packaging_calculator: allow custom value handler

Use _packaging_values_handler ctx key to pass your own handler
for specific on demand overrides.

stock_packaging_calculator/models/product.py

@@ -102,10 +102,13 @@ class Product(models.Model):
         """Produce a list of dictionaries of packaging info."""
         # TODO: refactor to handle fractional quantities (eg: 0.5 Kg)
         res = []
+        prepare_values = self.env.context.get(
+            "_packaging_values_handler", self._prepare_qty_by_packaging_values
+        )
         for pkg in pkg_by_qty:
             qty_per_pkg, qty = self._qty_by_pkg(pkg.qty, qty)
             if qty_per_pkg:
-                value = self._prepare_qty_by_packaging_values(pkg, qty_per_pkg)
+                value = prepare_values(pkg, qty_per_pkg)
                 if with_contained:
                     contained = None
                     if not pkg.is_unit:

stock_packaging_calculator/tests/test_packaging_calc.py

@@ -234,6 +234,24 @@ class TestCalc(SavepointCase):
             expected,
         )
 
+    def test_calc_custom_values(self):
+        """Test custom values handler."""
+        expected = [
+            {"my_qty": 1, "foo": self.pkg_pallet.name},
+            {"my_qty": 3, "foo": self.pkg_big_box.name},
+            {"my_qty": 1, "foo": self.pkg_box.name},
+            {"my_qty": 5, "foo": self.uom_unit.name},
+        ]
+        self.assertEqual(
+            self.product_a.with_context(
+                _packaging_values_handler=lambda pkg, qty_per_pkg: {
+                    "my_qty": qty_per_pkg,
+                    "foo": pkg.name,
+                }
+            ).product_qty_by_packaging(2655),
+            expected,
+        )
+
     def test_calc_sub1(self):
         """Test contained packaging behavior 1."""
         expected = [