BUG: Interchange protocol implementation handles empty dataframes incorrectly (#57175)

fix from_dataframe for empty dataframes

Author: MarcoGorelli

doc/source/whatsnew/v2.2.1.rst

@@ -29,6 +29,7 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
+- Fixed bug in :func:`pandas.api.interchange.from_dataframe` which was raising for empty inputs (:issue:`56700`)
 - Fixed bug in :meth:`DataFrame.__getitem__` for empty :class:`DataFrame` with Copy-on-Write enabled (:issue:`57130`)
 
 .. ---------------------------------------------------------------------------

pandas/core/interchange/buffer.py

@@ -23,7 +23,7 @@ def __init__(self, x: np.ndarray, allow_copy: bool = True) -> None:
  """
  Handle only regular columns (= numpy arrays) for now.
  """
- if not x.strides == (x.dtype.itemsize,):
+ if x.strides[0] and not x.strides == (x.dtype.itemsize,):
  # The protocol does not support strided buffers, so a copy is
  # necessary. If that's not allowed, we need to raise an exception.
  if allow_copy:

pandas/tests/interchange/test_impl.py

@@ -392,3 +392,12 @@ def test_large_string():
  result = pd.api.interchange.from_dataframe(df.__dataframe__())
  expected = pd.DataFrame({"a": ["x"]}, dtype="object")
  tm.assert_frame_equal(result, expected)
+
+
+def test_empty_dataframe():
+ # https://github.com/pandas-dev/pandas/issues/56700
+ df = pd.DataFrame({"a": []}, dtype="int8")
+ dfi = df.__dataframe__()
+ result = pd.api.interchange.from_dataframe(dfi, allow_copy=False)
+ expected = pd.DataFrame({"a": []}, dtype="int8")
+ tm.assert_frame_equal(result, expected)