Fix silently ignored AttributeError in custom accessors (#935)

Fixes GH933

Author: shoyer

xarray/core/extensions.py

@@ -1,5 +1,8 @@
+import traceback
+
 from .dataarray import DataArray
 from .dataset import Dataset
+from .pycompat import PY2
 
 
 class AccessorRegistrationError(Exception):
@@ -16,7 +19,16 @@ def __get__(self, obj, cls):
         if obj is None:
             # we're accessing the attribute of the class, i.e., Dataset.geo
             return self._accessor
-        accessor_obj = self._accessor(obj)
+        try:
+            accessor_obj = self._accessor(obj)
+        except AttributeError:
+            # __getattr__ on data object will swallow any AttributeErrors raised
+            # when initializing the accessor, so we need to raise as something
+            # else (GH933):
+            msg = 'error initializing %r accessor.' % self._name
+            if PY2:
+                msg += ' Full traceback:\n' + traceback.format_exc()
+            raise RuntimeError(msg)
         # Replace the property with the accessor object. Inspired by:
         # http://www.pydanny.com/cached-property.html
         # We need to use object.__setattr__ because we overwrite __setattr__ on

xarray/test/test_extensions.py

@@ -74,3 +74,14 @@ def test_pickle_dataarray(self):
         assert array.example_accessor is array.example_accessor
         array_restored = pickle.loads(pickle.dumps(array))
         assert array.identical(array_restored)
+
+    def test_broken_accessor(self):
+        # regression test for GH933
+
+        @xr.register_dataset_accessor('stupid_accessor')
+        class BrokenAccessor(object):
+            def __init__(self, xarray_obj):
+                raise AttributeError('broken')
+
+        with self.assertRaisesRegexp(RuntimeError, 'error initializing'):
+            xr.Dataset().stupid_accessor