Prevent count, radian, bit and dimensionless interconversion (#209)

Author: kroenlein

gemd/__version__.py

@@ -1 +1 @@
-__version__ = "1.18.2"
+__version__ = "1.18.3"

gemd/units/impl.py

@@ -227,8 +227,20 @@ def convert_units(value: float, starting_unit: str, final_unit: str) -> float:
     if starting_unit == final_unit:
         return value  # skip computation
     else:
-        resolved_final_unit = _REGISTRY.parse_units(final_unit)  # `to` bypasses preparser
-        return _REGISTRY.Quantity(value, starting_unit).to(resolved_final_unit).magnitude
+        resolved_value = _REGISTRY.Quantity(value, starting_unit)
+        resolved_final_unit = _REGISTRY.parse_units(final_unit)
+        # Make sure count, radian, bit, and non-dimensional don't accidentally interconvert
+        # https://pint.readthedocs.io/en/0.23/user/angular_frequency.html
+        root1 = _REGISTRY.get_root_units(resolved_value)[1]
+        root2 = _REGISTRY.get_root_units(resolved_final_unit)[1]
+        if root1 != root2:
+            raise IncompatibleUnitsError(
+                units1=resolved_value.units,
+                dim1=_REGISTRY.get_dimensionality(resolved_final_unit),
+                units2=final_unit,
+                dim2=_REGISTRY.get_dimensionality(resolved_final_unit)
+            )
+        return resolved_value.to(resolved_final_unit).magnitude
 
 
 @register_unit_format("clean")

tests/units/test_parser.py

@@ -5,8 +5,7 @@
 import pytest
 
 from gemd.units import parse_units, convert_units, get_base_units, change_definitions_file, \
-    UndefinedUnitError, DefinitionSyntaxError
-from gemd.units.impl import DEFAULT_FILE
+    UndefinedUnitError, DefinitionSyntaxError, IncompatibleUnitsError
 
 
 @pytest.mark.parametrize("return_unit", [True, False])
@@ -144,6 +143,13 @@ def test_conversion():
     assert -1e-8 < convert_units(100, 'g / 100 mL', 'g/cc') - 1 < 1e-8
     assert -1e-8 < convert_units(1, "g / 2.5 cm", "g / 25 mm") - 1 < 1e-8
 
+    # Verify that convert_units throws exceptions
+    with pytest.raises(IncompatibleUnitsError):
+        convert_units(1, 'mL', 'g')
+    with pytest.raises(IncompatibleUnitsError):
+        # https://pint.readthedocs.io/en/0.23/user/angular_frequency.html
+        convert_units(1, 'Hz', 'rpm')
+
 
 def test_get_base_units():
     """Test that base units & conversions make sense."""