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, 3 insertions, 3 deletions

diff --git a/lib/sqlalchemy/engine/row.py b/lib/sqlalchemy/engine/row.py
index 55d8c2249..b58b350e2 100644
--- a/lib/sqlalchemy/engine/row.py
+++ b/lib/sqlalchemy/engine/row.py
@@ -84,8 +84,8 @@ except ImportError:
         def _subscript_impl(self, key, ismapping):
             try:
                 rec = self._keymap[key]
-            except KeyError:
-                rec = self._parent._key_fallback(key)
+            except KeyError as ke:
+                rec = self._parent._key_fallback(key, ke)
             except TypeError:
                 # the non-C version detects a slice using TypeError.
                 # this is pretty inefficient for the slice use case
@@ -119,7 +119,7 @@ except ImportError:
             try:
                 return self._get_by_key_impl_mapping(name)
             except KeyError as e:
-                raise AttributeError(e.args[0])
+                util.raise_(AttributeError(e.args[0]), replace_context=e)
 
 
 class Row(BaseRow, collections_abc.Sequence):