Stringify correctly for non-str exception argument

Fixed issue where a non-string object sent to
:class:`_exc.SQLAlchemyError` or a subclass, as occurs with some third
party dialects, would fail to stringify correctly. Pull request
courtesy Andrzej Bartosiński.

Fixes: #5599
Closes: #5600
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5600
Pull-request-sha: cdccccc42a6ac8de771593a43ee8675bfd8dbeb6

Change-Id: Icd710d9015abc80f61a84893d75fbb33ee0fe46e

3 files changed, 36 insertions, 5 deletions

diff --git a/doc/build/changelog/unreleased_13/5599.rst b/doc/build/changelog/unreleased_13/5599.rst
new file mode 100644
index 000000000..c5a5dcc3a
--- /dev/null
+++ b/doc/build/changelog/unreleased_13/5599.rst
@@ -0,0 +1,8 @@
+.. change::
+    :tags: bug, engine
+    :tickets: 5599
+
+    Fixed issue where a non-string object sent to
+    :class:`_exc.SQLAlchemyError` or a subclass, as occurs with some third
+    party dialects, would fail to stringify correctly. Pull request
+    courtesy Andrzej Bartosiński.
diff --git a/lib/sqlalchemy/exc.py b/lib/sqlalchemy/exc.py
index c2308a5cc..b80bf9b01 100644
--- a/lib/sqlalchemy/exc.py
+++ b/lib/sqlalchemy/exc.py
@@ -56,10 +56,18 @@ class SQLAlchemyError(Exception):
         #
         if len(self.args) == 1:
             text = self.args[0]
+
             if as_unicode and isinstance(text, compat.binary_types):
-                return compat.decode_backslashreplace(text, "utf-8")
+                text = compat.decode_backslashreplace(text, "utf-8")
+            # This is for when the argument is not a string of any sort.
+            # Otherwise, converting this exception to string would fail for
+            # non-string arguments.
+            elif compat.py3k or not as_unicode:
+                text = str(text)
             else:
-                return self.args[0]
+                text = compat.text_type(text)
+
+            return text
         else:
             # this is not a normal case within SQLAlchemy but is here for
             # compatibility with Exception.args - the str() comes out as
diff --git a/test/engine/test_execute.py b/test/engine/test_execute.py
index 7e3893afd..73681ee51 100644
--- a/test/engine/test_execute.py
+++ b/test/engine/test_execute.py
@@ -58,6 +58,14 @@ class SomeException(Exception):
     pass
 
 
+class Foo(object):
+    def __str__(self):
+        return "foo"
+
+    def __unicode__(self):
+        return util.u("fóó")
+
+
 class ExecuteTest(fixtures.TablesTest):
     __backend__ = True
 
@@ -382,7 +390,7 @@ class ExecuteTest(fixtures.TablesTest):
             eq_(str(err), message)
 
             # unicode accessor decodes to utf-8
-            eq_(unicode(err), util.u("some message méil"))  # noqa
+            eq_(unicode(err), util.u("some message méil"))  # noqa F821
         else:
             eq_(str(err), util.u("some message méil"))
 
@@ -397,7 +405,7 @@ class ExecuteTest(fixtures.TablesTest):
             eq_(str(err), message)
 
             # unicode accessor decodes to utf-8
-            eq_(unicode(err), util.u("some message m\\xe9il"))  # noqa
+            eq_(unicode(err), util.u("some message m\\xe9il"))  # noqa F821
         else:
             eq_(str(err), util.u("some message m\\xe9il"))
 
@@ -408,10 +416,17 @@ class ExecuteTest(fixtures.TablesTest):
 
         err = tsa.exc.SQLAlchemyError(message)
         if util.py2k:
-            eq_(unicode(err), util.u("some message méil"))  # noqa
+            eq_(unicode(err), util.u("some message méil"))  # noqa F821
         else:
             eq_(str(err), util.u("some message méil"))
 
+    def test_stmt_exception_object_arg(self):
+        err = tsa.exc.SQLAlchemyError(Foo())
+        eq_(str(err), "foo")
+
+        if util.py2k:
+            eq_(unicode(err), util.u("fóó"))  # noqa F821
+
     def test_stmt_exception_str_multi_args(self):
         err = tsa.exc.SQLAlchemyError("some message", 206)
         eq_(str(err), "('some message', 206)")