Migrate parse_units to functools.cache

pint/facets/context/registry.py

@@ -40,7 +40,6 @@ def __init__(self, registry_cache: RegistryCache) -> None:
         self.dimensional_equivalents = registry_cache.dimensional_equivalents
         self.root_units = {}
         self.dimensionality = registry_cache.dimensionality
-        self.parse_unit = registry_cache.parse_unit
 
 
 class GenericContextRegistry(

pint/facets/plain/registry.py

@@ -128,9 +128,6 @@ def __init__(self) -> None:
         #: Maps dimensionality (UnitsContainer) to Units (UnitsContainer)
         self.dimensionality: dict[UnitsContainer, UnitsContainer] = {}
 
-        #: Cache the unit name associated to user input. ('mV' -> 'millivolt')
-        self.parse_unit: dict[str, UnitsContainer] = {}
-
     def __eq__(self, other: Any):
         if not isinstance(other, self.__class__):
             return False
@@ -589,6 +586,8 @@ def _build_cache(self, loaded_files=None) -> None:
             return
 
         self._cache = RegistryCache()
+        for func in (self._parse_single_unit, self._parse_units):
+            func.cache_clear()
 
         deps: dict[str, set[str]] = {
             name: set(definition.reference.keys()) if definition.reference else set()
@@ -1164,6 +1163,7 @@ def parse_units(
         units = self._parse_units(input_string, as_delta, case_sensitive)
         return self.Unit(units)
 
+    @functools.cache
     def _parse_units(
         self,
         input_string: str,
@@ -1174,12 +1174,9 @@ def _parse_units(
         the canonical names.
         """
 
-        cache = self._cache.parse_unit
         # Issue #1097: it is possible, when a unit was defined while a different context
         # was active, that the unit is in self._cache.parse_unit but not in self._units.
         # If this is the case, force self._units to be repopulated.
-        if as_delta and input_string in cache and input_string in self._units:
-            return cache[input_string]
 
         for p in self.preprocessors:
             input_string = p(input_string)
@@ -1207,9 +1204,6 @@ def _parse_units(
                     cname = "delta_" + cname
             ret = ret.add(cname, value)
 
-        if as_delta:
-            cache[input_string] = ret
-
         return ret
 
     def _eval_token(