fix: Empty RangeIndex Display (pydata#10594)

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>

Author: 3 people

xarray/indexes/range_index.py

@@ -20,8 +20,9 @@ class RangeCoordinateTransform(CoordinateTransform):
 
     start: float
     stop: float
+    _step: float | None
 
-    __slots__ = ("start", "stop")
+    __slots__ = ("_step", "start", "stop")
 
     def __init__(
         self,
@@ -39,6 +40,7 @@ def __init__(
 
         self.start = start
         self.stop = stop
+        self._step = None  # Will be calculated by property
 
     @property
     def coord_name(self) -> Hashable:
@@ -54,7 +56,13 @@ def size(self) -> int:
 
     @property
     def step(self) -> float:
-        return (self.stop - self.start) / self.size
+        if self._step is not None:
+            return self._step
+        if self.size > 0:
+            return (self.stop - self.start) / self.size
+        else:
+            # For empty arrays, default to 1.0
+            return 1.0
 
     def forward(self, dim_positions: dict[str, Any]) -> dict[Hashable, Any]:
         positions = dim_positions[self.dim]
@@ -81,12 +89,22 @@ def equals(
     def slice(self, sl: slice) -> "RangeCoordinateTransform":
         new_range = range(self.size)[sl]
         new_size = len(new_range)
+
         new_start = self.start + new_range.start * self.step
         new_stop = self.start + new_range.stop * self.step
 
-        return type(self)(
-            new_start, new_stop, new_size, self.coord_name, self.dim, dtype=self.dtype
+        result = type(self)(
+            new_start,
+            new_stop,
+            new_size,
+            self.coord_name,
+            self.dim,
+            dtype=self.dtype,
         )
+        if new_size == 0:
+            # For empty slices, preserve step from parent
+            result._step = self.step
+        return result
 
 
 class RangeIndex(CoordinateTransformIndex):

xarray/tests/test_range_index.py

@@ -166,6 +166,62 @@ def test_range_index_isel() -> None:
     assert_identical(actual, expected)
 
 
+def test_range_index_empty_slice() -> None:
+    """Test that empty slices of RangeIndex are printable and preserve step.
+
+    Regression test for https://github.com/pydata/xarray/issues/10547
+    """
+    # Test with linspace
+    n = 30
+    step = 1
+    da = xr.DataArray(np.zeros(n), dims=["x"])
+    da = da.assign_coords(
+        xr.Coordinates.from_xindex(RangeIndex.linspace(0, (n - 1) * step, n, dim="x"))
+    )
+
+    # This should not raise ZeroDivisionError
+    sub = da.isel(x=slice(0))
+    assert sub.sizes["x"] == 0
+
+    # Test that it's printable
+    repr_str = repr(sub)
+    assert "RangeIndex" in repr_str
+    assert "step=1" in repr_str
+
+    # Test with different step values
+    index = RangeIndex.arange(0, 10, 2.5, dim="y")
+    da2 = xr.DataArray(np.zeros(4), dims=["y"])
+    da2 = da2.assign_coords(xr.Coordinates.from_xindex(index))
+    empty = da2.isel(y=slice(0))
+
+    # Should preserve step
+    assert empty.sizes["y"] == 0
+    range_index_y = empty._indexes["y"]
+    assert isinstance(range_index_y, RangeIndex)
+    assert range_index_y.step == 2.5
+
+    # Test that it's printable
+    repr_str2 = repr(empty)
+    assert "RangeIndex" in repr_str2
+    assert "step=2.5" in repr_str2
+
+    # Test negative step
+    index3 = RangeIndex.arange(10, 0, -1, dim="z")
+    da3 = xr.DataArray(np.zeros(10), dims=["z"])
+    da3 = da3.assign_coords(xr.Coordinates.from_xindex(index3))
+    empty3 = da3.isel(z=slice(0))
+
+    assert empty3.sizes["z"] == 0
+    range_index_z = empty3._indexes["z"]
+    assert isinstance(range_index_z, RangeIndex)
+    assert range_index_z.step == -1.0
+
+    # Test that it's printable
+    repr_str3 = repr(empty3)
+    assert "RangeIndex" in repr_str3
+    assert "step=-1" in repr_str3
+
+
 def test_range_index_sel() -> None:
     ds = create_dataset_arange(0.0, 1.0, 0.1)