From cfac04da2a2152b01c4fc8e7c80cffa830595dec Mon Sep 17 00:00:00 2001
From: Mike Bayer <mike_mp@zzzcomputing.com>
Date: Tue, 16 Jul 2019 13:02:16 -0400
Subject: [PATCH] Intercept unresolveable comparator attrbute error for attr
 access

Fixed bug where a synonym created against a mapped attribute that does not
exist yet, as is the case when it refers to backref before mappers are
configured, would raise recursion errors when trying to test for attributes
on it which ultimately don't exist (as occurs when the classes are run
through Sphinx autodoc), as the unconfigured state of the synonym would put
it into an attribute not found loop.

Fixes: #4767
Change-Id: I9aade8628349fbf538181a0049416cec0a17179c
(cherry picked from commit 234723fa296c9fa7ac63f1c4d877edc7ba120edd)
---
 doc/build/changelog/unreleased_13/4767.rst | 11 ++++++++
 lib/sqlalchemy/orm/attributes.py           | 31 +++++++++++++++-------
 test/orm/test_mapper.py                    | 26 ++++++++++++++++++
 3 files changed, 58 insertions(+), 10 deletions(-)
 create mode 100644 doc/build/changelog/unreleased_13/4767.rst

diff --git a/doc/build/changelog/unreleased_13/4767.rst b/doc/build/changelog/unreleased_13/4767.rst
new file mode 100644
index 0000000000..04608ade97
--- /dev/null
+++ b/doc/build/changelog/unreleased_13/4767.rst
@@ -0,0 +1,11 @@
+.. change::
+    :tags: bug, orm
+    :tickets: 4767
+
+    Fixed bug where a synonym created against a mapped attribute that does not
+    exist yet, as is the case when it refers to backref before mappers are
+    configured, would raise recursion errors when trying to test for attributes
+    on it which ultimately don't exist (as occurs when the classes are run
+    through Sphinx autodoc), as the unconfigured state of the synonym would put
+    it into an attribute not found loop.
+
diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index 9d4a0d0823..a415a15fa4 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -365,23 +365,34 @@ def create_proxied_attribute(descriptor):
         def __getattr__(self, attribute):
             """Delegate __getattr__ to the original descriptor and/or
             comparator."""
-
             try:
                 return getattr(descriptor, attribute)
             except AttributeError:
+                if attribute == "comparator":
+                    raise AttributeError("comparator")
                 try:
-                    return getattr(self.comparator, attribute)
+                    # comparator itself might be unreachable
+                    comparator = self.comparator
                 except AttributeError:
                     raise AttributeError(
-                        "Neither %r object nor %r object associated with %s "
-                        "has an attribute %r"
-                        % (
-                            type(descriptor).__name__,
-                            type(self.comparator).__name__,
-                            self,
-                            attribute,
-                        )
+                        "Neither %r object nor unconfigured comparator "
+                        "object associated with %s has an attribute %r"
+                        % (type(descriptor).__name__, self, attribute)
                     )
+                else:
+                    try:
+                        return getattr(comparator, attribute)
+                    except AttributeError:
+                        raise AttributeError(
+                            "Neither %r object nor %r object "
+                            "associated with %s has an attribute %r"
+                            % (
+                                type(descriptor).__name__,
+                                type(comparator).__name__,
+                                self,
+                                attribute,
+                            )
+                        )
 
     Proxy.__name__ = type(descriptor).__name__ + "Proxy"
 
diff --git a/test/orm/test_mapper.py b/test/orm/test_mapper.py
index 6103bfa2fb..08f3eca2b8 100644
--- a/test/orm/test_mapper.py
+++ b/test/orm/test_mapper.py
@@ -1535,6 +1535,32 @@ class MapperTest(_fixtures.FixtureTest, AssertsCompiledSQL):
         eq_(attributes.instance_state(u1).attrs.x.history, ([5], (), ()))
         eq_(attributes.instance_state(u1).attrs.y.history, ([5], (), ()))
 
+    def test_synonym_nonexistent_attr(self):
+        # test [ticket:4767].
+        # synonym points to non-existent attrbute that hasn't been mapped yet.
+        users = self.tables.users
+
+        class User(object):
+            def _x(self):
+                return self.id
+
+            x = property(_x)
+
+        m = mapper(
+            User,
+            users,
+            properties={"x": synonym("some_attr", descriptor=User.x)},
+        )
+
+        # object gracefully handles this condition
+        assert not hasattr(User.x, "__name__")
+        assert not hasattr(User.x, "comparator")
+
+        m.add_property("some_attr", column_property(users.c.name))
+
+        assert not hasattr(User.x, "__name__")
+        assert hasattr(User.x, "comparator")
+
     def test_synonym_of_non_property_raises(self):
         from sqlalchemy.ext.associationproxy import association_proxy
 
-- 
2.47.2