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, 14 insertions, 11 deletions

diff --git a/lib/sqlalchemy/sql/base.py b/lib/sqlalchemy/sql/base.py
index a7324c45f..2d336360f 100644
--- a/lib/sqlalchemy/sql/base.py
+++ b/lib/sqlalchemy/sql/base.py
@@ -128,8 +128,8 @@ class _DialectArgView(util.collections_abc.MutableMapping):
     def _key(self, key):
         try:
             dialect, value_key = key.split("_", 1)
-        except ValueError:
-            raise KeyError(key)
+        except ValueError as err:
+            util.raise_(KeyError(key), replace_context=err)
         else:
             return dialect, value_key
 
@@ -138,17 +138,20 @@ class _DialectArgView(util.collections_abc.MutableMapping):
 
         try:
             opt = self.obj.dialect_options[dialect]
-        except exc.NoSuchModuleError:
-            raise KeyError(key)
+        except exc.NoSuchModuleError as err:
+            util.raise_(KeyError(key), replace_context=err)
         else:
             return opt[value_key]
 
     def __setitem__(self, key, value):
         try:
             dialect, value_key = self._key(key)
-        except KeyError:
-            raise exc.ArgumentError(
-                "Keys must be of the form <dialectname>_<argname>"
+        except KeyError as err:
+            util.raise_(
+                exc.ArgumentError(
+                    "Keys must be of the form <dialectname>_<argname>"
+                ),
+                replace_context=err,
             )
         else:
             self.obj.dialect_options[dialect][value_key] = value
@@ -634,17 +637,17 @@ class ColumnCollection(object):
     def __getitem__(self, key):
         try:
             return self._index[key]
-        except KeyError:
+        except KeyError as err:
             if isinstance(key, util.int_types):
-                raise IndexError(key)
+                util.raise_(IndexError(key), replace_context=err)
             else:
                 raise
 
     def __getattr__(self, key):
         try:
             return self._index[key]
-        except KeyError:
-            raise AttributeError(key)
+        except KeyError as err:
+            util.raise_(AttributeError(key), replace_context=err)
 
     def __contains__(self, key):
         if key not in self._index: