From c68ce57284606eacad05efa7a761e7fac5cb1f47 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Wed, 2 Sep 2020 22:43:02 +0200
Subject: [PATCH 01/20] use the application registry instead of creating a new
 one

---
 pint_xarray/accessors.py | 30 ++++++++++++------------------
 1 file changed, 12 insertions(+), 18 deletions(-)

diff --git a/pint_xarray/accessors.py b/pint_xarray/accessors.py
index 303aae14..4df7a101 100644
--- a/pint_xarray/accessors.py
+++ b/pint_xarray/accessors.py
@@ -82,13 +82,15 @@ def either_dict_or_kwargs(positional, keywords, method_name):
         return keywords
 
 
-def _get_registry(unit_registry, registry_kwargs):
+def get_registry(unit_registry):
     if unit_registry is None:
-        if registry_kwargs is None:
-            registry_kwargs = {}
-        registry_kwargs.update(force_ndarray=True)
-        # TODO should this registry object then be stored somewhere global?
-        unit_registry = pint.UnitRegistry(**registry_kwargs)
+        unit_registry = pint.get_application_registry()
+
+    if not unit_registry.force_ndarray_like and not unit_registry.force_ndarray:
+        raise ValueError(
+            "invalid registry. Please enable 'force_ndarray_like' or 'force_ndarray'."
+        )
+
     return unit_registry
 
 
@@ -120,9 +122,7 @@ class PintDataArrayAccessor:
     def __init__(self, da):
         self.da = da
 
-    def quantify(
-        self, units=None, unit_registry=None, registry_kwargs=None, **unit_kwargs
-    ):
+    def quantify(self, units=None, unit_registry=None, **unit_kwargs):
         """
         Attaches units to the DataArray.
 
@@ -155,8 +155,6 @@ def quantify(
         unit_registry : pint.UnitRegistry, optional
             Unit registry to be used for the units attached to this DataArray.
             If not given then a default registry will be created.
-        registry_kwargs : dict, optional
-            Keyword arguments to be passed to the unit registry.
         **unit_kwargs
             Keyword argument form of units.
 
@@ -197,7 +195,7 @@ def quantify(
 
         units = either_dict_or_kwargs(units, unit_kwargs, ".quantify")
 
-        registry = _get_registry(unit_registry, registry_kwargs)
+        registry = get_registry(unit_registry)
 
         unit_attrs = conversion.extract_unit_attributes(self.da)
         new_obj = conversion.strip_unit_attributes(self.da)
@@ -391,9 +389,7 @@ class PintDatasetAccessor:
     def __init__(self, ds):
         self.ds = ds
 
-    def quantify(
-        self, units=None, unit_registry=None, registry_kwargs=None, **unit_kwargs
-    ):
+    def quantify(self, units=None, unit_registry=None, **unit_kwargs):
         """
         Attaches units to each variable in the Dataset.
 
@@ -426,8 +422,6 @@ def quantify(
             Unit registry to be used for the units attached to each
             DataArray in this Dataset. If not given then a default
             registry will be created.
-        registry_kwargs : dict, optional
-            Keyword arguments to be passed to `pint.UnitRegistry`.
         **unit_kwargs
             Keyword argument form of ``units``.
 
@@ -464,7 +458,7 @@ def quantify(
             b        (x) int64 <Quantity([ 5 -2  1], 'decimeter')>
         """
         units = either_dict_or_kwargs(units, unit_kwargs, ".quantify")
-        registry = _get_registry(unit_registry, registry_kwargs)
+        registry = get_registry(unit_registry)
 
         unit_attrs = conversion.extract_unit_attributes(self.ds)
         new_obj = conversion.strip_unit_attributes(self.ds)

From 322400acdd3b4ac6ccf69b27751e43663be2a97d Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Wed, 2 Sep 2020 23:08:30 +0200
Subject: [PATCH 02/20] try to get the unit registry from units or existing
 quantities

---
 pint_xarray/accessors.py | 33 +++++++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/pint_xarray/accessors.py b/pint_xarray/accessors.py
index 4df7a101..c9728128 100644
--- a/pint_xarray/accessors.py
+++ b/pint_xarray/accessors.py
@@ -57,6 +57,14 @@ def zip_mappings(*mappings, fill_value=None):
     return zipped
 
 
+def merge_mappings(first, *mappings):
+    result = first.copy()
+    for mapping in mappings:
+        result.update(mapping)
+
+    return result
+
+
 def units_to_str_or_none(mapping):
     return {
         key: str(value) if isinstance(value, Unit) else value
@@ -82,9 +90,20 @@ def either_dict_or_kwargs(positional, keywords, method_name):
         return keywords
 
 
-def get_registry(unit_registry):
+def get_registry(unit_registry, units):
+    registries = {unit._REGISTRY for unit in units.values() if isinstance(unit, Unit)}
+
     if unit_registry is None:
-        unit_registry = pint.get_application_registry()
+        if not registries:
+            unit_registry = pint.get_application_registry()
+            registries.add(unit_registry)
+        elif len(registries) == 1:
+            (unit_registry,) = registries
+
+    if len(registries) > 1 or unit_registry not in registries:
+        raise ValueError(
+            "using multiple unit registries in the same object is not supported"
+        )
 
     if not unit_registry.force_ndarray_like and not unit_registry.force_ndarray:
         raise ValueError(
@@ -195,7 +214,10 @@ def quantify(self, units=None, unit_registry=None, **unit_kwargs):
 
         units = either_dict_or_kwargs(units, unit_kwargs, ".quantify")
 
-        registry = get_registry(unit_registry)
+        registry = get_registry(
+            unit_registry,
+            merge_mappings(units, conversion.extract_units(self.da)),
+        )
 
         unit_attrs = conversion.extract_unit_attributes(self.da)
         new_obj = conversion.strip_unit_attributes(self.da)
@@ -458,7 +480,10 @@ def quantify(self, units=None, unit_registry=None, **unit_kwargs):
             b        (x) int64 <Quantity([ 5 -2  1], 'decimeter')>
         """
         units = either_dict_or_kwargs(units, unit_kwargs, ".quantify")
-        registry = get_registry(unit_registry)
+        registry = get_registry(
+            unit_registry,
+            merge_mappings(units, conversion.extract_units(self.ds)),
+        )
 
         unit_attrs = conversion.extract_unit_attributes(self.ds)
         new_obj = conversion.strip_unit_attributes(self.ds)

From b21728f6320b9b6fbfdf400be52efb626be9c6c5 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Wed, 2 Sep 2020 23:12:33 +0200
Subject: [PATCH 03/20] use a module-local registry instead of the application
 registry

---
 pint_xarray/__init__.py  | 1 +
 pint_xarray/accessors.py | 5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/pint_xarray/__init__.py b/pint_xarray/__init__.py
index ef933c46..768b84b4 100644
--- a/pint_xarray/__init__.py
+++ b/pint_xarray/__init__.py
@@ -8,6 +8,7 @@
 from . import testing  # noqa: F401
 from . import formatting
 from .accessors import PintDataArrayAccessor, PintDatasetAccessor  # noqa: F401
+from .accessors import _registry as unit_registry  # noqa: F401
 
 try:
     __version__ = version("pint-xarray")
diff --git a/pint_xarray/accessors.py b/pint_xarray/accessors.py
index c9728128..a864adb6 100644
--- a/pint_xarray/accessors.py
+++ b/pint_xarray/accessors.py
@@ -18,6 +18,9 @@
     raise ImportError("NUMPY_EXPERIMENTAL_ARRAY_FUNCTION is not enabled")
 
 
+_registry = pint.UnitRegistry(force_ndarray_like=True)
+
+
 # TODO could/should we overwrite xr.open_dataset and xr.open_mfdataset to make
 # them apply units upon loading???
 # TODO could even override the decode_cf kwarg?
@@ -95,7 +98,7 @@ def get_registry(unit_registry, units):
 
     if unit_registry is None:
         if not registries:
-            unit_registry = pint.get_application_registry()
+            unit_registry = _registry
             registries.add(unit_registry)
         elif len(registries) == 1:
             (unit_registry,) = registries

From 4af2849b51bf027117c46ce5ff02bcc998755bef Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Wed, 2 Sep 2020 23:37:21 +0200
Subject: [PATCH 04/20] fix get_registry

---
 pint_xarray/accessors.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/pint_xarray/accessors.py b/pint_xarray/accessors.py
index a864adb6..adb0cec7 100644
--- a/pint_xarray/accessors.py
+++ b/pint_xarray/accessors.py
@@ -63,7 +63,9 @@ def zip_mappings(*mappings, fill_value=None):
 def merge_mappings(first, *mappings):
     result = first.copy()
     for mapping in mappings:
-        result.update(mapping)
+        result.update(
+            {key: value for key, value in mapping.items() if value is not None}
+        )
 
     return result
 
@@ -99,9 +101,9 @@ def get_registry(unit_registry, units):
     if unit_registry is None:
         if not registries:
             unit_registry = _registry
-            registries.add(unit_registry)
         elif len(registries) == 1:
             (unit_registry,) = registries
+    registries.add(unit_registry)
 
     if len(registries) > 1 or unit_registry not in registries:
         raise ValueError(

From 976837449e698d18ee144d2b2a4b3da69a7aac09 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Wed, 2 Sep 2020 23:37:35 +0200
Subject: [PATCH 05/20] fix the registry property of DataArray

---
 pint_xarray/accessors.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pint_xarray/accessors.py b/pint_xarray/accessors.py
index adb0cec7..5c2392e8 100644
--- a/pint_xarray/accessors.py
+++ b/pint_xarray/accessors.py
@@ -285,7 +285,7 @@ def dimensionality(self):
     @property
     def registry(self):
         # TODO is this a bad idea? (see GH issue #1071 in pint)
-        return self.data._REGISTRY
+        return getattr(self.da.data, "_REGISTRY", None)
 
     @registry.setter
     def registry(self, _):

From aa26ce2693798b405827a3e660cb4b4e78418143 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Thu, 3 Sep 2020 02:08:53 +0200
Subject: [PATCH 06/20] remove the registry_kwargs test

---
 pint_xarray/tests/test_accessors.py | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/pint_xarray/tests/test_accessors.py b/pint_xarray/tests/test_accessors.py
index 8867af8a..2dcd5df7 100644
--- a/pint_xarray/tests/test_accessors.py
+++ b/pint_xarray/tests/test_accessors.py
@@ -94,11 +94,6 @@ def test_error_on_nonsense_units(self, example_unitless_da):
         with pytest.raises(UndefinedUnitError):
             da.pint.quantify(units="aecjhbav")
 
-    def test_registry_kwargs(self, example_unitless_da):
-        orig = example_unitless_da
-        result = orig.pint.quantify(registry_kwargs={"auto_reduce_dimensions": True})
-        assert result.data._REGISTRY.auto_reduce_dimensions is True
-
 
 class TestDequantifyDataArray:
     def test_strip_units(self, example_quantity_da):

From e48b95d1e18ec9740e261349be923afdbe64431b Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 4 Sep 2020 17:39:25 +0200
Subject: [PATCH 07/20] merge the units in get_registry

---
 pint_xarray/accessors.py | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/pint_xarray/accessors.py b/pint_xarray/accessors.py
index 5c2392e8..4c8f4e61 100644
--- a/pint_xarray/accessors.py
+++ b/pint_xarray/accessors.py
@@ -95,7 +95,8 @@ def either_dict_or_kwargs(positional, keywords, method_name):
         return keywords
 
 
-def get_registry(unit_registry, units):
+def get_registry(unit_registry, new_units, existing_units):
+    units = merge_mappings(existing_units, new_units)
     registries = {unit._REGISTRY for unit in units.values() if isinstance(unit, Unit)}
 
     if unit_registry is None:
@@ -221,7 +222,8 @@ def quantify(self, units=None, unit_registry=None, **unit_kwargs):
 
         registry = get_registry(
             unit_registry,
-            merge_mappings(units, conversion.extract_units(self.da)),
+            units,
+            conversion.extract_units(self.da),
         )
 
         unit_attrs = conversion.extract_unit_attributes(self.da)
@@ -487,7 +489,8 @@ def quantify(self, units=None, unit_registry=None, **unit_kwargs):
         units = either_dict_or_kwargs(units, unit_kwargs, ".quantify")
         registry = get_registry(
             unit_registry,
-            merge_mappings(units, conversion.extract_units(self.ds)),
+            units,
+            conversion.extract_units(self.ds),
         )
 
         unit_attrs = conversion.extract_unit_attributes(self.ds)

From 6efdece698e429c079eb42c375dd1dcb9abc902d Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 4 Sep 2020 17:47:16 +0200
Subject: [PATCH 08/20] don't use the name of the DataArray to set the units

---
 docs/creation.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docs/creation.rst b/docs/creation.rst
index ecf6c29e..9af20b25 100644
--- a/docs/creation.rst
+++ b/docs/creation.rst
@@ -40,7 +40,7 @@ We can also override the units of a variable:
 
     In [4]: ds.pint.quantify(b="km")
 
-    In [5]: da.pint.quantify({"b": "degree"})
+    In [5]: da.pint.quantify("degree")
 
 Overriding works, even if there is no ``units`` attribute, so we could use this
 to attach units to a normal :py:class:`Dataset`:

From f890a36076f9f045c4e2973ca239018891fa8208 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 4 Sep 2020 17:47:41 +0200
Subject: [PATCH 09/20] remove a comma

---
 docs/creation.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docs/creation.rst b/docs/creation.rst
index 9af20b25..1eb49df1 100644
--- a/docs/creation.rst
+++ b/docs/creation.rst
@@ -42,7 +42,7 @@ We can also override the units of a variable:
 
     In [5]: da.pint.quantify("degree")
 
-Overriding works, even if there is no ``units`` attribute, so we could use this
+Overriding works even if there is no ``units`` attribute, so we could use this
 to attach units to a normal :py:class:`Dataset`:
 
 .. ipython::

From 02b8cc1a6e55a810f617c2ef7db2cfbb6fcd35ca Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 4 Sep 2020 17:48:31 +0200
Subject: [PATCH 10/20] mention that quantify will reject units from multiple
 registries

---
 docs/creation.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/docs/creation.rst b/docs/creation.rst
index 1eb49df1..f00c2e4f 100644
--- a/docs/creation.rst
+++ b/docs/creation.rst
@@ -53,6 +53,13 @@ to attach units to a normal :py:class:`Dataset`:
 Of course, we could use :py:class:`pint.Unit` instances instead of strings to
 specify units, too.
 
+.. note::
+
+    Unit objects tied to different registries cannot interact with each
+    other. In order to avoid this, :py:meth:`DataArray.pint.quantify` and
+    :py:meth:`Dataset.pint.quantify` will make sure only a single registry is
+    used per ``xarray`` object.
+
 If we wanted to change the units of the data of a :py:class:`DataArray`, we
 could do so using the :py:attr:`DataArray.name` attribute:
 

From b85d5c151e339b925d5c71a624f477e77387a23e Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 4 Sep 2020 17:48:50 +0200
Subject: [PATCH 11/20] document overriding the default registry

---
 docs/creation.rst | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/docs/creation.rst b/docs/creation.rst
index f00c2e4f..1040b1a3 100644
--- a/docs/creation.rst
+++ b/docs/creation.rst
@@ -83,6 +83,31 @@ have to first swap the dimensions:
        ...: )
        ...: da_with_units
 
+By default, :py:meth:`Dataset.pint.quantify` and
+:py:meth:`DataArray.pint.quantify` will use the unit registry at
+:py:obj:`pint_xarray.unit_registry`. If we want a different registry, we can
+either pass it as the ``unit_registry`` parameter:
+
+.. ipython::
+
+   In [10]: ureg = pint.UnitRegistry(force_ndarray_like=True)
+       ...: # set up the registry
+
+   In [11]: da.pint.quantify("degree", unit_registry=ureg)
+
+or overwrite the default registry:
+
+.. ipython::
+
+   In [12]: pint_xarray.unit_registry = ureg
+
+   In [13]: da.pint.quantify("degree")
+
+.. note::
+
+    To properly work with ``xarray``, the ``force_ndarray_like`` or
+    ``force_ndarray`` options have to be enabled on the custom registry.
+
 Saving with units
 -----------------
 In order to not lose the units when saving to disk, we first have to call the

From 1287eda82077800cb99979db7cbcfde7ed3b6de1 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 4 Sep 2020 17:51:07 +0200
Subject: [PATCH 12/20] rename the default registry to default_registry

---
 pint_xarray/__init__.py  | 2 +-
 pint_xarray/accessors.py | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/pint_xarray/__init__.py b/pint_xarray/__init__.py
index 768b84b4..f96f6ab6 100644
--- a/pint_xarray/__init__.py
+++ b/pint_xarray/__init__.py
@@ -8,7 +8,7 @@
 from . import testing  # noqa: F401
 from . import formatting
 from .accessors import PintDataArrayAccessor, PintDatasetAccessor  # noqa: F401
-from .accessors import _registry as unit_registry  # noqa: F401
+from .accessors import default_registry as unit_registry  # noqa: F401
 
 try:
     __version__ = version("pint-xarray")
diff --git a/pint_xarray/accessors.py b/pint_xarray/accessors.py
index e3396d3c..6013ec7c 100644
--- a/pint_xarray/accessors.py
+++ b/pint_xarray/accessors.py
@@ -8,7 +8,7 @@
 
 from . import conversion
 
-_registry = pint.UnitRegistry(force_ndarray_like=True)
+default_registry = pint.UnitRegistry(force_ndarray_like=True)
 
 # TODO could/should we overwrite xr.open_dataset and xr.open_mfdataset to make
 # them apply units upon loading???
@@ -90,7 +90,7 @@ def get_registry(unit_registry, new_units, existing_units):
 
     if unit_registry is None:
         if not registries:
-            unit_registry = _registry
+            unit_registry = default_registry
         elif len(registries) == 1:
             (unit_registry,) = registries
     registries.add(unit_registry)

From 5c4152bc29a28105faa00f87d14215a48150833a Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 4 Sep 2020 17:51:52 +0200
Subject: [PATCH 13/20] load pint into the docs namespace

---
 docs/creation.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/docs/creation.rst b/docs/creation.rst
index 1040b1a3..e8cd641a 100644
--- a/docs/creation.rst
+++ b/docs/creation.rst
@@ -9,6 +9,7 @@ Attaching units
     :suppress:
 
     import xarray as xr
+    import pint
 
 Usually, when loading data from disk we get a :py:class:`Dataset` or
 :py:class:`DataArray` with units in attributes:

From 8824f35077b231241d4f71bce8035b361fbb31ac Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Fri, 4 Sep 2020 17:54:00 +0200
Subject: [PATCH 14/20] also load pint_xarray

---
 docs/creation.rst | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/docs/creation.rst b/docs/creation.rst
index e8cd641a..fb1b1e00 100644
--- a/docs/creation.rst
+++ b/docs/creation.rst
@@ -8,8 +8,9 @@ Attaching units
 .. ipython:: python
     :suppress:
 
-    import xarray as xr
     import pint
+    import pint_xarray
+    import xarray as xr
 
 Usually, when loading data from disk we get a :py:class:`Dataset` or
 :py:class:`DataArray` with units in attributes:

From f9700a97ae0790cb07c92070d0febf0a7064b234 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Thu, 22 Oct 2020 16:13:50 +0200
Subject: [PATCH 15/20] add a function set up a registry for use with
 pint_xarray

---
 pint_xarray/accessors.py | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/pint_xarray/accessors.py b/pint_xarray/accessors.py
index 1c23b9df..0e870aa2 100644
--- a/pint_xarray/accessors.py
+++ b/pint_xarray/accessors.py
@@ -10,6 +10,32 @@
 
 default_registry = pint.UnitRegistry(force_ndarray_like=True)
 
+
+def setup_registry(registry=None):
+    """set up the given registry for use with pint_xarray
+
+    Namely, it enables ``force_ndarray_like`` to make sure results are always
+    duck arrays.
+
+    Parameters
+    ----------
+    registry : pint.UnitRegistry, optional
+        The registry to use. If not given, the application registry is used.
+
+    See also
+    --------
+    pint.get_application_registry
+    pint.set_application_registry
+    """
+    if registry is None:
+        registry = default_registry
+
+    if not registry.force_ndarray and not registry.force_ndarray_like:
+        registry.force_ndarray_like = True
+
+    return registry
+
+
 # TODO could/should we overwrite xr.open_dataset and xr.open_mfdataset to make
 # them apply units upon loading???
 # TODO could even override the decode_cf kwarg?

From b675ee38b6cadf889c10aa0e7c1dd2e302d6773f Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Thu, 22 Oct 2020 16:16:41 +0200
Subject: [PATCH 16/20] replace the default registry with the application
 registry

---
 pint_xarray/accessors.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pint_xarray/accessors.py b/pint_xarray/accessors.py
index 0e870aa2..ecb36168 100644
--- a/pint_xarray/accessors.py
+++ b/pint_xarray/accessors.py
@@ -8,7 +8,7 @@
 
 from . import conversion
 
-default_registry = pint.UnitRegistry(force_ndarray_like=True)
+default_registry = pint.get_application_registry()
 
 
 def setup_registry(registry=None):

From fffa6c72dc16b6f9477804fcf6d7fafd8806ef3a Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Thu, 22 Oct 2020 16:18:27 +0200
Subject: [PATCH 17/20] simplify the setup function

---
 pint_xarray/accessors.py | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/pint_xarray/accessors.py b/pint_xarray/accessors.py
index ecb36168..085ce819 100644
--- a/pint_xarray/accessors.py
+++ b/pint_xarray/accessors.py
@@ -8,10 +8,8 @@
 
 from . import conversion
 
-default_registry = pint.get_application_registry()
 
-
-def setup_registry(registry=None):
+def setup_registry(registry):
     """set up the given registry for use with pint_xarray
 
     Namely, it enables ``force_ndarray_like`` to make sure results are always
@@ -27,15 +25,14 @@ def setup_registry(registry=None):
     pint.get_application_registry
     pint.set_application_registry
     """
-    if registry is None:
-        registry = default_registry
-
     if not registry.force_ndarray and not registry.force_ndarray_like:
         registry.force_ndarray_like = True
 
     return registry
 
 
+default_registry = setup_registry(pint.get_application_registry())
+
 # TODO could/should we overwrite xr.open_dataset and xr.open_mfdataset to make
 # them apply units upon loading???
 # TODO could even override the decode_cf kwarg?

From ed8de4efae3023687838ff9119fec16d47c36c4f Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Thu, 22 Oct 2020 16:26:48 +0200
Subject: [PATCH 18/20] update the docstring

---
 pint_xarray/accessors.py | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/pint_xarray/accessors.py b/pint_xarray/accessors.py
index 085ce819..dbc1d27a 100644
--- a/pint_xarray/accessors.py
+++ b/pint_xarray/accessors.py
@@ -17,13 +17,8 @@ def setup_registry(registry):
 
     Parameters
     ----------
-    registry : pint.UnitRegistry, optional
-        The registry to use. If not given, the application registry is used.
-
-    See also
-    --------
-    pint.get_application_registry
-    pint.set_application_registry
+    registry : pint.UnitRegistry
+        The registry to modify
     """
     if not registry.force_ndarray and not registry.force_ndarray_like:
         registry.force_ndarray_like = True

From 20a3e3eb590a073ee4480836f4e22a183bea11c7 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Thu, 22 Oct 2020 17:13:35 +0200
Subject: [PATCH 19/20] update whats-new.rst

---
 docs/whats-new.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/docs/whats-new.rst b/docs/whats-new.rst
index 8c219a08..38961d66 100644
--- a/docs/whats-new.rst
+++ b/docs/whats-new.rst
@@ -13,3 +13,4 @@ What's new
 - fix the :py:attr:`DataArray.pint.units`, :py:attr:`DataArray.pint.magnitude`
   and :py:attr:`DataArray.pint.dimensionality` properties and add docstrings for
   all three. (:pull:`31`)
+- use ``pint``'s application registry as a module-global registry (:pull:`32`)

From dffd9d201e50b2bcb8c4da83363e6f19062254b3 Mon Sep 17 00:00:00 2001
From: Keewis <keewis@posteo.de>
Date: Thu, 22 Oct 2020 17:18:46 +0200
Subject: [PATCH 20/20] mention that the default registry is the application
 registry

---
 docs/creation.rst | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/docs/creation.rst b/docs/creation.rst
index fb1b1e00..37e9f9fb 100644
--- a/docs/creation.rst
+++ b/docs/creation.rst
@@ -87,8 +87,9 @@ have to first swap the dimensions:
 
 By default, :py:meth:`Dataset.pint.quantify` and
 :py:meth:`DataArray.pint.quantify` will use the unit registry at
-:py:obj:`pint_xarray.unit_registry`. If we want a different registry, we can
-either pass it as the ``unit_registry`` parameter:
+:py:obj:`pint_xarray.unit_registry` (the
+:py:func:`application registry <pint.get_application_registry>`). If we want a
+different registry, we can either pass it as the ``unit_registry`` parameter:
 
 .. ipython::