Allow metadata.reflect() to recover from unreflectable tables

Added support for views that are unreflectable due to stale
table definitions, when calling :meth:`.MetaData.reflect`; a warning
is emitted for the table that cannot respond to ``DESCRIBE``
but the operation succeeds.  The MySQL dialect now
raises UnreflectableTableError which is in turn caught by
MetaData.reflect().  Reflecting the view standalone raises
this error directly.

Change-Id: Id8005219d8e073c154cc84a873df911b4a6cf4d6
Fixes: #3871

1 files changed, 11 insertions, 4 deletions

diff --git a/lib/sqlalchemy/testing/assertions.py b/lib/sqlalchemy/testing/assertions.py
index 884556345..dfea33dc7 100644
--- a/lib/sqlalchemy/testing/assertions.py
+++ b/lib/sqlalchemy/testing/assertions.py
@@ -130,9 +130,16 @@ def _expect_warnings(exc_cls, messages, regex=True, assert_=True,
 
     real_warn = warnings.warn
 
-    def our_warn(msg, exception, *arg, **kw):
-        if not issubclass(exception, exc_cls):
-            return real_warn(msg, exception, *arg, **kw)
+    def our_warn(msg, *arg, **kw):
+        if isinstance(msg, exc_cls):
+            exception = msg
+            msg = str(exception)
+        elif arg:
+            exception = arg[0]
+        else:
+            exception = None
+        if not exception or not issubclass(exception, exc_cls):
+            return real_warn(msg, *arg, **kw)
 
         if not filters:
             return
@@ -143,7 +150,7 @@ def _expect_warnings(exc_cls, messages, regex=True, assert_=True,
                 seen.discard(filter_)
                 break
         else:
-            real_warn(msg, exception, *arg, **kw)
+            real_warn(msg, *arg, **kw)
 
     with mock.patch("warnings.warn", our_warn):
         yield