Ensure .engine is part of Connectable interface, implement as descriptor

Fixed bug where using reflection function such as :meth:`.MetaData.reflect`
with an :class:`.Engine` object that had execution options applied to it
would fail, as the resulting :class:`.OptionEngine` proxy object failed to
include a ``.engine`` attribute used within the reflection routines.

Fixes: #4754
Change-Id: I6c342af5c6db6fe362b9d25f3f26d6859f62f87a

2 files changed, 11 insertions, 1 deletions

diff --git a/lib/sqlalchemy/engine/base.py b/lib/sqlalchemy/engine/base.py
index 7aee1a73b..07d1b8f89 100644
--- a/lib/sqlalchemy/engine/base.py
+++ b/lib/sqlalchemy/engine/base.py
@@ -1899,13 +1899,16 @@ class Engine(Connectable, log.Identified):
         if logging_name:
             self.logging_name = logging_name
         self.echo = echo
-        self.engine = self
         log.instance_logger(self, echoflag=echo)
         if proxy:
             interfaces.ConnectionProxy._adapt_listener(self, proxy)
         if execution_options:
             self.update_execution_options(**execution_options)
 
+    @property
+    def engine(self):
+        return self
+
     def update_execution_options(self, **opt):
         r"""Update the default execution_options dictionary
         of this :class:`.Engine`.
diff --git a/lib/sqlalchemy/engine/interfaces.py b/lib/sqlalchemy/engine/interfaces.py
index 4806e72a5..aadadb845 100644
--- a/lib/sqlalchemy/engine/interfaces.py
+++ b/lib/sqlalchemy/engine/interfaces.py
@@ -1117,6 +1117,13 @@ class Connectable(object):
 
         """
 
+    engine = None
+    """The :class:`.Engine` instance referred to by this :class:`.Connectable`.
+
+    May be ``self`` if this is already an :class:`.Engine`.
+
+    """
+
     @util.deprecated(
         "1.3",
         "The :meth:`.Engine.contextual_connect` and "