Dimensions are transformed to base upon context parsing

3 files changed, 28 insertions, 13 deletions

diff --git a/pint/context.py b/pint/context.py
index 1ae4525..b4f2cbb 100644
--- a/pint/context.py
+++ b/pint/context.py
@@ -105,7 +105,7 @@ class Context(object):
         return context
 
     @classmethod
-    def from_lines(cls, lines):
+    def from_lines(cls, lines, to_base_func=None):
         header, lines = lines[0], lines[1:]
 
         r = _header_re.search(header)
@@ -141,10 +141,16 @@ class Context(object):
 
             if '<->' in rel:
                 src, dst = (ParserHelper.from_string(s) for s in rel.split('<->'))
+                if to_base_func:
+                    src = to_base_func(src)
+                    dst = to_base_func(dst)
                 ctx.add_transformation(src, dst, func)
                 ctx.add_transformation(dst, src, func)
             elif '->' in rel:
                 src, dst = (ParserHelper.from_string(s) for s in rel.split('->'))
+                if to_base_func:
+                    src = to_base_func(src)
+                    dst = to_base_func(dst)
                 ctx.add_transformation(src, dst, func)
             else:
                 raise ValueError('Relationships must be specified with <-> or ->.')
diff --git a/pint/testsuite/test_contexts.py b/pint/testsuite/test_contexts.py
index 116546b..9c4e4aa 100644
--- a/pint/testsuite/test_contexts.py
+++ b/pint/testsuite/test_contexts.py
@@ -599,13 +599,18 @@ class TestDefinedContexts(TestCase):
         with ureg.context('sp'):
             from pint.util import find_shortest_path
             for a, b in itertools.product(eq, eq):
-                da, db = Context.__keytransform__(a.dimensionality,
-                                                  b.dimensionality)
-                p = find_shortest_path(ureg._active_ctx.graph, da, db)
-                self.assertTrue(p)
-                msg = '{} <-> {}'.format(a, b)
-                self.assertAlmostEqualRelError(a, b, rel=.01, msg=msg)
-
-
-        #for a, b in itertools.product(eq, eq):
-        #    self.assertAlmostEqualRelError(a.to(b.units, ctx='sp'), b, rel=.01, msg=msg)
+                for x in range(2):
+                    if x == 1:
+                        a = a.to_base_units()
+                        b = b.to_base_units()
+                    da, db = Context.__keytransform__(a.dimensionality,
+                                                      b.dimensionality)
+                    p = find_shortest_path(ureg._active_ctx.graph, da, db)
+                    self.assertTrue(p)
+                    msg = '{} <-> {}'.format(a, b)
+                    # assertAlmostEqualRelError converts second to first
+                    self.assertAlmostEqualRelError(b, a, rel=.01, msg=msg)
+
+
+        for a, b in itertools.product(eq, eq):
+            self.assertAlmostEqualRelError(a.to(b.units, 'sp'), b, rel=.01, msg=msg)
diff --git a/pint/unit.py b/pint/unit.py
index 9f32416..fc34680 100644
--- a/pint/unit.py
+++ b/pint/unit.py
@@ -633,7 +633,8 @@ class UnitRegistry(object):
                 with open(file, encoding='utf-8') as fp:
                     return self.load_definitions(fp, is_resource)
             except Exception as e:
-                raise ValueError('While opening {}\n{}'.format(file, e.message))
+                msg = getattr(e, 'message', str(e))
+                raise ValueError('While opening {}\n{}'.format(file, msg))
 
         ifile = enumerate(file)
         for no, line in ifile:
@@ -655,7 +656,10 @@ class UnitRegistry(object):
                 for no, line in ifile:
                     line = line.strip()
                     if line.startswith('@end'):
-                        self.add_context(Context.from_lines(context))
+                        try:
+                            self.add_context(Context.from_lines(context, self.get_dimensionality))
+                        except KeyError as e:
+                            raise ValueError('Unknown dimension {}'.format(str(e)))
                         break
                     elif line.startswith('@'):
                         raise ValueError('In line {}, cannot nest @ directives:\n{}'.format(no, line))