer dtype in interchange from_dataframe

* wip

* wip

* add failing test

* wip

* simplify

* remove unnecessary assertion

* dont double-extract bit width

* Update pandas/core/interchange/from_dataframe.py

Co-authored-by: Stijn de Gooijer <stijn@degooijer.io>

---------

Co-authored-by: Stijn de Gooijer <stijn@degooijer.io>

pandas/core/interchange/from_dataframe.py

@@ -266,10 +266,9 @@ def string_column_to_ndarray(col: Column) -> tuple[np.ndarray, Any]:
 
     assert buffers["offsets"], "String buffers must contain offsets"
     # Retrieve the data buffer containing the UTF-8 code units
-    data_buff, protocol_data_dtype = buffers["data"]
+    data_buff, _ = buffers["data"]
     # We're going to reinterpret the buffer as uint8, so make sure we can do it safely
-    assert protocol_data_dtype[1] == 8
-    assert protocol_data_dtype[2] in (
+    assert col.dtype[2] in (
         ArrowCTypes.STRING,
         ArrowCTypes.LARGE_STRING,
     )  # format_str == utf-8
@@ -377,15 +376,16 @@ def datetime_column_to_ndarray(col: Column) -> tuple[np.ndarray | pd.Series, Any
     """
     buffers = col.get_buffers()
 
-    _, _, format_str, _ = col.dtype
-    dbuf, dtype = buffers["data"]
+    _, col_bit_width, format_str, _ = col.dtype
+    dbuf, _ = buffers["data"]
     # Consider dtype being `uint` to get number of units passed since the 01.01.1970
+
     data = buffer_to_ndarray(
         dbuf,
         (
-            DtypeKind.UINT,
-            dtype[1],
-            getattr(ArrowCTypes, f"UINT{dtype[1]}"),
+            DtypeKind.INT,
+            col_bit_width,
+            getattr(ArrowCTypes, f"INT{col_bit_width}"),
             Endianness.NATIVE,
         ),
         offset=col.offset,

pandas/tests/interchange/test_impl.py

@@ -18,6 +18,7 @@
     DtypeKind,
 )
 from pandas.core.interchange.from_dataframe import from_dataframe
+from pandas.core.interchange.utils import ArrowCTypes
 
 
 @pytest.fixture
@@ -340,3 +341,24 @@ def test_interchange_from_non_pandas_tz_aware(request):
         dtype="datetime64[us, Asia/Kathmandu]",
     )
     tm.assert_frame_equal(expected, result)
+
+
+def test_interchange_from_corrected_buffer_dtypes(monkeypatch) -> None:
+    # https://github.com/pandas-dev/pandas/issues/54781
+    df = pd.DataFrame({"a": ["foo", "bar"]}).__dataframe__()
+    interchange = df.__dataframe__()
+    column = interchange.get_column_by_name("a")
+    buffers = column.get_buffers()
+    buffers_data = buffers["data"]
+    buffer_dtype = buffers_data[1]
+    buffer_dtype = (
+        DtypeKind.UINT,
+        8,
+        ArrowCTypes.UINT8,
+        buffer_dtype[3],
+    )
+    buffers["data"] = (buffers_data[0], buffer_dtype)
+    column.get_buffers = lambda: buffers
+    interchange.get_column_by_name = lambda _: column
+    monkeypatch.setattr(df, "__dataframe__", lambda allow_copy: interchange)
+    pd.api.interchange.from_dataframe(df)