Backport PR pandas-dev#56788: Bug: Interchange protocol implementation does not allow for empty string columns

Author: roadrollerdafjorst

doc/source/whatsnew/v2.2.0.rst

@@ -936,6 +936,7 @@ Other
 - Bug in :func:`cut` and :func:`qcut` with ``datetime64`` dtype values with non-nanosecond units incorrectly returning nanosecond-unit bins (:issue:`56101`)
 - Bug in :func:`cut` incorrectly allowing cutting of timezone-aware datetimes with timezone-naive bins (:issue:`54964`)
 - Bug in :func:`infer_freq` and :meth:`DatetimeIndex.inferred_freq` with weekly frequencies and non-nanosecond resolutions (:issue:`55609`)
+- Bug in :func:`pd.api.interchange.from_dataframe` where it raised  ``NotImplementedError`` when handling empty string columns (:issue:`56703`)
 - Bug in :meth:`DataFrame.apply` where passing ``raw=True`` ignored ``args`` passed to the applied function (:issue:`55009`)
 - Bug in :meth:`DataFrame.from_dict` which would always sort the rows of the created :class:`DataFrame`.  (:issue:`55683`)
 - Bug in :meth:`DataFrame.sort_index` when passing ``axis="columns"`` and ``ignore_index=True`` raising a ``ValueError`` (:issue:`56478`)
@@ -944,7 +945,6 @@ Other
 - Bug in the error message when assigning an empty :class:`DataFrame` to a column (:issue:`55956`)
 - Bug when time-like strings were being cast to :class:`ArrowDtype` with ``pyarrow.time64`` type (:issue:`56463`)
 
-
 .. ---------------------------------------------------------------------------
 .. _whatsnew_220.contributors:
 

pandas/core/interchange/column.py

@@ -116,7 +116,7 @@ def dtype(self) -> tuple[DtypeKind, int, str, str]:
                 Endianness.NATIVE,
             )
         elif is_string_dtype(dtype):
-            if infer_dtype(self._col) == "string":
+            if infer_dtype(self._col) in ("string", "empty"):
                 return (
                     DtypeKind.STRING,
                     8,

pandas/tests/interchange/test_impl.py

@@ -364,6 +364,14 @@ def test_interchange_from_corrected_buffer_dtypes(monkeypatch) -> None:
     pd.api.interchange.from_dataframe(df)
 
 
+def test_empty_string_column():
+    # https://github.com/pandas-dev/pandas/issues/56703
+    df = pd.DataFrame({"a": []}, dtype=str)
+    df2 = df.__dataframe__()
+    result = pd.api.interchange.from_dataframe(df2)
+    tm.assert_frame_equal(df, result)
+
+
 def test_large_string():
     # GH#56702
     pytest.importorskip("pyarrow")