Ensure all nested exception throws have a cause

Applied an explicit "cause" to most if not all internally raised exceptions
that are raised from within an internal exception catch, to avoid
misleading stacktraces that suggest an error within the handling of an
exception.  While it would be preferable to suppress the internally caught
exception in the way that the ``__suppress_context__`` attribute would,
there does not as yet seem to be a way to do this without suppressing an
enclosing user constructed context, so for now it exposes the internally
caught exception as the cause so that full information about the context
of the error is maintained.

Fixes: #4849
Change-Id: I55a86b29023675d9e5e49bc7edc5a2dc0bcd4751

1 files changed, 8 insertions, 7 deletions

diff --git a/lib/sqlalchemy/ext/indexable.py b/lib/sqlalchemy/ext/indexable.py
index f2e0501bb..6eb7e1185 100644
--- a/lib/sqlalchemy/ext/indexable.py
+++ b/lib/sqlalchemy/ext/indexable.py
@@ -223,7 +223,8 @@ The above query will render::
 """  # noqa
 from __future__ import absolute_import
 
-from sqlalchemy import inspect
+from .. import inspect
+from .. import util
 from ..ext.hybrid import hybrid_property
 from ..orm.attributes import flag_modified
 
@@ -301,9 +302,9 @@ class index_property(hybrid_property):  # noqa
                 self.datatype = dict
         self.onebased = onebased
 
-    def _fget_default(self):
+    def _fget_default(self, err=None):
         if self.default == self._NO_DEFAULT_ARGUMENT:
-            raise AttributeError(self.attr_name)
+            util.raise_(AttributeError(self.attr_name), replace_context=err)
         else:
             return self.default
 
@@ -314,8 +315,8 @@ class index_property(hybrid_property):  # noqa
             return self._fget_default()
         try:
             value = column_value[self.index]
-        except (KeyError, IndexError):
-            return self._fget_default()
+        except (KeyError, IndexError) as err:
+            return self._fget_default(err)
         else:
             return value
 
@@ -337,8 +338,8 @@ class index_property(hybrid_property):  # noqa
             raise AttributeError(self.attr_name)
         try:
             del column_value[self.index]
-        except KeyError:
-            raise AttributeError(self.attr_name)
+        except KeyError as err:
+            util.raise_(AttributeError(self.attr_name), replace_context=err)
         else:
             setattr(instance, attr_name, column_value)
             flag_modified(instance, attr_name)