ignore datetime units (#241)

* add a predicate to check if a unit is for datetime decoding

* skip the unit attributes if the unit string is a datetime unit

* don't strip datetime units

* make sure to only try matching unit strings

* add tests for the modification

* what's new entry

* mention this behavior in the docstring of `.quantify`

Author: keewis

docs/whats-new.rst

@@ -5,7 +5,6 @@ What's new
 0.4 (*unreleased*)
 ------------------
 - adopt `SPEC0 <https://scientific-python.org/specs/spec-0000>`_ (:pull:`228`)
-  By `Justus Magin <https://github.com/keewis>`_.
 
   This means that the supported versions change:
 
@@ -18,8 +17,11 @@ What's new
    pint                0.16             0.19
   ============ ==============  ==============
 
+  By `Justus Magin <https://github.com/keewis>`_.
 - add support for python 3.11 (:pull:`228`)
   By `Justus Magin <https://github.com/keewis>`_.
+- ignore datetime units on attributes (:pull:`241`)
+  By `Justus Magin <https://github.com/keewis>`_.
 
 0.3 (27 Jul 2022)
 -----------------

pint_xarray/accessors.py

@@ -258,6 +258,12 @@ def quantify(self, units=_default, unit_registry=None, **unit_kwargs):
             ``xarray`` changes the way it implements indexes, these
             units will be set as attributes.
 
+        .. note::
+            Also note that datetime units (i.e. ones that match
+            ``{units} since {date}``) in unit attributes will be
+            ignored, to avoid interfering with ``xarray``'s datetime
+            encoding / decoding.
+
         Parameters
         ----------
         units : unit-like or mapping of hashable to unit-like, optional
@@ -984,6 +990,12 @@ def quantify(self, units=_default, unit_registry=None, **unit_kwargs):
             ``xarray`` changes the way it implements indexes, these
             units will be set as attributes.
 
+        .. note::
+            Also note that datetime units (i.e. ones that match
+            ``{units} since {date}``) in unit attributes will be
+            ignored, to avoid interfering with ``xarray``'s datetime
+            encoding / decoding.
+
         Parameters
         ----------
         units : mapping of hashable to unit-like, optional

pint_xarray/conversion.py

@@ -1,4 +1,5 @@
 import itertools
+import re
 
 import pint
 from xarray import DataArray, Dataset, IndexVariable, Variable
@@ -11,6 +12,14 @@
 slice_attributes = ("start", "stop", "step")
 temporary_name = "<this-array>"
 
+time_units_re = r"\w+"
+datetime_re = r"\d{4}-\d{2}-\d{2}(?:[ T]\d{2}:\d{2}:\d{2}(?:\.\d+)?)?"
+datetime_units_re = re.compile(rf"{time_units_re} since {datetime_re}")
+
+
+def is_datetime_unit(unit):
+    return isinstance(unit, str) and datetime_units_re.match(unit) is not None
+
 
 def array_attach_units(data, unit):
     """attach a unit to the data
@@ -266,7 +275,11 @@ def extract_units(obj):
 
 
 def extract_unit_attributes_dataset(obj, attr="units"):
-    return {name: var.attrs.get(attr, None) for name, var in obj.variables.items()}
+    all_units = {name: var.attrs.get(attr, None) for name, var in obj.variables.items()}
+
+    return {
+        name: unit for name, unit in all_units.items() if not is_datetime_unit(unit)
+    }
 
 
 def extract_unit_attributes(obj, attr="units"):
@@ -308,6 +321,9 @@ def strip_units(obj):
 def strip_unit_attributes_dataset(obj, attr="units"):
     new_obj = obj.copy()
     for var in new_obj.variables.values():
+        if is_datetime_unit(var.attrs.get(attr, "")):
+            continue
+
         var.attrs.pop(attr, None)
 
     return new_obj

pint_xarray/tests/test_conversion.py

@@ -482,6 +482,11 @@ def test_extract_units(self, type, units):
                 {"a": "K", "b": "hPa", "x": "m", "u": "s"},
                 id="Dataset",
             ),
+            pytest.param(
+                Dataset(coords={"t": ("t", [], {"units": "seconds since 2000-01-01"})}),
+                {},
+                id="datetime_unit",
+            ),
         ),
     )
     def test_extract_unit_attributes(self, obj, expected):
@@ -546,6 +551,11 @@ def test_strip_units(self, obj, expected):
                 {"a": "K", "b": "hPa", "x": "m", "u": "s"},
                 id="Dataset",
             ),
+            pytest.param(
+                Dataset(coords={"t": ("t", [], {"units": "seconds since 2000-01-01"})}),
+                {},
+                id="datetime_unit",
+            ),
         ),
     )
     def test_strip_unit_attributes(self, obj, expected):