adding a units module

when we want to produce JSON-LD packages from other
formats we often have only unit symbols like `kg`, `m3`
etc. available. But in the openLCA JSON-LD format
each unit is contained in a unit group and flow
amounts are not only given in a unit but in a unit of
a specific quantity (e.g. `m3 volume` vs.
`m3 norm volume`). So when we have a unit symbol for
an amount we need to get the references to the entities
of the corresponding classes:

* Unit
* UnitGroup
* FlowProperty

```
import olca.units as units

units.unit_ref(unit_symbol: str) -> Ref
units.group_ref(unit_symbol: str) -> Ref
units.property_ref(unit_symbol: str) -> Ref
```

we store the inits as a data file of this package. it
is a simple CSV file that can be generated via the
following Python script in openLCA

```python
import org.openlca.app.db.Database as DB
import org.openlca.io.UnitMapping as UnitMapping

import codecs
import csv

with codecs.open("C:/Users/Besitzer/Desktop/units.csv", "wb", encoding="utf-8") as f:
  w = csv.writer(f)
  w.writerow([
    "unit name",
    "unit uuid",
    "unit group name",
    "unit group uuid",
    "flow property name",
    "flow property uuid"
  ])
  mapping = UnitMapping.createDefault(DB.get())
  for uname in mapping.getUnits():
    group = mapping.getUnitGroup(uname)
    unit = group.getUnit(uname)
    prop = mapping.getFlowProperty(uname)
    w.writerow([
      uname,
      unit.refId,
      group.name,
      group.refId,
      prop.name,
      prop.refId
    ])

.gitignore

@@ -1,5 +1,7 @@
-.idea/
-*/__pycache__/
-*.egg-info/
-build/
-dist/
+/.idea/
+/*/__pycache__/
+/*.egg-info/
+/build/
+/dist/
+/env/
+/.vscode/

MANIFEST.in

@@ -0,0 +1 @@
+include olca/units/units.csv

olca/units/__init__.py

@@ -0,0 +1,54 @@
+import csv
+import os.path as path
+
+from typing import Optional
+
+import olca
+
+_unit_refs = None
+_group_refs = None
+_prop_refs = None
+
+
+def _init():
+    global _unit_refs, _group_refs, _prop_refs
+    _unit_refs = {}
+    _group_refs = {}
+    _prop_refs = {}
+
+    fpath = path.join(path.dirname(__file__), 'units.csv')
+    with open(fpath, 'r', encoding='utf-8') as f:
+        r = csv.reader(f)
+        next(r)
+        for row in r:
+            unit = row[0]
+            _unit_refs[unit] = olca.ref(olca.Unit, row[1], unit)
+            _group_refs[unit] = olca.ref(olca.UnitGroup, row[3], row[2])
+            _prop_refs[unit] = olca.ref(olca.FlowProperty, row[5], row[4])
+
+
+def unit_ref(symbol: str) -> Optional[olca.Ref]:
+    """Get the unit reference (an instance of olca.Ref) for the given unit
+       symbol (a string like 'kg'). Returns `None` if the given unit is
+       unknown."""
+    if _unit_refs is None:
+        _init()
+    return _unit_refs.get(symbol)
+
+
+def group_ref(symbol: str) -> Optional[olca.Ref]:
+    """Get the unit group reference (an instance of olca.Ref) for the given
+       unit symbol (a string like 'kg'). Returns `None` if the given unit is
+       unknown."""
+    if _group_refs is None:
+        _init()
+    return _group_refs.get(symbol)
+
+
+def property_ref(symbol: str) -> Optional[olca.Ref]:
+    """Get the reference (an instance of olca.Ref) to the default flow property
+       (quantity) for the given unit symbol (a string like 'kg'). Returns
+       `None` if the given unit is unknown."""
+    if _prop_refs is None:
+        _init()
+    return _prop_refs.get(symbol)

requirements.txt

@@ -0,0 +1 @@
+requests>=2.22.0

setup.py

@@ -9,12 +9,13 @@
 
 setup(
     name='olca-ipc',
-    version='0.0.7',
+    version='0.0.8',
     description='A Python package for calling openLCA functions from Python.',
     long_description=long_description,
     url='https://github.com/GreenDelta/olca-ipc.py',
     packages=['olca'],
     install_requires=['requests'],
+    include_package_data=True,
     keywords=['openLCA', 'life cycle assessment', 'LCA'],
     classifiers=[
         "Development Status :: 2 - Pre-Alpha",

tests/test_units.py

@@ -0,0 +1,31 @@
+import unittest
+
+import olca.units as units
+
+
+class TestUnits(unittest.TestCase):
+
+    def test_unit_ref(self):
+        ref = units.unit_ref('m2')
+        self.assertEqual(
+            '3ce61faa-5716-41c1-aef6-b5920054acc9',
+            ref.id),
+        self.assertEqual('m2', ref.name)
+
+    def test_group_ref(self):
+        ref = units.group_ref('m2')
+        self.assertEqual(
+            '93a60a57-a3c8-18da-a746-0800200c9a66',
+            ref.id),
+        self.assertEqual('Units of area', ref.name)
+
+    def test_prop_ref(self):
+        ref = units.property_ref('m2')
+        self.assertEqual(
+            '93a60a56-a3c8-19da-a746-0800200c9a66',
+            ref.id),
+        self.assertEqual('Area', ref.name)
+
+
+if __name__ == '__main__':
+    unittest.main()