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, 9 insertions, 6 deletions

diff --git a/lib/sqlalchemy/orm/interfaces.py b/lib/sqlalchemy/orm/interfaces.py
index f75c7d3ba..57c192a5d 100644
--- a/lib/sqlalchemy/orm/interfaces.py
+++ b/lib/sqlalchemy/orm/interfaces.py
@@ -557,12 +557,15 @@ class StrategizedProperty(MapperProperty):
         try:
             return self._strategies[key]
         except KeyError:
-            cls = self._strategy_lookup(self, *key)
-            # this previously was setting self._strategies[cls], that's
-            # a bad idea; should use strategy key at all times because every
-            # strategy has multiple keys at this point
-            self._strategies[key] = strategy = cls(self, key)
-            return strategy
+            pass
+
+        # run outside to prevent transfer of exception context
+        cls = self._strategy_lookup(self, *key)
+        # this previously was setting self._strategies[cls], that's
+        # a bad idea; should use strategy key at all times because every
+        # strategy has multiple keys at this point
+        self._strategies[key] = strategy = cls(self, key)
+        return strategy
 
     def setup(self, context, query_entity, path, adapter, **kwargs):
         loader = self._get_context_loader(context, path)