From 5affeb7a535ac78645f1b3740af126762dcf8104 Mon Sep 17 00:00:00 2001
From: Giacomo Caria <44147817+gcaria@users.noreply.github.com>
Date: Mon, 8 Nov 2021 23:28:31 +0100
Subject: [PATCH 1/3] Fix bug and add test

---
 xarray/core/variable.py        |  2 ++
 xarray/tests/test_dataarray.py | 10 ++++++++++
 2 files changed, 12 insertions(+)

diff --git a/xarray/core/variable.py b/xarray/core/variable.py
index a96adb31e64..64b73bfeacb 100644
--- a/xarray/core/variable.py
+++ b/xarray/core/variable.py
@@ -2418,6 +2418,8 @@ def _inplace_binary_op(self, other, f):
         if dims != self.dims:
             raise ValueError("dimensions cannot change for in-place operations")
         with np.errstate(all="ignore"):
+            if isinstance(self_data, np.ndarray):
+                self.values = f(self_data.copy(), other_data)
             self.values = f(self_data, other_data)
         return self
 
diff --git a/xarray/tests/test_dataarray.py b/xarray/tests/test_dataarray.py
index 53c650046e7..cc3f638ea1c 100644
--- a/xarray/tests/test_dataarray.py
+++ b/xarray/tests/test_dataarray.py
@@ -1987,6 +1987,16 @@ def test_inplace_math_basics(self):
         assert_array_equal(b.values, x)
         assert source_ndarray(b.values) is x
 
+    def test_inplace_math_error(self):
+        data = np.random.rand(4)
+        times = np.arange(4)
+        foo = DataArray(data, coords=[times], dims=["time"])
+        b = times.copy()
+        with pytest.raises(ValueError, match=r"Cannot assign to the .values"):
+            foo.coords["time"] += 1
+        # Check error throwing prevented inplace operation
+        assert_array_equal(foo.coords["time"], b)
+
     def test_inplace_math_automatic_alignment(self):
         a = DataArray(range(5), [("x", range(5))])
         b = DataArray(range(1, 6), [("x", range(1, 6))])

From 48c6ee2242649caf8441cf759c6338bc44ae1fa1 Mon Sep 17 00:00:00 2001
From: Giacomo Caria <44147817+gcaria@users.noreply.github.com>
Date: Mon, 8 Nov 2021 23:59:28 +0100
Subject: [PATCH 2/3] Add comment

---
 xarray/core/variable.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xarray/core/variable.py b/xarray/core/variable.py
index 64b73bfeacb..3b2f5b4638e 100644
--- a/xarray/core/variable.py
+++ b/xarray/core/variable.py
@@ -2418,6 +2418,8 @@ def _inplace_binary_op(self, other, f):
         if dims != self.dims:
             raise ValueError("dimensions cannot change for in-place operations")
         with np.errstate(all="ignore"):
+            # Give a chance to Variable.values.setter to throw error if needed
+            # without updating self_data inplace
             if isinstance(self_data, np.ndarray):
                 self.values = f(self_data.copy(), other_data)
             self.values = f(self_data, other_data)

From b3541198d9a489aecf80e889aa1681c6210da059 Mon Sep 17 00:00:00 2001
From: Giacomo Caria <44147817+gcaria@users.noreply.github.com>
Date: Tue, 9 Nov 2021 19:53:35 +0100
Subject: [PATCH 3/3] Edit only _inplace_binary_op

---
 xarray/core/variable.py        | 9 +++++----
 xarray/tests/test_dataarray.py | 4 +++-
 xarray/tests/test_variable.py  | 8 ++++++++
 3 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/xarray/core/variable.py b/xarray/core/variable.py
index 3b2f5b4638e..52125ec4113 100644
--- a/xarray/core/variable.py
+++ b/xarray/core/variable.py
@@ -2418,10 +2418,6 @@ def _inplace_binary_op(self, other, f):
         if dims != self.dims:
             raise ValueError("dimensions cannot change for in-place operations")
         with np.errstate(all="ignore"):
-            # Give a chance to Variable.values.setter to throw error if needed
-            # without updating self_data inplace
-            if isinstance(self_data, np.ndarray):
-                self.values = f(self_data.copy(), other_data)
             self.values = f(self_data, other_data)
         return self
 
@@ -2817,6 +2813,11 @@ def name(self):
     def name(self, value):
         raise AttributeError("cannot modify name of IndexVariable in-place")
 
+    def _inplace_binary_op(self, other, f):
+        raise TypeError(
+            "Values of an IndexVariable are immutable and can not be modified inplace"
+        )
+
 
 # for backwards compatibility
 Coordinate = utils.alias(IndexVariable, "Coordinate")
diff --git a/xarray/tests/test_dataarray.py b/xarray/tests/test_dataarray.py
index cc3f638ea1c..5e9c1b87ce2 100644
--- a/xarray/tests/test_dataarray.py
+++ b/xarray/tests/test_dataarray.py
@@ -1992,7 +1992,9 @@ def test_inplace_math_error(self):
         times = np.arange(4)
         foo = DataArray(data, coords=[times], dims=["time"])
         b = times.copy()
-        with pytest.raises(ValueError, match=r"Cannot assign to the .values"):
+        with pytest.raises(
+            TypeError, match=r"Values of an IndexVariable are immutable"
+        ):
             foo.coords["time"] += 1
         # Check error throwing prevented inplace operation
         assert_array_equal(foo.coords["time"], b)
diff --git a/xarray/tests/test_variable.py b/xarray/tests/test_variable.py
index 9c0e45c5da9..3267af8b45b 100644
--- a/xarray/tests/test_variable.py
+++ b/xarray/tests/test_variable.py
@@ -1656,6 +1656,14 @@ def test_inplace_math(self):
         with pytest.raises(ValueError, match=r"dimensions cannot change"):
             v += Variable("y", np.arange(5))
 
+    def test_inplace_math_error(self):
+        x = np.arange(5)
+        v = IndexVariable(["x"], x)
+        with pytest.raises(
+            TypeError, match=r"Values of an IndexVariable are immutable"
+        ):
+            v += 1
+
     def test_reduce(self):
         v = Variable(["x", "y"], self.d, {"ignored": "attributes"})
         assert_identical(v.reduce(np.std, "x"), Variable(["y"], self.d.std(axis=0)))