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, 15 insertions, 8 deletions

diff --git a/lib/sqlalchemy/ext/compiler.py b/lib/sqlalchemy/ext/compiler.py
index c27907cdc..b8b6f8dc0 100644
--- a/lib/sqlalchemy/ext/compiler.py
+++ b/lib/sqlalchemy/ext/compiler.py
@@ -398,6 +398,7 @@ Example usage::
 
 """
 from .. import exc
+from .. import util
 from ..sql import sqltypes
 from ..sql import visitors
 
@@ -422,10 +423,13 @@ def compiles(class_, *specs):
                 def _wrap_existing_dispatch(element, compiler, **kw):
                     try:
                         return existing_dispatch(element, compiler, **kw)
-                    except exc.UnsupportedCompilationError:
-                        raise exc.CompileError(
-                            "%s construct has no default "
-                            "compilation handler." % type(element)
+                    except exc.UnsupportedCompilationError as uce:
+                        util.raise_(
+                            exc.CompileError(
+                                "%s construct has no default "
+                                "compilation handler." % type(element)
+                            ),
+                            from_=uce,
                         )
 
                 existing.specs["default"] = _wrap_existing_dispatch
@@ -470,10 +474,13 @@ class _dispatcher(object):
         if not fn:
             try:
                 fn = self.specs["default"]
-            except KeyError:
-                raise exc.CompileError(
-                    "%s construct has no default "
-                    "compilation handler." % type(element)
+            except KeyError as ke:
+                util.raise_(
+                    exc.CompileError(
+                        "%s construct has no default "
+                        "compilation handler." % type(element)
+                    ),
+                    replace_context=ke,
                 )
 
         # if compilation includes add_to_result_map, collect add_to_result_map