BUG: dropna changes index even when nothing dropped

doc/source/whatsnew/v1.4.0.rst

@@ -433,6 +433,7 @@ Indexing
 - Bug in :meth:`DataFrame.drop` where the error message did not show missing labels with commas when raising ``KeyError`` (:issue:`42881`)
 - Bug in :meth:`DataFrame.query` where method calls in query strings led to errors when the ``numexpr`` package was installed. (:issue:`22435`)
 - Bug in :meth:`DataFrame.nlargest` and :meth:`Series.nlargest` where sorted result did not count indexes containing ``np.nan`` (:issue:`28984`)
+- Bug in :meth:`DataFrame.dropna` where :class:`RangeIndex` loses its type even when nothing is dropped (:issue:`41965`)
 - Bug in indexing on a non-unique object-dtype :class:`Index` with an NA scalar (e.g. ``np.nan``) (:issue:`43711`)
 -
 

pandas/core/indexes/range.py

@@ -16,7 +16,10 @@
 import numpy as np
 
 from pandas._libs import index as libindex
-from pandas._libs.lib import no_default
+from pandas._libs.lib import (
+ maybe_indices_to_slice,
+ no_default,
+)
 from pandas._typing import (
  Dtype,
  npt,
@@ -433,8 +436,19 @@ def delete(self, loc) -> Int64Index: # type: ignore[override]
 
  def take(
  self, indices, axis: int = 0, allow_fill: bool = True, fill_value=None, **kwargs
- ) -> Int64Index:
+ ) -> Int64Index | RangeIndex:
  with rewrite_exception("Int64Index", type(self).__name__):
+ if (
+ fill_value is None
+ and isinstance(indices, np.ndarray)
+ and indices.dtype == int
+ ):
+ try:
+ slc = maybe_indices_to_slice(indices, len(indices))
+ return self[slc]
+ except ValueError:
+ # Buffer dtype mismatch, expected 'intp_t' but got something else
+ pass
  return self._int64index.take(
  indices,
  axis=axis,
@@ -770,7 +784,8 @@ def _concat(self, indexes: list[Index], name: Hashable) -> Index:
  if start is None:
  # This is set by the first non-empty index
  start = rng.start
- if step is None and len(rng) > 1:
+ if step is None and (len(rng) > 1 or rng.step < 0):
+ # GH #41965 for decreasing RangeIndex, like range(0, -1, -1)
  step = rng.step
  elif step is None:
  # First non-empty index had only one element

pandas/tests/frame/methods/test_dropna.py

@@ -243,3 +243,9 @@ def test_dropna_pos_args_deprecation(self):
  result = df.dropna(1)
  expected = DataFrame({"a": [1, 2, 3]})
  tm.assert_frame_equal(result, expected)
+
+ def test_dropna_retains_RangeIndex_when_nothing_dropped(self):
+ # GH#41965
+ expected = DataFrame({"a": range(10)})
+ result = expected.dropna()
+ tm.assert_frame_equal(result, expected, check_index_type=True)

pandas/tests/reshape/concat/test_concat.py

@@ -698,3 +698,16 @@ def test_concat_posargs_deprecation():
  result = concat([df, df2], 0)
  expected = DataFrame([[1, 2, 3], [4, 5, 6]], index=["a", "b"])
  tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+ "ser2_index", [pd.RangeIndex(0, -1, -1), pd.RangeIndex(0, 1, 1)]
+)
+def test_concat_series_with_rangeindex(ser2_index):
+ # GH#41965
+ ser1 = Series([], index=pd.RangeIndex(0), dtype="int64")
+ ser2 = Series([0], index=ser2_index, dtype="int64")
+ # expected should be exactly ser2, as ser1 is empty
+ expected = ser2.copy()
+ result = concat([ser1, ser2])
+ tm.assert_series_equal(expected, result, check_index_type=True)