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, 7 insertions, 4 deletions

diff --git a/lib/sqlalchemy/processors.py b/lib/sqlalchemy/processors.py
index 67f1564ec..8618d5e2a 100644
--- a/lib/sqlalchemy/processors.py
+++ b/lib/sqlalchemy/processors.py
@@ -32,10 +32,13 @@ def str_to_datetime_processor_factory(regexp, type_):
         else:
             try:
                 m = rmatch(value)
-            except TypeError:
-                raise ValueError(
-                    "Couldn't parse %s string '%r' "
-                    "- value is not a string." % (type_.__name__, value)
+            except TypeError as err:
+                util.raise_(
+                    ValueError(
+                        "Couldn't parse %s string '%r' "
+                        "- value is not a string." % (type_.__name__, value)
+                    ),
+                    from_=err,
                 )
             if m is None:
                 raise ValueError(