From 3719ba7ca6d66dbb1c7a105907beb13e8d826e37 Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Fri, 19 Apr 2024 11:11:25 -0400
Subject: [PATCH 01/15] test expand_dims doesn't create Index

---
 xarray/tests/test_dataset.py | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/xarray/tests/test_dataset.py b/xarray/tests/test_dataset.py
index a948fafc815..54a7b0a6f87 100644
--- a/xarray/tests/test_dataset.py
+++ b/xarray/tests/test_dataset.py
@@ -3431,6 +3431,23 @@ def test_expand_dims_kwargs_python36plus(self) -> None:
         )
         assert_identical(other_way_expected, other_way)
 
+    def test_expand_dims_creates_indexvariable(self):
+        # data variables should not gain an index ever
+        ds = Dataset({"a": 0})
+        for flag in [True, False]:
+            expanded = ds.expand_dims("x", create_1d_index=flag)
+            expected = Dataset({"a": ("x", [0])})
+            assert_identical(expanded, expected)
+            assert expanded.indexes == {}
+
+        # coordinate variables should gain an index only if create_1d_index is True (the default)
+        ds = Dataset(coords={"x": 0})
+        expanded = ds.expand_dims("x")
+        expected = Dataset({"x": ("x", [0])})
+        assert_identical(expanded, expected)
+        expanded_no_index = ds.expand_dims("x", create_1d_index=False)
+        assert expanded_no_index.indexes == {}
+
     @requires_pandas_version_two
     def test_expand_dims_non_nanosecond_conversion(self) -> None:
         # Regression test for https://github.com/pydata/xarray/issues/7493#issuecomment-1953091000

From 018e74bb1c94141cd86502bfa2ac964a44f28860 Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Fri, 19 Apr 2024 11:40:06 -0400
Subject: [PATCH 02/15] add option to not create 1D index in expand_dims

---
 xarray/core/dataset.py | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/xarray/core/dataset.py b/xarray/core/dataset.py
index 96f3be00995..233b7833785 100644
--- a/xarray/core/dataset.py
+++ b/xarray/core/dataset.py
@@ -4497,6 +4497,7 @@ def expand_dims(
         self,
         dim: None | Hashable | Sequence[Hashable] | Mapping[Any, Any] = None,
         axis: None | int | Sequence[int] = None,
+        create_1d_index: bool = True,
         **dim_kwargs: Any,
     ) -> Self:
         """Return a new object with an additional axis (or axes) inserted at
@@ -4506,6 +4507,8 @@ def expand_dims(
         If dim is already a scalar coordinate, it will be promoted to a 1D
         coordinate consisting of a single value.
 
+        The 
+        
         Parameters
         ----------
         dim : hashable, sequence of hashable, mapping, or None
@@ -4521,6 +4524,8 @@ def expand_dims(
             multiple axes are inserted. In this case, dim arguments should be
             same length list. If axis=None is passed, all the axes will be
             inserted to the start of the result array.
+        create_1d_index : bool, default is True
+            Whether to create new PandasIndex objects for new 1D coordinate variables.
         **dim_kwargs : int or sequence or ndarray
             The keywords are arbitrary dimensions being inserted and the values
             are either the lengths of the new dims (if int is given), or their
@@ -4640,6 +4645,8 @@ def expand_dims(
                 # save the coordinates to the variables dict, and set the
                 # value within the dim dict to the length of the iterable
                 # for later use.
+
+                # TODO should we have an option to not create a variable here?
                 index = PandasIndex(v, k)
                 indexes[k] = index
                 variables.update(index.create_variables())
@@ -4678,11 +4685,16 @@ def expand_dims(
                     variables[k] = v.set_dims(dict(all_dims))
             else:
                 if k not in variables:
-                    # If dims includes a label of a non-dimension coordinate,
-                    # it will be promoted to a 1D coordinate with a single value.
-                    index, index_vars = create_default_index_implicit(v.set_dims(k))
-                    indexes[k] = index
-                    variables.update(index_vars)
+                    if create_1d_index:
+                        # If dims includes a label of a non-dimension coordinate,
+                        # it will be promoted to a 1D coordinate with a single value.
+                        index, index_vars = create_default_index_implicit(v.set_dims(k))
+                        indexes[k] = index
+                        variables.update(index_vars)
+                    else:
+                        # create 1D variable without creating a new index
+                        new_1d_var = v.set_dims(k)
+                        variables.update({k: new_1d_var})
 
         return self._replace_with_new_dims(
             variables, coord_names=coord_names, indexes=indexes

From f680505d3dd630bc8fa0a200e5d9efde7cf8ada9 Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Fri, 19 Apr 2024 22:47:07 -0400
Subject: [PATCH 03/15] refactor tests to consider data variables and
 coordinate variables separately

---
 xarray/tests/test_dataset.py | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/xarray/tests/test_dataset.py b/xarray/tests/test_dataset.py
index 54a7b0a6f87..74c4af1790d 100644
--- a/xarray/tests/test_dataset.py
+++ b/xarray/tests/test_dataset.py
@@ -3431,21 +3431,34 @@ def test_expand_dims_kwargs_python36plus(self) -> None:
         )
         assert_identical(other_way_expected, other_way)
 
-    def test_expand_dims_creates_indexvariable(self):
+    @pytest.mark.parametrize("create_1d_index_flag", [True, False])
+    def test_expand_dims_create_index_data_variable(self, create_1d_index_flag):
         # data variables should not gain an index ever
-        ds = Dataset({"a": 0})
-        for flag in [True, False]:
-            expanded = ds.expand_dims("x", create_1d_index=flag)
-            expected = Dataset({"a": ("x", [0])})
-            assert_identical(expanded, expected)
-            assert expanded.indexes == {}
+        ds = Dataset({"x": 0})
+        expanded = ds.expand_dims("x", create_1d_index=create_1d_index_flag)
+
+        # TODO I can't just create the expected dataset directly using constructor because of GH issue 8959
+        # expected = Dataset(data_vars={"x": ("x", [0])})
+        expected = Dataset({"x": ("x", [0])}).drop_indexes("x").reset_coords("x")
+
+        # TODO also can't just assert equivalence because it will fail internal invariants default indexes checks
+        # assert_identical(expanded, expected)
+        assert expected.data_vars == {"x": Variable(data=[0], dims=["x"])}
+        assert expanded.indexes == {}
 
+    def test_expand_dims_create_index_coordinate_variable(self):
         # coordinate variables should gain an index only if create_1d_index is True (the default)
         ds = Dataset(coords={"x": 0})
         expanded = ds.expand_dims("x")
         expected = Dataset({"x": ("x", [0])})
         assert_identical(expanded, expected)
+
         expanded_no_index = ds.expand_dims("x", create_1d_index=False)
+        expected = Dataset(coords={"x": ("x", [0])}).drop_indexes("x")
+
+        # TODO also can't just assert equivalence because it will fail internal invariants default indexes checks
+        # assert_identical(expanded, expected)
+        assert expanded_no_index.coords == {"x": Variable(data=[0], dims=["x"])}
         assert expanded_no_index.indexes == {}
 
     @requires_pandas_version_two

From f10509aa983a4baae66163030b26827ef6c11344 Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Fri, 19 Apr 2024 23:22:11 -0400
Subject: [PATCH 04/15] fix bug causing new test to fail

---
 xarray/core/dataset.py | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/xarray/core/dataset.py b/xarray/core/dataset.py
index 233b7833785..9752afbd749 100644
--- a/xarray/core/dataset.py
+++ b/xarray/core/dataset.py
@@ -4507,8 +4507,9 @@ def expand_dims(
         If dim is already a scalar coordinate, it will be promoted to a 1D
         coordinate consisting of a single value.
 
-        The 
-        
+        The automatic creation of indexes to back new 1D coordinate variables
+        controlled by the create_1d_index kwarg.
+
         Parameters
         ----------
         dim : hashable, sequence of hashable, mapping, or None
@@ -4525,7 +4526,7 @@ def expand_dims(
             same length list. If axis=None is passed, all the axes will be
             inserted to the start of the result array.
         create_1d_index : bool, default is True
-            Whether to create new PandasIndex objects for new 1D coordinate variables.
+            Whether to create new PandasIndex objects for any new 1D coordinate variables.
         **dim_kwargs : int or sequence or ndarray
             The keywords are arbitrary dimensions being inserted and the values
             are either the lengths of the new dims (if int is given), or their
@@ -4685,7 +4686,7 @@ def expand_dims(
                     variables[k] = v.set_dims(dict(all_dims))
             else:
                 if k not in variables:
-                    if create_1d_index:
+                    if k in coord_names and create_1d_index:
                         # If dims includes a label of a non-dimension coordinate,
                         # it will be promoted to a 1D coordinate with a single value.
                         index, index_vars = create_default_index_implicit(v.set_dims(k))

From 8152c0a40c4386eb01cd1974769527ae5c85a570 Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Fri, 19 Apr 2024 23:51:28 -0400
Subject: [PATCH 05/15] test index auto-creation when iterable passed as new
 coordinate values

---
 xarray/tests/test_dataset.py | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/xarray/tests/test_dataset.py b/xarray/tests/test_dataset.py
index 74c4af1790d..feee69bf49a 100644
--- a/xarray/tests/test_dataset.py
+++ b/xarray/tests/test_dataset.py
@@ -11,6 +11,7 @@
 from typing import Any, Literal
 
 import numpy as np
+import numpy.testing as npt
 import pandas as pd
 import pytest
 from pandas.core.indexes.datetimes import DatetimeIndex
@@ -3461,6 +3462,22 @@ def test_expand_dims_create_index_coordinate_variable(self):
         assert expanded_no_index.coords == {"x": Variable(data=[0], dims=["x"])}
         assert expanded_no_index.indexes == {}
 
+    def test_expand_dims_create_index_from_iterable(self):
+        ds = Dataset(coords={"x": 0})
+        expanded = ds.expand_dims(x=[0, 1])
+        expected = Dataset({"x": ("x", [0, 1])})
+        assert_identical(expanded, expected)
+
+        expanded_no_index = ds.expand_dims(x=[0, 1], create_1d_index=False)
+        expected = Dataset(coords={"x": ("x", [0, 1])}).drop_indexes("x")
+
+        # TODO also can't just assert equivalence because it will fail internal invariants default indexes checks
+        # assert_identical(expanded, expected)
+        assert list(expanded_no_index.coords) == ["x"]
+        assert isinstance(expanded_no_index.coords["x"].variable, Variable)
+        npt.assert_array_equal(expanded_no_index.coords["x"].data, np.array([0, 1]))
+        assert expanded_no_index.indexes == {}
+
     @requires_pandas_version_two
     def test_expand_dims_non_nanosecond_conversion(self) -> None:
         # Regression test for https://github.com/pydata/xarray/issues/7493#issuecomment-1953091000

From aa813cff0d6011af81cb4a15c87e9857f35c3d26 Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Sat, 20 Apr 2024 00:06:27 -0400
Subject: [PATCH 06/15] make test for iterable pass

---
 xarray/core/dataset.py | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/xarray/core/dataset.py b/xarray/core/dataset.py
index 9752afbd749..25cf8263bf5 100644
--- a/xarray/core/dataset.py
+++ b/xarray/core/dataset.py
@@ -4647,10 +4647,13 @@ def expand_dims(
                 # value within the dim dict to the length of the iterable
                 # for later use.
 
-                # TODO should we have an option to not create a variable here?
-                index = PandasIndex(v, k)
-                indexes[k] = index
-                variables.update(index.create_variables())
+                if create_1d_index:
+                    index = PandasIndex(v, k)
+                    indexes[k] = index
+                    name_and_new_1d_var = index.create_variables()
+                else:
+                    name_and_new_1d_var = {k: Variable(data=v, dims=k)}
+                variables.update(name_and_new_1d_var)
                 coord_names.add(k)
                 dim[k] = variables[k].size
             elif isinstance(v, int):

From e78de7d5e93649114d9aaa916bf0f4185882be9c Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Sat, 20 Apr 2024 00:36:37 -0400
Subject: [PATCH 07/15] added kwarg to dataarray

---
 xarray/core/dataarray.py | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/xarray/core/dataarray.py b/xarray/core/dataarray.py
index 509962ff80d..818275dedd3 100644
--- a/xarray/core/dataarray.py
+++ b/xarray/core/dataarray.py
@@ -2557,6 +2557,7 @@ def expand_dims(
         self,
         dim: None | Hashable | Sequence[Hashable] | Mapping[Any, Any] = None,
         axis: None | int | Sequence[int] = None,
+        create_1d_index: bool = True,
         **dim_kwargs: Any,
     ) -> Self:
         """Return a new object with an additional axis (or axes) inserted at
@@ -2566,6 +2567,9 @@ def expand_dims(
         If dim is already a scalar coordinate, it will be promoted to a 1D
         coordinate consisting of a single value.
 
+        The automatic creation of indexes to back new 1D coordinate variables
+        controlled by the create_1d_index kwarg.
+
         Parameters
         ----------
         dim : Hashable, sequence of Hashable, dict, or None, optional
@@ -2581,6 +2585,8 @@ def expand_dims(
             multiple axes are inserted. In this case, dim arguments should be
             same length list. If axis=None is passed, all the axes will be
             inserted to the start of the result array.
+        create_1d_index : bool, default is True
+            Whether to create new PandasIndex objects for any new 1D coordinate variables.
         **dim_kwargs : int or sequence or ndarray
             The keywords are arbitrary dimensions being inserted and the values
             are either the lengths of the new dims (if int is given), or their
@@ -2644,7 +2650,9 @@ def expand_dims(
             dim = {dim: 1}
 
         dim = either_dict_or_kwargs(dim, dim_kwargs, "expand_dims")
-        ds = self._to_temp_dataset().expand_dims(dim, axis)
+        ds = self._to_temp_dataset().expand_dims(
+            dim, axis, create_1d_index=create_1d_index
+        )
         return self._from_temp_dataset(ds)
 
     def set_index(

From b1329cc08c2b08cf032c6a6ddb4da8ff99bbbee7 Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Sat, 20 Apr 2024 01:22:51 -0400
Subject: [PATCH 08/15] whatsnew

---
 doc/whats-new.rst | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index 2332f7f236b..ffa6f562e41 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -29,6 +29,9 @@ New Features
   for example, will retain the object.  However, one cannot do operations that are not possible on the `ExtensionArray`
   then, such as broadcasting.
   By `Ilan Gold <https://github.com/ilan-gold>`_.
+- Added the option to avoid automatically creating 1D pandas indexes in :py:meth:`Dataset.expand_dims()`, by passing the new kwarg
+  `create_1d_index=False`. (:pull:`8960`) 
+  By `Tom Nicholas <https://github.com/TomNicholas>`_.
 
 Breaking changes
 ~~~~~~~~~~~~~~~~

From a9f7e0c7c177f16371819963f16388537fead51d Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Sat, 20 Apr 2024 05:23:22 +0000
Subject: [PATCH 09/15] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 doc/whats-new.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index ffa6f562e41..7ecbc8fc3e6 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -30,7 +30,7 @@ New Features
   then, such as broadcasting.
   By `Ilan Gold <https://github.com/ilan-gold>`_.
 - Added the option to avoid automatically creating 1D pandas indexes in :py:meth:`Dataset.expand_dims()`, by passing the new kwarg
-  `create_1d_index=False`. (:pull:`8960`) 
+  `create_1d_index=False`. (:pull:`8960`)
   By `Tom Nicholas <https://github.com/TomNicholas>`_.
 
 Breaking changes

From 62e750f46c55209b904cc14ece174fa58cce546a Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Fri, 26 Apr 2024 11:31:59 -0400
Subject: [PATCH 10/15] update tests to use private versions of assertions

---
 xarray/tests/test_dataset.py | 22 ++++++++--------------
 1 file changed, 8 insertions(+), 14 deletions(-)

diff --git a/xarray/tests/test_dataset.py b/xarray/tests/test_dataset.py
index feee69bf49a..4b1e2141750 100644
--- a/xarray/tests/test_dataset.py
+++ b/xarray/tests/test_dataset.py
@@ -11,7 +11,6 @@
 from typing import Any, Literal
 
 import numpy as np
-import numpy.testing as npt
 import pandas as pd
 import pytest
 from pandas.core.indexes.datetimes import DatetimeIndex
@@ -3438,13 +3437,10 @@ def test_expand_dims_create_index_data_variable(self, create_1d_index_flag):
         ds = Dataset({"x": 0})
         expanded = ds.expand_dims("x", create_1d_index=create_1d_index_flag)
 
-        # TODO I can't just create the expected dataset directly using constructor because of GH issue 8959
-        # expected = Dataset(data_vars={"x": ("x", [0])})
+        # TODO Can't just create the expected dataset directly using constructor because of GH issue 8959
         expected = Dataset({"x": ("x", [0])}).drop_indexes("x").reset_coords("x")
 
-        # TODO also can't just assert equivalence because it will fail internal invariants default indexes checks
-        # assert_identical(expanded, expected)
-        assert expected.data_vars == {"x": Variable(data=[0], dims=["x"])}
+        assert_identical(expanded, expected, check_default_indexes=False)
         assert expanded.indexes == {}
 
     def test_expand_dims_create_index_coordinate_variable(self):
@@ -3455,11 +3451,11 @@ def test_expand_dims_create_index_coordinate_variable(self):
         assert_identical(expanded, expected)
 
         expanded_no_index = ds.expand_dims("x", create_1d_index=False)
+
+        # TODO Can't just create the expected dataset directly using constructor because of GH issue 8959
         expected = Dataset(coords={"x": ("x", [0])}).drop_indexes("x")
 
-        # TODO also can't just assert equivalence because it will fail internal invariants default indexes checks
-        # assert_identical(expanded, expected)
-        assert expanded_no_index.coords == {"x": Variable(data=[0], dims=["x"])}
+        assert_identical(expanded_no_index, expected, check_default_indexes=False)
         assert expanded_no_index.indexes == {}
 
     def test_expand_dims_create_index_from_iterable(self):
@@ -3469,13 +3465,11 @@ def test_expand_dims_create_index_from_iterable(self):
         assert_identical(expanded, expected)
 
         expanded_no_index = ds.expand_dims(x=[0, 1], create_1d_index=False)
+
+        # TODO Can't just create the expected dataset directly using constructor because of GH issue 8959
         expected = Dataset(coords={"x": ("x", [0, 1])}).drop_indexes("x")
 
-        # TODO also can't just assert equivalence because it will fail internal invariants default indexes checks
-        # assert_identical(expanded, expected)
-        assert list(expanded_no_index.coords) == ["x"]
-        assert isinstance(expanded_no_index.coords["x"].variable, Variable)
-        npt.assert_array_equal(expanded_no_index.coords["x"].data, np.array([0, 1]))
+        assert_identical(expanded, expected, check_default_indexes=False)
         assert expanded_no_index.indexes == {}
 
     @requires_pandas_version_two

From f86c82f1c9efdf0b207f1fb537aea2cefe287c82 Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Fri, 26 Apr 2024 11:34:37 -0400
Subject: [PATCH 11/15] create_1d_index->create_index

---
 xarray/core/dataarray.py     | 10 ++++------
 xarray/core/dataset.py       | 10 +++++-----
 xarray/tests/test_dataset.py | 12 ++++++------
 3 files changed, 15 insertions(+), 17 deletions(-)

diff --git a/xarray/core/dataarray.py b/xarray/core/dataarray.py
index 818275dedd3..41c9af1bb10 100644
--- a/xarray/core/dataarray.py
+++ b/xarray/core/dataarray.py
@@ -2557,7 +2557,7 @@ def expand_dims(
         self,
         dim: None | Hashable | Sequence[Hashable] | Mapping[Any, Any] = None,
         axis: None | int | Sequence[int] = None,
-        create_1d_index: bool = True,
+        create_index: bool = True,
         **dim_kwargs: Any,
     ) -> Self:
         """Return a new object with an additional axis (or axes) inserted at
@@ -2568,7 +2568,7 @@ def expand_dims(
         coordinate consisting of a single value.
 
         The automatic creation of indexes to back new 1D coordinate variables
-        controlled by the create_1d_index kwarg.
+        controlled by the create_index kwarg.
 
         Parameters
         ----------
@@ -2585,7 +2585,7 @@ def expand_dims(
             multiple axes are inserted. In this case, dim arguments should be
             same length list. If axis=None is passed, all the axes will be
             inserted to the start of the result array.
-        create_1d_index : bool, default is True
+        create_index : bool, default is True
             Whether to create new PandasIndex objects for any new 1D coordinate variables.
         **dim_kwargs : int or sequence or ndarray
             The keywords are arbitrary dimensions being inserted and the values
@@ -2650,9 +2650,7 @@ def expand_dims(
             dim = {dim: 1}
 
         dim = either_dict_or_kwargs(dim, dim_kwargs, "expand_dims")
-        ds = self._to_temp_dataset().expand_dims(
-            dim, axis, create_1d_index=create_1d_index
-        )
+        ds = self._to_temp_dataset().expand_dims(dim, axis, create_index=create_index)
         return self._from_temp_dataset(ds)
 
     def set_index(
diff --git a/xarray/core/dataset.py b/xarray/core/dataset.py
index 25cf8263bf5..a2bc3505366 100644
--- a/xarray/core/dataset.py
+++ b/xarray/core/dataset.py
@@ -4497,7 +4497,7 @@ def expand_dims(
         self,
         dim: None | Hashable | Sequence[Hashable] | Mapping[Any, Any] = None,
         axis: None | int | Sequence[int] = None,
-        create_1d_index: bool = True,
+        create_index: bool = True,
         **dim_kwargs: Any,
     ) -> Self:
         """Return a new object with an additional axis (or axes) inserted at
@@ -4508,7 +4508,7 @@ def expand_dims(
         coordinate consisting of a single value.
 
         The automatic creation of indexes to back new 1D coordinate variables
-        controlled by the create_1d_index kwarg.
+        controlled by the create_index kwarg.
 
         Parameters
         ----------
@@ -4525,7 +4525,7 @@ def expand_dims(
             multiple axes are inserted. In this case, dim arguments should be
             same length list. If axis=None is passed, all the axes will be
             inserted to the start of the result array.
-        create_1d_index : bool, default is True
+        create_index : bool, default is True
             Whether to create new PandasIndex objects for any new 1D coordinate variables.
         **dim_kwargs : int or sequence or ndarray
             The keywords are arbitrary dimensions being inserted and the values
@@ -4647,7 +4647,7 @@ def expand_dims(
                 # value within the dim dict to the length of the iterable
                 # for later use.
 
-                if create_1d_index:
+                if create_index:
                     index = PandasIndex(v, k)
                     indexes[k] = index
                     name_and_new_1d_var = index.create_variables()
@@ -4689,7 +4689,7 @@ def expand_dims(
                     variables[k] = v.set_dims(dict(all_dims))
             else:
                 if k not in variables:
-                    if k in coord_names and create_1d_index:
+                    if k in coord_names and create_index:
                         # If dims includes a label of a non-dimension coordinate,
                         # it will be promoted to a 1D coordinate with a single value.
                         index, index_vars = create_default_index_implicit(v.set_dims(k))
diff --git a/xarray/tests/test_dataset.py b/xarray/tests/test_dataset.py
index 4b1e2141750..e4251ced7a4 100644
--- a/xarray/tests/test_dataset.py
+++ b/xarray/tests/test_dataset.py
@@ -3431,11 +3431,11 @@ def test_expand_dims_kwargs_python36plus(self) -> None:
         )
         assert_identical(other_way_expected, other_way)
 
-    @pytest.mark.parametrize("create_1d_index_flag", [True, False])
-    def test_expand_dims_create_index_data_variable(self, create_1d_index_flag):
+    @pytest.mark.parametrize("create_index_flag", [True, False])
+    def test_expand_dims_create_index_data_variable(self, create_index_flag):
         # data variables should not gain an index ever
         ds = Dataset({"x": 0})
-        expanded = ds.expand_dims("x", create_1d_index=create_1d_index_flag)
+        expanded = ds.expand_dims("x", create_index=create_index_flag)
 
         # TODO Can't just create the expected dataset directly using constructor because of GH issue 8959
         expected = Dataset({"x": ("x", [0])}).drop_indexes("x").reset_coords("x")
@@ -3444,13 +3444,13 @@ def test_expand_dims_create_index_data_variable(self, create_1d_index_flag):
         assert expanded.indexes == {}
 
     def test_expand_dims_create_index_coordinate_variable(self):
-        # coordinate variables should gain an index only if create_1d_index is True (the default)
+        # coordinate variables should gain an index only if create_index is True (the default)
         ds = Dataset(coords={"x": 0})
         expanded = ds.expand_dims("x")
         expected = Dataset({"x": ("x", [0])})
         assert_identical(expanded, expected)
 
-        expanded_no_index = ds.expand_dims("x", create_1d_index=False)
+        expanded_no_index = ds.expand_dims("x", create_index=False)
 
         # TODO Can't just create the expected dataset directly using constructor because of GH issue 8959
         expected = Dataset(coords={"x": ("x", [0])}).drop_indexes("x")
@@ -3464,7 +3464,7 @@ def test_expand_dims_create_index_from_iterable(self):
         expected = Dataset({"x": ("x", [0, 1])})
         assert_identical(expanded, expected)
 
-        expanded_no_index = ds.expand_dims(x=[0, 1], create_1d_index=False)
+        expanded_no_index = ds.expand_dims(x=[0, 1], create_index=False)
 
         # TODO Can't just create the expected dataset directly using constructor because of GH issue 8959
         expected = Dataset(coords={"x": ("x", [0, 1])}).drop_indexes("x")

From d5d90fdd27673537d47d7acae883571e0604ee91 Mon Sep 17 00:00:00 2001
From: Tom Nicholas <tom@cworthy.org>
Date: Fri, 26 Apr 2024 09:38:47 -0600
Subject: [PATCH 12/15] Update doc/whats-new.rst

Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>
---
 doc/whats-new.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index ce05e41243a..2231f0a7d4c 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -30,7 +30,7 @@ New Features
   then, such as broadcasting.
   By `Ilan Gold <https://github.com/ilan-gold>`_.
 - Added the option to avoid automatically creating 1D pandas indexes in :py:meth:`Dataset.expand_dims()`, by passing the new kwarg
-  `create_1d_index=False`. (:pull:`8960`)
+  `create_index=False`. (:pull:`8960`)
   By `Tom Nicholas <https://github.com/TomNicholas>`_.
 
 Breaking changes

From 7e8f895c2746e2a75979b5675967b434aef09ee6 Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Fri, 26 Apr 2024 23:31:08 -0400
Subject: [PATCH 13/15] warn if create_index=True but no index created because
 dimension variable was a data var not a coord

---
 xarray/core/dataset.py       | 7 +++++++
 xarray/tests/test_dataset.py | 7 ++++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/xarray/core/dataset.py b/xarray/core/dataset.py
index a2bc3505366..3985d2cad1a 100644
--- a/xarray/core/dataset.py
+++ b/xarray/core/dataset.py
@@ -4696,6 +4696,13 @@ def expand_dims(
                         indexes[k] = index
                         variables.update(index_vars)
                     else:
+                        if create_index:
+                            warnings.warn(
+                                f"No index created for dimension {k} because variable {k} is not a coordinate. "
+                                f"To create an index for {k}, please first call `.set_coords({k})` on this object.",
+                                UserWarning,
+                            )
+
                         # create 1D variable without creating a new index
                         new_1d_var = v.set_dims(k)
                         variables.update({k: new_1d_var})
diff --git a/xarray/tests/test_dataset.py b/xarray/tests/test_dataset.py
index e4251ced7a4..17ef63d826e 100644
--- a/xarray/tests/test_dataset.py
+++ b/xarray/tests/test_dataset.py
@@ -3435,7 +3435,12 @@ def test_expand_dims_kwargs_python36plus(self) -> None:
     def test_expand_dims_create_index_data_variable(self, create_index_flag):
         # data variables should not gain an index ever
         ds = Dataset({"x": 0})
-        expanded = ds.expand_dims("x", create_index=create_index_flag)
+
+        if create_index_flag:
+            with pytest.warns(UserWarning, match="No index created"):
+                expanded = ds.expand_dims("x", create_index=create_index_flag)
+        else:
+            expanded = ds.expand_dims("x", create_index=create_index_flag)
 
         # TODO Can't just create the expected dataset directly using constructor because of GH issue 8959
         expected = Dataset({"x": ("x", [0])}).drop_indexes("x").reset_coords("x")

From ed85446d54ea8791f9224b71da833fd0f5d9c1eb Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Fri, 26 Apr 2024 23:32:53 -0400
Subject: [PATCH 14/15] add string marks in warning message

---
 xarray/core/dataset.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xarray/core/dataset.py b/xarray/core/dataset.py
index 3985d2cad1a..4f9125a1ab0 100644
--- a/xarray/core/dataset.py
+++ b/xarray/core/dataset.py
@@ -4699,7 +4699,7 @@ def expand_dims(
                         if create_index:
                             warnings.warn(
                                 f"No index created for dimension {k} because variable {k} is not a coordinate. "
-                                f"To create an index for {k}, please first call `.set_coords({k})` on this object.",
+                                f"To create an index for {k}, please first call `.set_coords('{k}')` on this object.",
                                 UserWarning,
                             )
 

From 6bde1587a0da18f856c1f1e12425475ce8f99d30 Mon Sep 17 00:00:00 2001
From: TomNicholas <tom@cworthy.org>
Date: Sat, 27 Apr 2024 12:27:15 -0400
Subject: [PATCH 15/15] fix bad merge

---
 xarray/tests/test_dataset.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/xarray/tests/test_dataset.py b/xarray/tests/test_dataset.py
index 79e4d74f84a..40cf85484da 100644
--- a/xarray/tests/test_dataset.py
+++ b/xarray/tests/test_dataset.py
@@ -3476,7 +3476,6 @@ def test_expand_dims_create_index_from_iterable(self):
         assert_identical(expanded, expected, check_default_indexes=False)
         assert expanded_no_index.indexes == {}
 
-    @requires_pandas_version_two
     def test_expand_dims_non_nanosecond_conversion(self) -> None:
         # Regression test for https://github.com/pydata/xarray/issues/7493#issuecomment-1953091000
         with pytest.warns(UserWarning, match="non-nanosecond precision"):