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

5 files changed, 28 insertions, 1 deletions

diff --git a/doc/build/changelog/unreleased_13/4754.rst b/doc/build/changelog/unreleased_13/4754.rst
new file mode 100644
index 000000000..d8a3375cc
--- /dev/null
+++ b/doc/build/changelog/unreleased_13/4754.rst
@@ -0,0 +1,8 @@
+.. change::
+    :tags: bug, engine
+    :tickets: 4754
+
+    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.
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 "
diff --git a/test/engine/test_execute.py b/test/engine/test_execute.py
index 20c8ae659..335c3a487 100644
--- a/test/engine/test_execute.py
+++ b/test/engine/test_execute.py
@@ -635,10 +635,14 @@ class ExecuteTest(fixtures.TestBase):
             options={"execution_options": {"base": "x1"}}
         )
 
+        is_(eng.engine, eng)
+
         eng1 = eng.execution_options(foo="b1")
+        is_(eng1.engine, eng1)
         eng2 = eng.execution_options(foo="b2")
         eng1a = eng1.execution_options(bar="a1")
         eng2a = eng2.execution_options(foo="b3", bar="a2")
+        is_(eng2a.engine, eng2a)
 
         eq_(eng._execution_options, {"base": "x1"})
         eq_(eng1._execution_options, {"base": "x1", "foo": "b1"})
diff --git a/test/engine/test_reflection.py b/test/engine/test_reflection.py
index 3dc3a4461..1cabf8963 100644
--- a/test/engine/test_reflection.py
+++ b/test/engine/test_reflection.py
@@ -1198,6 +1198,11 @@ class ReflectionTest(fixtures.TestBase, ComparesTables):
     def test_reflect_uses_bind_engine_reflect(self):
         self._test_reflect_uses_bind(lambda e: MetaData().reflect(e))
 
+    def test_reflect_uses_bind_option_engine_reflect(self):
+        self._test_reflect_uses_bind(
+            lambda e: MetaData().reflect(e.execution_options(foo="bar"))
+        )
+
     @testing.provide_metadata
     def test_reflect_all(self):
         existing = testing.db.table_names()