[pull] main from pydata:main

doc/whats-new.rst

@@ -33,6 +33,12 @@ Bug Fixes
   a ``zarr_format=3`` store with ``use_zarr_fill_value_as_mask=False``, so it is no
   longer silently lost on round-trip (:issue:`10269`).
   By `Davis Bennett <https://github.com/d-v-b>`_.
+- :py:meth:`~xarray.indexes.RangeIndex.arange` now preserves the requested
+  ``step`` instead of silently re-deriving it from ``(stop - start) / size``, so
+  its values match :py:func:`numpy.arange` when ``step`` does not evenly divide
+  the interval. Strided slicing of a :py:class:`~xarray.indexes.RangeIndex` now
+  preserves the step as well (:issue:`11325`).
+  By `mokashang <https://github.com/mokashang>`_.
 - Fix :py:func:`decode_cf` failing on integer-encoded time arrays that contain
   NaT when running against numpy 2.5+.
   By `Ian Hunt-Isaak <https://github.com/ianhi>`_.

xarray/indexes/range_index.py

@@ -32,6 +32,7 @@ def __init__(
         coord_name: Hashable,
         dim: str,
         dtype: Any = None,
+        step: float | None = None,
     ):
         if dtype is None:
             dtype = np.dtype(np.float64)
@@ -40,7 +41,7 @@ def __init__(
 
         self.start = start
         self.stop = stop
-        self._step = None  # Will be calculated by property
+        self._step = step
 
     @property
     def coord_name(self) -> Hashable:
@@ -121,21 +122,23 @@ def slice(self, sl: slice) -> "RangeCoordinateTransform":
         new_range = range(self.size)[sl]
         new_size = len(new_range)
 
+        # A slice scales the spacing by its own step, e.g. ``[::2]`` doubles it.
+        # Preserve the exact resulting step instead of letting it be re-derived
+        # from ``(stop - start) / size``, which would be wrong whenever the
+        # spacing does not evenly divide the interval. See GH11325.
+        new_step = self.step * new_range.step
         new_start = self.start + new_range.start * self.step
-        new_stop = self.start + new_range.stop * self.step
+        new_stop = new_start + new_size * new_step
 
-        result = type(self)(
+        return type(self)(
             new_start,
             new_stop,
             new_size,
             self.coord_name,
             self.dim,
             dtype=self.dtype,
+            step=new_step,
         )
-        if new_size == 0:
-            # For empty slices, preserve step from parent
-            result._step = self.step
-        return result
 
 
 class RangeIndex(CoordinateTransformIndex):
@@ -278,8 +281,13 @@ def arange(
 
         size = math.ceil((stop - start) / step)
 
+        # Snap ``stop`` to ``start + size * step`` and keep the exact ``step`` so
+        # that the materialized values match ``numpy.arange`` even when ``step``
+        # does not evenly divide ``stop - start``. See GH11325.
+        stop = start + size * step
+        # Snap `stop` to `start + size * step`
         transform = RangeCoordinateTransform(
-            start, stop, size, coord_name, dim, dtype=dtype
+            start, stop, size, coord_name, dim, dtype=dtype, step=step
         )
 
         return cls(transform)

xarray/tests/__init__.py

@@ -131,10 +131,10 @@ def _importorskip(
 has_bottleneck, requires_bottleneck = _importorskip("bottleneck")
 has_rasterio, requires_rasterio = _importorskip("rasterio")
 has_zarr, requires_zarr = _importorskip("zarr")
-has_zarr_v3, requires_zarr_v3 = _importorskip("zarr", "3.0.0")
+requires_zarr_v3 = requires_zarr
 has_zarr_v3_dtypes, requires_zarr_v3_dtypes = _importorskip("zarr", "3.1.0")
 has_zarr_v3_async_oindex, requires_zarr_v3_async_oindex = _importorskip("zarr", "3.1.2")
-if has_zarr_v3:
+if has_zarr:
     import zarr
 
     # manual update by checking attrs for now
@@ -197,14 +197,7 @@ def _importorskip(
     "zarr_format",
     [
         pytest.param(2, id="zarr_format=2"),
-        pytest.param(
-            3,
-            marks=pytest.mark.skipif(
-                not has_zarr_v3,
-                reason="zarr-python v2 cannot understand the zarr v3 format",
-            ),
-            id="zarr_format=3",
-        ),
+        pytest.param(3, id="zarr_format=3"),
     ],
 )