From 35aee00e45006df7d4148ee76c02bb70fc56a96d Mon Sep 17 00:00:00 2001
From: Bouwe Andela <b.andela@esciencecenter.nl>
Date: Mon, 30 Mar 2026 11:28:13 +0200
Subject: [PATCH 1/2] Optimize add_bounds

---
 cf_xarray/helpers.py | 27 ++++++++++-----------------
 1 file changed, 10 insertions(+), 17 deletions(-)

diff --git a/cf_xarray/helpers.py b/cf_xarray/helpers.py
index 5ca5b1b9..8b66e51d 100644
--- a/cf_xarray/helpers.py
+++ b/cf_xarray/helpers.py
@@ -17,25 +17,18 @@ def _guess_bounds_1d(da, dim):
     """
     if dim not in da.dims:
         (dim,) = da.cf.axes[dim]
-    ADDED_INDEX = False
-    if dim not in da.coords:
-        # For proper alignment in the lines below, we need an index on dim.
-        da = da.assign_coords({dim: da[dim]})
-        ADDED_INDEX = True
 
     bound_position = 0.5
-    diff = da.diff(dim).pad({dim: (1, 1)}, mode="edge").reset_index(dim)
-    lower = (
-        da.reset_index(dim) - bound_position * diff.isel({dim: slice(0, -1)})
-    ).assign_coords({dim: da[dim]})
-    upper = (
-        da.reset_index(dim) + bound_position * diff.isel({dim: slice(1, None)})
-    ).assign_coords({dim: da[dim]})
-    result = xr.concat([lower, upper], dim="bounds").transpose(..., "bounds")
-
-    if ADDED_INDEX:
-        result = result.drop_vars(dim)
-    return result.drop_attrs(deep=False)
+
+    diff = da.diff(dim).pad({dim: (1, 1)}, mode="edge")
+    lower = da.data - bound_position * diff.isel({dim: slice(0, -1)}).data
+    upper = da.data + bound_position * diff.isel({dim: slice(1, None)}).data
+    return (
+        da.expand_dims({"bounds": 2})
+        .copy(deep=False, data=[lower, upper])
+        .transpose(..., "bounds")
+        .drop_attrs(deep=False)
+    )
 
 
 def _guess_bounds_2d(da, dims):

From f466765d32ac9b8e0ceaa1c3cc4a31fbdcbcfc16 Mon Sep 17 00:00:00 2001
From: Bouwe Andela <b.andela@esciencecenter.nl>
Date: Wed, 1 Apr 2026 08:38:37 +0200
Subject: [PATCH 2/2] Preserve array type

---
 cf_xarray/helpers.py             | 13 +++++++++----
 cf_xarray/tests/test_accessor.py | 16 ++++++++++++++++
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/cf_xarray/helpers.py b/cf_xarray/helpers.py
index 8b66e51d..108a9c81 100644
--- a/cf_xarray/helpers.py
+++ b/cf_xarray/helpers.py
@@ -21,11 +21,16 @@ def _guess_bounds_1d(da, dim):
     bound_position = 0.5
 
     diff = da.diff(dim).pad({dim: (1, 1)}, mode="edge")
-    lower = da.data - bound_position * diff.isel({dim: slice(0, -1)}).data
-    upper = da.data + bound_position * diff.isel({dim: slice(1, None)}).data
+    lower = da.copy(
+        deep=False,
+        data=da.data - bound_position * diff.isel({dim: slice(0, -1)}).data,
+    )
+    upper = da.copy(
+        deep=False,
+        data=da.data + bound_position * diff.isel({dim: slice(1, None)}).data,
+    )
     return (
-        da.expand_dims({"bounds": 2})
-        .copy(deep=False, data=[lower, upper])
+        xr.concat([lower, upper], dim="bounds")
         .transpose(..., "bounds")
         .drop_attrs(deep=False)
     )
diff --git a/cf_xarray/tests/test_accessor.py b/cf_xarray/tests/test_accessor.py
index b9ef3ed9..ce9c7a78 100644
--- a/cf_xarray/tests/test_accessor.py
+++ b/cf_xarray/tests/test_accessor.py
@@ -3,6 +3,7 @@
 import warnings
 from textwrap import dedent
 
+import dask.array
 import matplotlib as mpl
 import numpy as np
 import pandas as pd
@@ -839,6 +840,21 @@ def test_add_bounds(dims):
     _check_unchanged(original, ds)
 
 
+def test_add_bounds_preserves_array_type() -> None:
+    # Test that the array type of the bounds variable is the same as the original variable.
+    ds = airds
+    original = ds.copy(deep=True)
+    ds = ds.drop_indexes("lat").rename_dims(lat="x")
+    ds["lat"] = ds.lat.copy(data=dask.array.asarray(ds.lat.data))
+    added = ds.cf.add_bounds("lat")
+
+    assert isinstance(added.lat.data, dask.array.Array)
+    assert isinstance(added.lat_bounds.data, dask.array.Array)
+
+    assert isinstance(ds.lat.data, dask.array.Array)
+    _check_unchanged(original, ds)
+
+
 def test_add_irregularly_spaced_bounds_do_not_overlap() -> None:
     # Test that added bounds with irregular spacing do not overlap.
     ds = airds