return NotImplemented for binary special methods

This patch fixes the behaviour or special binary methods (e.g. __add__) by
returning NotImplemented upon failure rather than raising DimensionError. See
https://docs.python.org/3.6/library/constants.html#NotImplemented.

nutils/SI.py

@@ -414,40 +414,47 @@ def __dotarg(op, *args, **kwargs):
 
     ## DEFINE OPERATORS
 
-    def op(name, with_reverse=False, *, __table=__DISPATCH_TABLE):
-        dispatch = __table[name]
-        op = getattr(operator, name)
-        ret = lambda *args: dispatch(op, *args)
-        if with_reverse:
-            ret = ret, lambda self, other: dispatch(op, other, self)
-        return ret
+    def op(name, __table=__DISPATCH_TABLE):
+        dispatch, op = __table[name], getattr(operator, name)
+        return lambda *args: dispatch(op, *args)
 
     __getitem__ = op('getitem')
     __setitem__ = op('setitem')
     __neg__ = op('neg')
     __pos__ = op('pos')
     __abs__ = op('abs')
+
+    def op(name, __table=__DISPATCH_TABLE):
+        dispatch, op = __table[name], getattr(operator, name)
+        return lambda *args: _try_or_noimp(dispatch, op, *args)
+
     __lt__ = op('lt')
     __le__ = op('le')
     __eq__ = op('eq')
     __ne__ = op('ne')
     __gt__ = op('gt')
     __ge__ = op('ge')
-    __add__, __radd__ = op('add', True)
-    __sub__, __rsub__ = op('sub', True)
-    __mul__, __rmul__ = op('mul', True)
-    __matmul__, __rmatmul__ = op('matmul', True)
-    __truediv, __rtruediv__ = op('truediv', True)
-    __mod__, __rmod__ = op('mod', True)
-    __pow__, __rpow__ = op('pow', True)
+
+    def op(name, __table=__DISPATCH_TABLE):
+        dispatch, op = __table[name], getattr(operator, name)
+        return lambda self, other: _try_or_noimp(dispatch, op, self, other), \
+               lambda self, other: _try_or_noimp(dispatch, op, other, self)
+
+    __add__, __radd__ = op('add')
+    __sub__, __rsub__ = op('sub')
+    __mul__, __rmul__ = op('mul')
+    __matmul__, __rmatmul__ = op('matmul')
+    __truediv, __rtruediv__ = op('truediv')
+    __mod__, __rmod__ = op('mod')
+    __pow__, __rpow__ = op('pow')
+
+    del op
 
     def __truediv__(self, other):
         if type(other) is str:
             return self.__value / self.__class__(other).__value
         return self.__truediv(other)
 
-    del op
-
     ## DISPATCH THIRD PARTY CALLS
 
     def __array_ufunc__(self, ufunc, method, *inputs, **kwargs):
@@ -504,6 +511,13 @@ def _split_factors(s):
             isnumer = False
 
 
+def _try_or_noimp(func, *args):
+    try:
+        return func(*args)
+    except DimensionError:
+        return NotImplemented
+
+
 ## SI DIMENSIONS
 
 Dimensionless = Dimension.from_powers({})

tests/test_SI.py

@@ -104,13 +104,13 @@ def test_power(self):
     def test_add(self):
         self.assertEqual(SI.Mass('2kg') + SI.Mass('3kg'), SI.Mass('5kg'))
         self.assertEqual(numpy.add(SI.Mass('2kg'), SI.Mass('3kg')), SI.Mass('5kg'))
-        with self.assertRaisesRegex(SI.DimensionError, r'incompatible arguments for add: \[M\], \[L\]'):
+        with self.assertRaisesRegex(TypeError, r"unsupported operand type\(s\) for \+: '\[M\]' and '\[L\]'"):
             SI.Mass('2kg') + SI.Length('3m')
 
     def test_sub(self):
         self.assertEqual(SI.Mass('2kg') - SI.Mass('3kg'), SI.Mass('-1kg'))
         self.assertEqual(numpy.subtract(SI.Mass('2kg'), SI.Mass('3kg')), SI.Mass('-1kg'))
-        with self.assertRaisesRegex(SI.DimensionError, r'incompatible arguments for sub: \[M\], \[L\]'):
+        with self.assertRaisesRegex(TypeError, r"unsupported operand type\(s\) for \-: '\[M\]' and '\[L\]'"):
             SI.Mass('2kg') - SI.Length('3m')
 
     def test_hypot(self):