From e5a6632e1f879726ddf30a53eec8cf2ddb3b04b0 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 17 Jan 2020 14:33:45 +0100
Subject: [PATCH 01/20] get fillna tests to pass

---
 xarray/core/duck_array_ops.py | 2 +-
 xarray/tests/test_units.py    | 7 +------
 2 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/xarray/core/duck_array_ops.py b/xarray/core/duck_array_ops.py
index 98b371ab7c3..3770ccd0868 100644
--- a/xarray/core/duck_array_ops.py
+++ b/xarray/core/duck_array_ops.py
@@ -261,7 +261,7 @@ def where_method(data, cond, other=dtypes.NA):
 
 
 def fillna(data, other):
-    return where(isnull(data), other, data)
+    return where(notnull(data), data, other)
 
 
 def concatenate(arrays, axis=0):
diff --git a/xarray/tests/test_units.py b/xarray/tests/test_units.py
index 2cb1550c088..f16815b64c5 100644
--- a/xarray/tests/test_units.py
+++ b/xarray/tests/test_units.py
@@ -1545,12 +1545,7 @@ def test_missing_value_detection(self, func):
                 unit_registry.dimensionless, DimensionalityError, id="dimensionless"
             ),
             pytest.param(unit_registry.s, DimensionalityError, id="incompatible_unit"),
-            pytest.param(
-                unit_registry.cm,
-                None,
-                id="compatible_unit",
-                marks=pytest.mark.xfail(reason="converts to fill value's unit"),
-            ),
+            pytest.param(unit_registry.cm, None, id="compatible_unit"),
             pytest.param(unit_registry.m, None, id="identical_unit"),
         ),
     )

From 9414fe36d62c0898669868f203a10f2223dce82a Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 17 Jan 2020 14:38:35 +0100
Subject: [PATCH 02/20] get the _getitem_with_mask tests to pass

---
 xarray/core/variable.py    | 6 +++++-
 xarray/tests/test_units.py | 9 +--------
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/xarray/core/variable.py b/xarray/core/variable.py
index 0a9d0767b77..31ec0831c35 100644
--- a/xarray/core/variable.py
+++ b/xarray/core/variable.py
@@ -742,7 +742,11 @@ def _getitem_with_mask(self, key, fill_value=dtypes.NA):
 
             data = as_indexable(self._data)[actual_indexer]
             mask = indexing.create_mask(indexer, self.shape, data)
-            data = duck_array_ops.where(mask, fill_value, data)
+            if isinstance(mask, bool):
+                mask = not mask
+            else:
+                mask = ~mask
+            data = duck_array_ops.where(mask, data, fill_value)
         else:
             # array cannot be indexed along dimensions of size 0, so just
             # build the mask directly instead.
diff --git a/xarray/tests/test_units.py b/xarray/tests/test_units.py
index f16815b64c5..06993c0a873 100644
--- a/xarray/tests/test_units.py
+++ b/xarray/tests/test_units.py
@@ -1756,14 +1756,7 @@ def test_1d_math(self, func, unit, error, dtype):
                 unit_registry.dimensionless, DimensionalityError, id="dimensionless"
             ),
             pytest.param(unit_registry.s, DimensionalityError, id="incompatible_unit"),
-            pytest.param(
-                unit_registry.cm,
-                None,
-                id="compatible_unit",
-                marks=pytest.mark.xfail(
-                    reason="getitem_with_mask converts to the unit of other"
-                ),
-            ),
+            pytest.param(unit_registry.cm, None, id="compatible_unit"),
             pytest.param(unit_registry.m, None, id="identical_unit"),
         ),
     )

From 12d2fe4b98eba684e3496a893edc1ae6571e5919 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 17 Jan 2020 22:51:16 +0100
Subject: [PATCH 03/20] silence the behavior change warning of pint

---
 xarray/tests/test_units.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/xarray/tests/test_units.py b/xarray/tests/test_units.py
index 06993c0a873..80a75b9e6ba 100644
--- a/xarray/tests/test_units.py
+++ b/xarray/tests/test_units.py
@@ -1,4 +1,5 @@
 import operator
+import warnings
 
 import numpy as np
 import pandas as pd
@@ -18,6 +19,13 @@
 unit_registry = pint.UnitRegistry(force_ndarray=True)
 Quantity = unit_registry.Quantity
 
+
+# silence pint's BehaviorChangeWarning
+with warnings.catch_warnings():
+    warnings.simplefilter("ignore")
+    Quantity([])
+
+
 pytestmark = [
     pytest.mark.skipif(
         not IS_NEP18_ACTIVE, reason="NUMPY_EXPERIMENTAL_ARRAY_FUNCTION is not enabled"

From 077d67abcf976c4ba6089813104bb09051c8ef31 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 17 Jan 2020 22:53:01 +0100
Subject: [PATCH 04/20] don't use 0 as fill value since that has special
 behaviour

---
 xarray/tests/test_units.py | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/xarray/tests/test_units.py b/xarray/tests/test_units.py
index 80a75b9e6ba..c0dfd1f51ff 100644
--- a/xarray/tests/test_units.py
+++ b/xarray/tests/test_units.py
@@ -1543,12 +1543,7 @@ def test_missing_value_detection(self, func):
     @pytest.mark.parametrize(
         "unit,error",
         (
-            pytest.param(
-                1,
-                DimensionalityError,
-                id="no_unit",
-                marks=pytest.mark.xfail(reason="uses 0 as a replacement"),
-            ),
+            pytest.param(1, DimensionalityError, id="no_unit"),
             pytest.param(
                 unit_registry.dimensionless, DimensionalityError, id="dimensionless"
             ),
@@ -1558,7 +1553,7 @@ def test_missing_value_detection(self, func):
         ),
     )
     def test_missing_value_fillna(self, unit, error):
-        value = 0
+        value = 10
         array = (
             np.array(
                 [

From e03fac86e081435c79f6c9f3cf18c75557a8bef1 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Sun, 19 Jan 2020 19:33:09 +0100
Subject: [PATCH 05/20] use concat as a class method

---
 xarray/tests/test_units.py | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/xarray/tests/test_units.py b/xarray/tests/test_units.py
index c0dfd1f51ff..90cef388604 100644
--- a/xarray/tests/test_units.py
+++ b/xarray/tests/test_units.py
@@ -1944,16 +1944,19 @@ def test_concat(self, unit, error, dtype):
 
         if error is not None:
             with pytest.raises(error):
-                variable.concat(other)
+                xr.Variable.concat([variable, other], dim="y")
 
             return
 
         units = extract_units(variable)
         expected = attach_units(
-            strip_units(variable).concat(strip_units(convert_units(other, units))),
+            xr.Variable.concat(
+                [strip_units(variable), strip_units(convert_units(other, units))],
+                dim="y",
+            ),
             units,
         )
-        actual = variable.concat(other)
+        actual = xr.Variable.concat([variable, other], dim="y")
 
         assert_units_equal(expected, actual)
         xr.testing.assert_identical(expected, actual)

From 6b65a76aea21d3287bc054bd2755fc65daeae85a Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Sun, 19 Jan 2020 23:34:43 +0100
Subject: [PATCH 06/20] use np.pad after trimming instead of concatenating a
 filled array

---
 xarray/core/variable.py    | 28 ++++++++++------------------
 xarray/tests/test_units.py |  7 +------
 2 files changed, 11 insertions(+), 24 deletions(-)

diff --git a/xarray/core/variable.py b/xarray/core/variable.py
index 31ec0831c35..5a7ceed52bf 100644
--- a/xarray/core/variable.py
+++ b/xarray/core/variable.py
@@ -1103,24 +1103,16 @@ def _shift_one_dim(self, dim, count, fill_value=dtypes.NA):
         else:
             dtype = self.dtype
 
-        shape = list(self.shape)
-        shape[axis] = min(abs(count), shape[axis])
-
-        if isinstance(trimmed_data, dask_array_type):
-            chunks = list(trimmed_data.chunks)
-            chunks[axis] = (shape[axis],)
-            full = functools.partial(da.full, chunks=chunks)
-        else:
-            full = np.full
-
-        filler = full(shape, fill_value, dtype=dtype)
-
-        if count > 0:
-            arrays = [filler, trimmed_data]
-        else:
-            arrays = [trimmed_data, filler]
-
-        data = duck_array_ops.concatenate(arrays, axis)
+        width = min(abs(count), self.shape[axis])
+        dim_pad = (width, 0) if count >= 0 else (0, width)
+        pads = [(0, 0) if d != dim else dim_pad for d in self.dims]
+
+        data = np.pad(
+            trimmed_data.astype(dtype),
+            pads,
+            mode="constant",
+            constant_values=fill_value,
+        )
 
         if isinstance(data, dask_array_type):
             # chunked data should come out with the same chunks; this makes
diff --git a/xarray/tests/test_units.py b/xarray/tests/test_units.py
index 90cef388604..35cde972f2b 100644
--- a/xarray/tests/test_units.py
+++ b/xarray/tests/test_units.py
@@ -1840,12 +1840,7 @@ def test_squeeze(self, dtype):
             ),
             method("reduce", np.std, "x"),
             method("round", 2),
-            pytest.param(
-                method("shift", {"x": -2}),
-                marks=pytest.mark.xfail(
-                    reason="trying to concatenate ndarray to quantity"
-                ),
-            ),
+            method("shift", {"x": -2}),
             method("transpose", "y", "x"),
         ),
         ids=repr,

From 88320ef488072a66afb3d175889e8838d00e4718 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Sun, 19 Jan 2020 23:41:05 +0100
Subject: [PATCH 07/20] rewrite the concat test to pass appropriate arrays

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

diff --git a/xarray/tests/test_units.py b/xarray/tests/test_units.py
index 35cde972f2b..f8d56b342f1 100644
--- a/xarray/tests/test_units.py
+++ b/xarray/tests/test_units.py
@@ -1915,7 +1915,6 @@ def test_unstack(self, dtype):
         assert_units_equal(expected, actual)
         xr.testing.assert_identical(expected, actual)
 
-    @pytest.mark.xfail(reason="ignores units")
     @pytest.mark.parametrize(
         "unit,error",
         (
@@ -1930,12 +1929,12 @@ def test_unstack(self, dtype):
     )
     def test_concat(self, unit, error, dtype):
         array1 = (
-            np.linspace(0, 5, 3 * 10).reshape(3, 10).astype(dtype) * unit_registry.m
+            np.linspace(0, 5, 9 * 10).reshape(3, 6, 5).astype(dtype) * unit_registry.m
         )
-        array2 = np.linspace(5, 10, 10 * 2).reshape(10, 2).astype(dtype) * unit
+        array2 = np.linspace(5, 10, 10 * 3).reshape(3, 2, 5).astype(dtype) * unit
 
-        variable = xr.Variable(("x", "y"), array1)
-        other = xr.Variable(("y", "z"), array2)
+        variable = xr.Variable(("x", "y", "z"), array1)
+        other = xr.Variable(("x", "y", "z"), array2)
 
         if error is not None:
             with pytest.raises(error):

From 1e07dce4b8fd59057d22ae3d7884a1114c78e22a Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Mon, 20 Jan 2020 02:38:45 +0100
Subject: [PATCH 08/20] use da.pad when dealing with dask arrays

---
 xarray/core/variable.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/xarray/core/variable.py b/xarray/core/variable.py
index 5a7ceed52bf..314f7096fe0 100644
--- a/xarray/core/variable.py
+++ b/xarray/core/variable.py
@@ -1107,7 +1107,12 @@ def _shift_one_dim(self, dim, count, fill_value=dtypes.NA):
         dim_pad = (width, 0) if count >= 0 else (0, width)
         pads = [(0, 0) if d != dim else dim_pad for d in self.dims]
 
-        data = np.pad(
+        if isinstance(trimmed_data, dask_array_type):
+            pad_func = da.pad
+        else:
+            pad_func = np.pad
+
+        data = pad_func(
             trimmed_data.astype(dtype),
             pads,
             mode="constant",

From ce572de9012261c259a09db46035704a0fbf8725 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Wed, 22 Jan 2020 20:12:42 +0100
Subject: [PATCH 09/20] mark the failing pad tests as xfail when on a current
 pint version

---
 xarray/tests/test_units.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xarray/tests/test_units.py b/xarray/tests/test_units.py
index abbe20a6355..12e574f9219 100644
--- a/xarray/tests/test_units.py
+++ b/xarray/tests/test_units.py
@@ -1,3 +1,4 @@
+from distutils.version import LooseVersion
 import operator
 import warnings
 
@@ -2029,7 +2030,8 @@ def test_no_conflicts(self, unit, dtype):
                 DimensionalityError,
                 id="no_unit",
                 marks=pytest.mark.xfail(
-                    reason="is not treated the same as dimensionless"
+                    LooseVersion(pint.__version__) < LooseVersion("0.10.2"),
+                    reason="bug in pint's implementation of np.pad",
                 ),
             ),
             pytest.param(

From 3d16f2e6c40c9b984f207b9b54c127a25ee4a4ca Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Wed, 5 Feb 2020 15:58:05 +0100
Subject: [PATCH 10/20] update whats-new.rst

---
 doc/whats-new.rst | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index 7fa70d0b67a..f0c7f99c02d 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -24,6 +24,8 @@ Breaking changes
 
 New Features
 ~~~~~~~~~~~~
+- implement pint support. (:issue:`3594`, :pull:`3706`)
+  By `Justus Magin <https://github.com/keewis>`_.
 
 Bug fixes
 ~~~~~~~~~

From a8cf968f6d009f8e1e22867c5b0d266feacc6946 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Wed, 5 Feb 2020 16:08:00 +0100
Subject: [PATCH 11/20] fix the import order

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

diff --git a/xarray/tests/test_units.py b/xarray/tests/test_units.py
index ca9b87ad978..bf605a08d46 100644
--- a/xarray/tests/test_units.py
+++ b/xarray/tests/test_units.py
@@ -1,6 +1,6 @@
-from distutils.version import LooseVersion
 import operator
 import warnings
+from distutils.version import LooseVersion
 
 import numpy as np
 import pandas as pd

From 98e2b40654ee9cabe7c4c43443088b4c0aad31ba Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Wed, 5 Feb 2020 17:32:56 +0100
Subject: [PATCH 12/20] test using pint master

---
 ci/azure/install.yml | 1 +
 1 file changed, 1 insertion(+)

diff --git a/ci/azure/install.yml b/ci/azure/install.yml
index e11a8b54db3..9ae6dcdb102 100644
--- a/ci/azure/install.yml
+++ b/ci/azure/install.yml
@@ -29,6 +29,7 @@ steps:
         git+https://github.com/zarr-developers/zarr \
         git+https://github.com/Unidata/cftime \
         git+https://github.com/mapbox/rasterio \
+        git+https://github.com/hgrecco/pint
         git+https://github.com/pydata/bottleneck
   condition: eq(variables['UPSTREAM_DEV'], 'true')
   displayName: Install upstream dev dependencies

From 05946b1ef25683a38a594df4138780ad46c073bb Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Wed, 5 Feb 2020 17:43:36 +0100
Subject: [PATCH 13/20] fix the install command

---
 ci/azure/install.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ci/azure/install.yml b/ci/azure/install.yml
index 9ae6dcdb102..958e3c180fa 100644
--- a/ci/azure/install.yml
+++ b/ci/azure/install.yml
@@ -29,7 +29,7 @@ steps:
         git+https://github.com/zarr-developers/zarr \
         git+https://github.com/Unidata/cftime \
         git+https://github.com/mapbox/rasterio \
-        git+https://github.com/hgrecco/pint
+        git+https://github.com/hgrecco/pint \
         git+https://github.com/pydata/bottleneck
   condition: eq(variables['UPSTREAM_DEV'], 'true')
   displayName: Install upstream dev dependencies

From 8c490ee0edb1133d6fe7571bce5c3e5c68e4d9b0 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Wed, 5 Feb 2020 18:45:27 +0100
Subject: [PATCH 14/20] reimplement the pad test to really work with units

---
 xarray/tests/test_units.py | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/xarray/tests/test_units.py b/xarray/tests/test_units.py
index bf605a08d46..a19e253a849 100644
--- a/xarray/tests/test_units.py
+++ b/xarray/tests/test_units.py
@@ -2025,6 +2025,43 @@ def test_no_conflicts(self, unit, dtype):
 
         assert expected == actual
 
+    def test_pad(self, dtype):
+        data = np.arange(4 * 3 * 2).reshape(4, 3, 2).astype(dtype) * unit_registry.m
+        v = xr.Variable(["x", "y", "z"], data)
+
+        xr_args = [{"x": (2, 1)}, {"y": (0, 3)}, {"x": (3, 1), "z": (2, 0)}]
+        np_args = [
+            ((2, 1), (0, 0), (0, 0)),
+            ((0, 0), (0, 3), (0, 0)),
+            ((3, 1), (0, 0), (2, 0)),
+        ]
+        for xr_arg, np_arg in zip(xr_args, np_args):
+            actual = v.pad_with_fill_value(**xr_arg)
+            expected = xr.Variable(
+                v.dims,
+                np.pad(
+                    v.data.astype(float),
+                    np_arg,
+                    mode="constant",
+                    constant_values=np.nan,
+                ),
+            )
+            xr.testing.assert_identical(expected, actual)
+            assert_units_equal(expected, actual)
+            assert isinstance(actual._data, type(v._data))
+
+        # for the boolean array, we pad False
+        data = np.full_like(data, False, dtype=bool).reshape(4, 3, 2)
+        v = xr.Variable(["x", "y", "z"], data)
+        for xr_arg, np_arg in zip(xr_args, np_args):
+            actual = v.pad_with_fill_value(fill_value=data.flat[0], **xr_arg)
+            expected = xr.Variable(
+                v.dims,
+                np.pad(v.data, np_arg, mode="constant", constant_values=v.data.flat[0]),
+            )
+            xr.testing.assert_identical(actual, expected)
+            assert_units_equal(expected, actual)
+
     @pytest.mark.parametrize(
         "unit,error",
         (

From f6eca88ba43a3c71e4f16e3e7f260a42abbc78f1 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 7 Feb 2020 12:27:13 +0100
Subject: [PATCH 15/20] use np.logical_not instead

---
 xarray/core/variable.py | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/xarray/core/variable.py b/xarray/core/variable.py
index 912d53f811c..17e88d1c84a 100644
--- a/xarray/core/variable.py
+++ b/xarray/core/variable.py
@@ -742,11 +742,7 @@ def _getitem_with_mask(self, key, fill_value=dtypes.NA):
 
             data = as_indexable(self._data)[actual_indexer]
             mask = indexing.create_mask(indexer, self.shape, data)
-            if isinstance(mask, bool):
-                mask = not mask
-            else:
-                mask = ~mask
-            data = duck_array_ops.where(mask, data, fill_value)
+            data = duck_array_ops.where(np.logical_not(mask), data, fill_value)
         else:
             # array cannot be indexed along dimensions of size 0, so just
             # build the mask directly instead.

From 72241e50daabb9d39d2870d546833ee0f93b1fd2 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 7 Feb 2020 12:32:32 +0100
Subject: [PATCH 16/20] use duck_array_ops to provide pad

---
 xarray/core/duck_array_ops.py | 1 +
 xarray/core/variable.py       | 7 +------
 2 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/xarray/core/duck_array_ops.py b/xarray/core/duck_array_ops.py
index 991c5985b2b..e0ed0f7db95 100644
--- a/xarray/core/duck_array_ops.py
+++ b/xarray/core/duck_array_ops.py
@@ -121,6 +121,7 @@ def notnull(data):
 isin = _dask_or_eager_func("isin", array_args=slice(2))
 take = _dask_or_eager_func("take")
 broadcast_to = _dask_or_eager_func("broadcast_to")
+pad = _dask_or_eager_func("pad")
 
 _concatenate = _dask_or_eager_func("concatenate", list_of_args=True)
 _stack = _dask_or_eager_func("stack", list_of_args=True)
diff --git a/xarray/core/variable.py b/xarray/core/variable.py
index 17e88d1c84a..6d1429df235 100644
--- a/xarray/core/variable.py
+++ b/xarray/core/variable.py
@@ -1103,12 +1103,7 @@ def _shift_one_dim(self, dim, count, fill_value=dtypes.NA):
         dim_pad = (width, 0) if count >= 0 else (0, width)
         pads = [(0, 0) if d != dim else dim_pad for d in self.dims]
 
-        if isinstance(trimmed_data, dask_array_type):
-            pad_func = da.pad
-        else:
-            pad_func = np.pad
-
-        data = pad_func(
+        data = duck_array_ops.pad(
             trimmed_data.astype(dtype),
             pads,
             mode="constant",

From 2ebfa6b5a29185fed0c58451072521b5f0c6f33f Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 7 Feb 2020 18:35:02 +0100
Subject: [PATCH 17/20] add comments explaining the order of the arguments to
 where

---
 xarray/core/duck_array_ops.py | 2 ++
 xarray/core/variable.py       | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/xarray/core/duck_array_ops.py b/xarray/core/duck_array_ops.py
index e0ed0f7db95..5d16de09c3d 100644
--- a/xarray/core/duck_array_ops.py
+++ b/xarray/core/duck_array_ops.py
@@ -262,6 +262,8 @@ def where_method(data, cond, other=dtypes.NA):
 
 
 def fillna(data, other):
+    # we need to pass data first so pint has a chance of returning the
+    # correct unit
     return where(notnull(data), data, other)
 
 
diff --git a/xarray/core/variable.py b/xarray/core/variable.py
index 6d1429df235..ecd44f25207 100644
--- a/xarray/core/variable.py
+++ b/xarray/core/variable.py
@@ -742,6 +742,8 @@ def _getitem_with_mask(self, key, fill_value=dtypes.NA):
 
             data = as_indexable(self._data)[actual_indexer]
             mask = indexing.create_mask(indexer, self.shape, data)
+            # we need to invert the mask in order to pass data first. This helps
+            # pint to choose the correct unit
             data = duck_array_ops.where(np.logical_not(mask), data, fill_value)
         else:
             # array cannot be indexed along dimensions of size 0, so just

From 0a0c2d3c55d136ad79e84ec09155018ba201bdf5 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Sun, 23 Feb 2020 16:26:14 +0100
Subject: [PATCH 18/20] mark the flipped parameter changes with a todo

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

diff --git a/xarray/core/duck_array_ops.py b/xarray/core/duck_array_ops.py
index 5d16de09c3d..06e12e83abd 100644
--- a/xarray/core/duck_array_ops.py
+++ b/xarray/core/duck_array_ops.py
@@ -264,6 +264,7 @@ def where_method(data, cond, other=dtypes.NA):
 def fillna(data, other):
     # we need to pass data first so pint has a chance of returning the
     # correct unit
+    # TODO: revert after https://github.com/hgrecco/pint/issues/1019 is fixed
     return where(notnull(data), data, other)
 
 
diff --git a/xarray/core/variable.py b/xarray/core/variable.py
index ecd44f25207..058b7bf52d4 100644
--- a/xarray/core/variable.py
+++ b/xarray/core/variable.py
@@ -744,6 +744,7 @@ def _getitem_with_mask(self, key, fill_value=dtypes.NA):
             mask = indexing.create_mask(indexer, self.shape, data)
             # we need to invert the mask in order to pass data first. This helps
             # pint to choose the correct unit
+            # TODO: revert after https://github.com/hgrecco/pint/issues/1019 is fixed
             data = duck_array_ops.where(np.logical_not(mask), data, fill_value)
         else:
             # array cannot be indexed along dimensions of size 0, so just

From 4cadc52f78e609160e761e0cf711845eb6bcf235 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Sun, 23 Feb 2020 16:30:11 +0100
Subject: [PATCH 19/20] skip the identical tests

---
 xarray/tests/test_units.py | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/xarray/tests/test_units.py b/xarray/tests/test_units.py
index a19e253a849..737872c9c00 100644
--- a/xarray/tests/test_units.py
+++ b/xarray/tests/test_units.py
@@ -1594,13 +1594,7 @@ def test_missing_value_fillna(self, unit, error):
             pytest.param(1, id="no_unit"),
             pytest.param(unit_registry.dimensionless, id="dimensionless"),
             pytest.param(unit_registry.s, id="incompatible_unit"),
-            pytest.param(
-                unit_registry.cm,
-                id="compatible_unit",
-                marks=pytest.mark.xfail(
-                    reason="checking for identical units does not work properly, yet"
-                ),
-            ),
+            pytest.param(unit_registry.cm, id="compatible_unit",),
             pytest.param(unit_registry.m, id="identical_unit"),
         ),
     )
@@ -1611,7 +1605,17 @@ def test_missing_value_fillna(self, unit, error):
             pytest.param(True, id="with_conversion"),
         ),
     )
-    @pytest.mark.parametrize("func", (method("equals"), method("identical")), ids=repr)
+    @pytest.mark.parametrize(
+        "func",
+        (
+            method("equals"),
+            pytest.param(
+                method("identical"),
+                marks=pytest.mark.skip(reason="behaviour of identical is unclear"),
+            ),
+        ),
+        ids=repr,
+    )
     def test_comparisons(self, func, unit, convert_data, dtype):
         array = np.linspace(0, 1, 9).astype(dtype)
         quantity1 = array * unit_registry.m

From b51caa42d05566d1fe41d8bd155a9f2195b9e96a Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Sun, 23 Feb 2020 16:31:05 +0100
Subject: [PATCH 20/20] remove the warnings filter

---
 xarray/tests/test_units.py | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/xarray/tests/test_units.py b/xarray/tests/test_units.py
index 737872c9c00..75e743c3455 100644
--- a/xarray/tests/test_units.py
+++ b/xarray/tests/test_units.py
@@ -1,5 +1,4 @@
 import operator
-import warnings
 from distutils.version import LooseVersion
 
 import numpy as np
@@ -22,12 +21,6 @@
 Quantity = unit_registry.Quantity
 
 
-# silence pint's BehaviorChangeWarning
-with warnings.catch_warnings():
-    warnings.simplefilter("ignore")
-    Quantity([])
-
-
 pytestmark = [
     pytest.mark.skipif(
         not IS_NEP18_ACTIVE, reason="NUMPY_EXPERIMENTAL_ARRAY_FUNCTION is not enabled"