From 569f904d45aeb5d0abb925e9875140afd61e3da4 Mon Sep 17 00:00:00 2001
From: "Lumberbot (aka Jack)"
 <39504233+meeseeksmachine@users.noreply.github.com>
Date: Wed, 8 Nov 2023 01:08:46 +0100
Subject: [PATCH] Backport PR #55227 on branch 2.1.x (BUG: Interchange object
 data buffer has the wrong dtype / from_dataframe incorrect ) (#55863)

Backport PR #55227: BUG: Interchange object data buffer has the wrong dtype / from_dataframe incorrect

Co-authored-by: Marco Edward Gorelli <marcogorelli@protonmail.com>
Co-authored-by: MarcoGorelli <33491632+MarcoGorelli@users.noreply.github.com>
---
 pandas/core/interchange/from_dataframe.py | 16 ++++++++--------
 pandas/tests/interchange/test_impl.py     | 22 ++++++++++++++++++++++
 2 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/pandas/core/interchange/from_dataframe.py b/pandas/core/interchange/from_dataframe.py
index 214fbf9f36435..d45ae37890ba7 100644
--- a/pandas/core/interchange/from_dataframe.py
+++ b/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,
diff --git a/pandas/tests/interchange/test_impl.py b/pandas/tests/interchange/test_impl.py
index 61d3edcbb2964..97a388569e261 100644
--- a/pandas/tests/interchange/test_impl.py
+++ b/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)