From c4c9866064f03646c686d7e08b00aeb203c35c19 Mon Sep 17 00:00:00 2001
From: Shane Harvey <shane.harvey@mongodb.com>
Date: Fri, 4 Aug 2017 01:45:00 -0700
Subject: [PATCH] bpo-31107: Fix copyreg mangled slot names calculation.
 (#2989)

---
 Lib/copyreg.py                                         |  6 +++++-
 Lib/test/test_copyreg.py                               | 10 ++++++++++
 Misc/ACKS                                              |  1 +
 .../Library/2017-08-02-12-48-15.bpo-31107.1t2hn5.rst   |  2 ++
 4 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2017-08-02-12-48-15.bpo-31107.1t2hn5.rst

diff --git a/Lib/copyreg.py b/Lib/copyreg.py
index ed29d719def..bbe1af4e2e7 100644
--- a/Lib/copyreg.py
+++ b/Lib/copyreg.py
@@ -128,7 +128,11 @@ def _slotnames(cls):
                         continue
                     # mangled names
                     elif name.startswith('__') and not name.endswith('__'):
-                        names.append('_%s%s' % (c.__name__, name))
+                        stripped = c.__name__.lstrip('_')
+                        if stripped:
+                            names.append('_%s%s' % (stripped, name))
+                        else:
+                            names.append(name)
                     else:
                         names.append(name)
 
diff --git a/Lib/test/test_copyreg.py b/Lib/test/test_copyreg.py
index 52e887cb36c..e3f1cd81aab 100644
--- a/Lib/test/test_copyreg.py
+++ b/Lib/test/test_copyreg.py
@@ -16,6 +16,12 @@ class WithWeakref(object):
 class WithPrivate(object):
     __slots__ = ('__spam',)
 
+class _WithLeadingUnderscoreAndPrivate(object):
+    __slots__ = ('__spam',)
+
+class ___(object):
+    __slots__ = ('__spam',)
+
 class WithSingleString(object):
     __slots__ = 'spam'
 
@@ -104,6 +110,10 @@ class CopyRegTestCase(unittest.TestCase):
         self.assertEqual(copyreg._slotnames(WithWeakref), [])
         expected = ['_WithPrivate__spam']
         self.assertEqual(copyreg._slotnames(WithPrivate), expected)
+        expected = ['_WithLeadingUnderscoreAndPrivate__spam']
+        self.assertEqual(copyreg._slotnames(_WithLeadingUnderscoreAndPrivate),
+                         expected)
+        self.assertEqual(copyreg._slotnames(___), ['__spam'])
         self.assertEqual(copyreg._slotnames(WithSingleString), ['spam'])
         expected = ['eggs', 'spam']
         expected.sort()
diff --git a/Misc/ACKS b/Misc/ACKS
index f1f6c147f49..ec7d481a587 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -598,6 +598,7 @@ David Harrigan
 Brian Harring
 Jonathan Hartley
 Travis B. Hartwell
+Shane Harvey
 Larry Hastings
 Tim Hatch
 Shane Hathaway
diff --git a/Misc/NEWS.d/next/Library/2017-08-02-12-48-15.bpo-31107.1t2hn5.rst b/Misc/NEWS.d/next/Library/2017-08-02-12-48-15.bpo-31107.1t2hn5.rst
new file mode 100644
index 00000000000..3c2a15528d8
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2017-08-02-12-48-15.bpo-31107.1t2hn5.rst
@@ -0,0 +1,2 @@
+Fix `copyreg._slotnames()` mangled attribute calculation for classes whose
+name begins with an underscore. Patch by Shane Harvey.