First attempt at ripping out agnostic code into ifcopenshell.api namespace. See #1399.

src/ifcopenshell-python/ifcopenshell/api/unit/assign_unit.py

@@ -0,0 +1,76 @@
+import ifcopenshell.util.unit
+
+
+class Usecase():
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {
+            "length": {
+                "is_metric": True,
+                "raw": "MILLIMETERS"
+            },
+            "area": {
+                "is_metric": True,
+                "raw": "METERS"
+            },
+            "volume": {
+                "is_metric": True,
+                "raw": "METERS"
+            },
+        }
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        for unit_type, data in self.settings.items():
+            if data["is_metric"]:
+                data["ifc"] = self.create_metric_unit(unit_type, data)
+            else:
+                data["ifc"] = self.create_imperial_unit(unit_type, data)
+        unit_assignment = self.file.by_type("IfcUnitAssignment")
+        if unit_assignment:
+            unit_assignment = unit_assignment[0]
+            # TODO: handle unit rewriting, which is complicated
+        else:
+            unit_assignment = self.file.createIfcUnitAssignment([u["ifc"] for u in self.settings.values()])
+            self.file.by_type("IfcProject")[0].UnitsInContext = unit_assignment
+        return unit_assignment
+
+    def create_metric_unit(self, unit_type, data):
+        type_prefix = ""
+        if unit_type == "area":
+            type_prefix = "SQUARE_"
+        elif unit_type == "volume":
+            type_prefix = "CUBIC_"
+        return self.file.createIfcSIUnit(
+            None,
+            "{}UNIT".format(unit_type.upper()),
+            ifcopenshell.util.unit.get_prefix(data["raw"]),
+            type_prefix + ifcopenshell.util.unit.get_unit_name(data["raw"]),
+        )
+
+    def create_imperial_unit(self, unit_type, data):
+        if unit_type == "length":
+            dimensional_exponents = self.file.createIfcDimensionalExponents(1, 0, 0, 0, 0, 0, 0)
+            name_prefix = ""
+        elif unit_type == "area":
+            dimensional_exponents = self.file.createIfcDimensionalExponents(2, 0, 0, 0, 0, 0, 0)
+            name_prefix = "square"
+        elif unit_type == "volume":
+            dimensional_exponents = self.file.createIfcDimensionalExponents(3, 0, 0, 0, 0, 0, 0)
+            name_prefix = "cubic"
+        si_unit = self.file.createIfcSIUnit(
+            None,
+            "{}UNIT".format(unit_type.upper()),
+            None,
+            "{}METRE".format(name_prefix.upper() + "_" if name_prefix else ""),
+        )
+        if data["raw"] == "INCHES":
+            name = "{}inch".format(name_prefix + " " if name_prefix else "")
+        elif data["raw"] == "FEET":
+            name = "{}foot".format(name_prefix + " " if name_prefix else "")
+        value_component = self.file.create_entity("IfcReal", **{"wrappedValue": ifcopenshell.util.unit.si_conversions[name]})
+        conversion_factor = self.file.createIfcMeasureWithUnit(value_component, si_unit)
+        return self.file.createIfcConversionBasedUnit(
+            dimensional_exponents, "{}UNIT".format(unit_type.upper()), name, conversion_factor
+        )

src/ifcopenshell-python/ifcopenshell/api/unit/data.py

@@ -0,0 +1,20 @@
+class Data:
+    is_loaded = False
+    units = {}
+
+    @classmethod
+    def purge(cls):
+        cls.is_loaded = False
+        cls.units = {}
+
+    @classmethod
+    def load(cls, file):
+        if not file:
+            return
+        unit_assignment = file.by_type("IfcUnitAssignment")
+        if not unit_assignment:
+            return
+        for unit in unit_assignment[0].Units:
+            pass
+        # TODO: implement along with UI
+        cls.is_loaded = True