Rewrite unit formatter for pint 0.24 and earlier

cf_xarray/tests/test_units.py

@@ -75,12 +75,14 @@ def test_udunits_power_syntax_parse_units():
         ("m ** -1", "m-1"),
         ("m ** 2 / s ** 2", "m2 s-2"),
         ("m ** 3 / (kg * s ** 2)", "m3 kg-1 s-2"),
+        ("", ""),
     ),
 )
 def test_udunits_format(units, expected):
     u = ureg.parse_units(units)
 
     assert f"{u:cf}" == expected
+    assert f"{u:~cf}" == expected
 
 
 @pytest.mark.parametrize(

cf_xarray/units.py

@@ -4,62 +4,56 @@
 import re
 
 import pint
-from pint import (  # noqa: F401
-    DimensionalityError,
-    UndefinedUnitError,
-    UnitStrippedWarning,
-)
+from packaging.version import Version
 
 from .utils import emit_user_level_warning
 
-# from `xclim`'s unit support module with permission of the maintainers
-try:
 
-    @pint.register_unit_format("cf")
-    def short_formatter(unit, registry, **options):
-        """Return a CF-compliant unit string from a `pint` unit.
-
-        Parameters
-        ----------
-        unit : pint.UnitContainer
-            Input unit.
-        registry : pint.UnitRegistry
-            the associated registry
-        **options
-            Additional options (may be ignored)
-
-        Returns
-        -------
-        out : str
-            Units following CF-Convention, using symbols.
-        """
-        import re
-
-        # convert UnitContainer back to Unit
-        unit = registry.Unit(unit)
-        # Print units using abbreviations (millimeter -> mm)
-        s = f"{unit:~D}"
-
-        # Search and replace patterns
-        pat = r"(?P<inverse>(?:1 )?/ )?(?P<unit>\w+)(?: \*\* (?P<pow>\d))?"
-
-        def repl(m):
-            i, u, p = m.groups()
-            p = p or (1 if i else "")
-            neg = "-" if i else ""
-
-            return f"{u}{neg}{p}"
-
-        out, n = re.subn(pat, repl, s)
-
-        # Remove multiplications
-        out = out.replace(" * ", " ")
-        # Delta degrees:
-        out = out.replace("Δ°", "delta_deg")
-        return out.replace("percent", "%")
+@pint.register_unit_format("cf")
+def short_formatter(unit, registry, **options):
+    """Return a CF-compliant unit string from a `pint` unit.
+
+    Parameters
+    ----------
+    unit : pint.UnitContainer
+        Input unit.
+    registry : pint.UnitRegistry
+        The associated registry
+    **options
+        Additional options (may be ignored)
+
+    Returns
+    -------
+    out : str
+        Units following CF-Convention, using symbols.
+    """
+    # pint 0.24.1 gives this for dimensionless units
+    if unit == {"dimensionless": 1}:
+        return ""
+
+    # If u is a name, get its symbol (same as pint's "~" pre-formatter)
+    # otherwise, assume a symbol (pint should have already raised on invalid units before this)
+    unit = pint.util.UnitsContainer(
+        {
+            registry._get_symbol(u) if u in registry._units else u: exp
+            for u, exp in unit.items()
+        }
+    )
+
+    # Change in formatter signature in pint 0.24
+    if Version(pint.__version__) < Version("0.24"):
+        args = (unit.items(),)
+    else:
+        # Numerators splitted from denominators
+        args = (
+            ((u, e) for u, e in unit.items() if e >= 0),
+            ((u, e) for u, e in unit.items() if e < 0),
+        )
+
+    out = pint.formatter(*args, as_ratio=False, product_fmt=" ", power_fmt="{}{}")
+    # To avoid potentiel unicode problems in netCDF. In both case, this unit is not recognized by udunits
+    return out.replace("Δ°", "delta_deg")
 
-except ImportError:
-    pass
 
 # ------
 # Reused with modification from MetPy under the terms of the BSD 3-Clause License.

doc/units.md

@@ -16,7 +16,7 @@ hide-toc: true
 
 The xarray ecosystem supports unit-aware arrays using  [pint](https://pint.readthedocs.io) and [pint-xarray](https://pint-xarray.readthedocs.io). Some changes are required to make these packages work well with [UDUNITS format recommended by the CF conventions](http://cfconventions.org/Data/cf-conventions/cf-conventions-1.8/cf-conventions.html#units).
 
-`cf_xarray` makes those recommended changes when you `import cf_xarray.units`. These changes allow pint to parse and format UDUNIT units strings, and add several custom units like `degrees_north` for latitude, `psu` for ocean salinity, etc.
+`cf_xarray` makes those recommended changes when you `import cf_xarray.units`. These changes allow pint to parse and format UDUNIT units strings, and add several custom units like `degrees_north` for latitude, `psu` for ocean salinity, etc.  Be aware that pint supports some units that UDUNITS does not recognize but `cf-xarray` will not try to detect them and raise an error. For example, a temperature subtraction returns "delta_degC" units in pint, which does not exist in UDUNITS.
 
 ## Formatting units
 
@@ -27,5 +27,5 @@ from pint import application_registry as ureg
 import cf_xarray.units
 
 u = ureg.Unit("m ** 3 / s ** 2")
-f"{u:~cf}"
+f"{u:cf}" # or {u:~cf}, both return the same short format
 ```

pyproject.toml

@@ -22,7 +22,7 @@ dependencies = [
 dynamic = ["version"]
 
 [project.optional-dependencies]
-all = ["matplotlib", "pint", "shapely", "regex", "rich", "pooch"]
+all = ["matplotlib", "pint >=0.18, !=0.24.0", "shapely", "regex", "rich", "pooch"]
 
 [project.urls]
 homepage = "https://cf-xarray.readthedocs.io"