Merge pull request #101 from mmarras/mmlarras-unitconversion-context

try unit conversion instead of literal dimensionality check #99

src/quantityfield/helper.py

@@ -10,10 +10,14 @@ def check_matching_unit_dimension(
     If not
     :raise DimensionalityError
     """
-
     base_unit = getattr(ureg, base_units)
+    # create a pint quantity by multiplying unit with magnitude of 1
+    base_quant = 1 * base_unit
 
     for unit_string in units_to_check:
         unit = getattr(ureg, unit_string)
-        if unit.dimensionality != base_unit.dimensionality:
-            raise DimensionalityError(base_unit, unit)
+        # try to convert base qunatity to new unit, this also work for ureg.context
+        try:
+            base_quant.to(unit)
+        except DimensionalityError as e:
+            raise DimensionalityError(base_unit, unit) from e

tests/test_helper.py

@@ -1,6 +1,6 @@
 from django.test import TestCase
 
-from pint import DimensionalityError
+from pint import Context, DimensionalityError
 
 import quantityfield.fields as fields
 import quantityfield.helper as helper
@@ -28,3 +28,80 @@ def test_get_prep_value(self):
         field = fields.IntegerQuantityField("meter")
         with self.assertRaises(ValueError):
             field.get_prep_value("foobar")
+
+
+class TestContextHandling(TestCase):
+    """Class to test ureg.context for compatible units as described in issue #99.
+    pint allows users to define a special context for unit conversions,
+    e.g. on earth a mass can directly be converted to a force given the
+    acceleration 'constant' on earth.
+    We will test the unit compatibility via the helper function for both
+    both context activated globally via ureg and within a with block. Finally
+    test the conversion integrated inside an IntegerQuantityField. Also the
+    negatives are tested: without the context, mass should not be accepted
+    as a matching unit for force.
+    """
+
+    def setUp(self):
+        """Setup a pint context in the UnitRegistry."""
+        # Define a context where mass is equated to force via earth's
+        # standard acceleration of gravity and vice versa
+        # (https://en.wikipedia.org/wiki/Standard_gravity)
+        self.context = Context("earth")
+        # mass -> force
+        self.context.add_transformation(
+            "[mass]", "[force]", lambda ureg, x: x * ureg.gravity
+        )
+        # force -> mass
+        self.context.add_transformation(
+            "[force]", "[mass]", lambda ureg, x: x / ureg.gravity
+        )
+        ureg.add_context(self.context)
+
+    def test_context_global(self):
+        """Activate ureg.context globally and test conversion compatibility directly."""
+        # Activate context globally and test
+        ureg.enable_contexts("earth")
+        helper.check_matching_unit_dimension(ureg, "kg", ["newton", "kN", "ton"])
+        ureg.disable_contexts()
+
+    def test_context_with_block(self):
+        """Activate ureg.context in with block and test conversion compatibility
+        directly."""
+        # Use context with the 'with' statement
+        with ureg.context("earth"):
+            helper.check_matching_unit_dimension(ureg, "kg", ["newton", "kN", "ton"])
+
+    def test_invalid_context(self):
+        """Negative test: Conversion mass to force should fail without context."""
+        with self.assertRaises(DimensionalityError):
+            helper.check_matching_unit_dimension(ureg, "kg", ["newton", "kN", "ton"])
+
+    def test_field_w_context_global(self):
+        """Negative test: Conversion mass to force should fail without context."""
+        ureg.enable_contexts("earth")
+        self.field = fields.IntegerQuantityField(
+            base_units="kg", unit_choices=["newton", "kN", "ton"]
+        )
+        ureg.disable_contexts()
+
+    def test_field_w_context_block(self):
+        """Activate ureg.context globally and test conversion compatibility complete
+        Field."""
+        with ureg.context("earth"):
+            self.field = fields.IntegerQuantityField(
+                base_units="kg", unit_choices=["newton", "kN", "ton"]
+            )
+
+    def test_invalid_field_wo_context(self):
+        """Negative test: Conversion mass to force should fail without context."""
+        with self.assertRaises(DimensionalityError):
+            self.field = fields.IntegerQuantityField(
+                base_units="kg", unit_choices=["newton", "kN", "ton"]
+            )
+
+    def tearDown(self):
+        """Disable and remove the contexts to not interfere with other tests."""
+        # Clean up by disabling and removing the context
+        ureg.disable_contexts()
+        ureg.remove_context("earth")