Enrico/bug context

CHANGES

@@ -4,6 +4,7 @@ Pint Changelog
 0.25 (unreleased)
 -----------------
 
+- Fixed bug when using context and added regression test.
 - Fix the default behaviour for pint-convert (cli) for importing uncertainties package (PR #2032, Issue #2016)
 - Added mu and mc as alternatives for SI micro prefix
 - Added ℓ as alternative for liter
@@ -15,6 +16,7 @@ Pint Changelog
 0.24.1 (2024-06-24)
 -----------------
 
+- Fixed bug when using context and added regression test. (This PR)
 - Fix custom formatter needing the registry object. (PR #2011)
 - Support python 3.9 following difficulties installing with NumPy 2. (PR #2019)
 - Fix default formatting of dimensionless unit issue. (PR #2012)

pint/facets/context/registry.py

@@ -244,6 +244,7 @@ def enable_contexts(
 
         # Finally we add them to the active context.
         self._active_ctx.insert_contexts(*contexts)
+        self._build_cache()
         self._switch_context_cache_and_units()
 
     def disable_contexts(self, n: int | None = None) -> None:

pint/testsuite/regressions/conv.txt

@@ -0,0 +1,14 @@
+# Density
+density_lng = 2.244 * cubic_metre_lng / metric_ton
+
+# Calorific value
+cv_lng = 1.22 * toe / metric_ton * 1.054261895
+
+metric_tons_per_second = metric_ton / s
+
+@context lng
+    [volume] / [time] -> [mass] / [time]: value / density_lng
+    [mass] / [time] -> [volume] / [time]: value * density_lng
+    [mass] / [time] -> [energy] / [time]: value * cv_lng
+    [energy] / [time] -> [mass] / [time]: value / cv_lng
+@end

pint/testsuite/regressions/test_contexts.py

@@ -0,0 +1,10 @@
+from pint import UnitRegistry
+
+
+class TestRegressions:
+    def test_lng_conversion(self):
+        ureg = UnitRegistry()
+        ureg.load_definitions("pint/testsuite/regressions/conv.txt")
+        ureg.enable_contexts("lng")
+        u = ureg.Unit("watt")
+        assert u.compatible_units()