preserve contextual error when patching the loader (#18)

resolves #17

oyaml.py

@@ -17,7 +17,12 @@ def map_representer(dumper, data):
 
 def map_constructor(loader, node):
     loader.flatten_mapping(node)
-    return OrderedDict(loader.construct_pairs(node))
+    pairs = loader.construct_pairs(node)
+    try:
+        return OrderedDict(pairs)
+    except TypeError:
+        loader.construct_mapping(node)  # trigger any contextual error
+        raise
 
 
 if pyyaml.safe_dump is pyyaml.dump:

setup.py

@@ -2,7 +2,7 @@
 
 setup(
     name="oyaml",
-    version="0.8",
+    version="0.9",
     description="Ordered YAML: drop-in replacement for PyYAML which preserves dict ordering",
     long_description=open("README.rst").read(),
     author="Wim Glenn",

test_oyaml.py

@@ -2,6 +2,7 @@
 from types import GeneratorType
 
 import pytest
+from yaml.constructor import ConstructorError
 from yaml.representer import RepresenterError
 
 import oyaml as yaml
@@ -186,3 +187,8 @@ def test_merge():
     assert map2 == expected
     assert map3 == expected
     assert map4 == expected
+
+
+def test_unhashable_error_context():
+    with pytest.raises(ConstructorError, match=r".*line.*column.*"):
+        yaml.safe_load("{foo: bar}: baz")