initial work, basic unit class

README.md

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+personification of force and pure energy

bia/test_units.py

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+from .units import Mass
+
+
+def test_basic_conversions():
+    a = Mass(2, 'kg')
+    kg = a.as_unit('kg')
+    assert kg.scalar == 2
+    assert kg.unit == 'kg'
+
+    lb = a.as_unit('lb')
+    assert abs(lb.scalar - 4.40925) < 0.0001
+    assert lb.unit == 'lb'
+
+    g = a.as_unit('g')
+    assert abs(g.scalar - 2000) < 0.0001
+    assert g.unit == 'g'
+
+    assert g.as_unit('lb').scalar == a.as_unit('lb').scalar

bia/units.py

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+from collections import defaultdict
+
+
+class UnitError(ValueError):
+    pass
+
+
+class ConversionError(ValueError):
+    pass
+
+
+class Unit(object):
+    _mapping = {}
+
+    def __init__(self, n, unit):
+        self.scalar = n
+        if unit not in self._mapping:
+            raise ValueError('invalid unit {} for {}'.format(unit, self.__class__.__name__))
+        self.unit = unit
+
+    def as_unit(self, unit):
+        if self.unit == unit:
+            return self.__class__(self.scalar, self.unit)
+
+        try:
+            if self.unit == self._base:
+                factor = self._mapping[unit]
+            else:
+                factor = self._mapping[unit] / self._mapping[self.unit]
+        except KeyError:
+            raise ConversionError('cannot convert from {} to {}'.format(self.unit, unit))
+
+        return self.__class__(self.scalar * factor, unit)
+
+    def __str__(self):
+        return '{}{}'.format(self.scalar, self.unit)
+
+    def __repr__(self):
+        return 'U({}, {!r})'.format(self.scalar, self.unit)
+
+    def __cmp__(self):
+        pass
+
+
+class Mass(Unit):
+    _base = 'kg'
+
+    # 1kg ==
+    _mapping = {
+        'kg': 1.0,
+        'lb': 2.20462,
+        'g': 1000.0,
+    }