diff --git a/Doc/library/weakref.rst b/Doc/library/weakref.rst
index 46bfc9d5a5d..9c655871b5e 100644
--- a/Doc/library/weakref.rst
+++ b/Doc/library/weakref.rst
@@ -209,6 +209,14 @@ methods of :class:`WeakKeyDictionary` objects.
    .. versionadded:: 2.5
 
 
+.. class:: WeakSet([elements])
+
+   Set class that keeps weak references to its elements.  An element will be
+   discarded when no strong reference to it exists any more.
+
+   .. versionadded:: 2.7
+
+
 .. data:: ReferenceType
 
    The type object for weak references objects.
diff --git a/Lib/_weakrefset.py b/Lib/_weakrefset.py
new file mode 100644
index 00000000000..90e07d4768b
--- /dev/null
+++ b/Lib/_weakrefset.py
@@ -0,0 +1,212 @@
+# Access WeakSet through the weakref module.
+# This code is separated-out because it is needed
+# by abc.py to load everything else at startup.
+
+from _weakref import ref
+
+__all__ = ['WeakSet']
+
+
+class _IterationGuard(object):
+    # This context manager registers itself in the current iterators of the
+    # weak container, such as to delay all removals until the context manager
+    # exits.
+    # This technique should be relatively thread-safe (since sets are).
+
+    def __init__(self, weakcontainer):
+        # Don't create cycles
+        self.weakcontainer = ref(weakcontainer)
+
+    def __enter__(self):
+        w = self.weakcontainer()
+        if w is not None:
+            w._iterating.add(self)
+        return self
+
+    def __exit__(self, e, t, b):
+        w = self.weakcontainer()
+        if w is not None:
+            s = w._iterating
+            s.remove(self)
+            if not s:
+                w._commit_removals()
+
+
+class WeakSet(object):
+    def __init__(self, data=None):
+        self.data = set()
+        def _remove(item, selfref=ref(self)):
+            self = selfref()
+            if self is not None:
+                if self._iterating:
+                    self._pending_removals.append(item)
+                else:
+                    self.data.discard(item)
+        self._remove = _remove
+        # A list of keys to be removed
+        self._pending_removals = []
+        self._iterating = set()
+        if data is not None:
+            self.update(data)
+
+    def _commit_removals(self):
+        l = self._pending_removals
+        discard = self.data.discard
+        while l:
+            discard(l.pop())
+
+    def __iter__(self):
+        with _IterationGuard(self):
+            for itemref in self.data:
+                item = itemref()
+                if item is not None:
+                    yield item
+
+    def __len__(self):
+        return sum(x() is not None for x in self.data)
+
+    def __contains__(self, item):
+        return ref(item) in self.data
+
+    def __reduce__(self):
+        return (self.__class__, (list(self),),
+                getattr(self, '__dict__', None))
+
+    __hash__ = None
+
+    def add(self, item):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.add(ref(item, self._remove))
+
+    def clear(self):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.clear()
+
+    def copy(self):
+        return self.__class__(self)
+
+    def pop(self):
+        if self._pending_removals:
+            self._commit_removals()
+        while True:
+            try:
+                itemref = self.data.pop()
+            except KeyError:
+                raise KeyError('pop from empty WeakSet')
+            item = itemref()
+            if item is not None:
+                return item
+
+    def remove(self, item):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.remove(ref(item))
+
+    def discard(self, item):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.discard(ref(item))
+
+    def update(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        if isinstance(other, self.__class__):
+            self.data.update(other.data)
+        else:
+            for element in other:
+                self.add(element)
+
+    def __ior__(self, other):
+        self.update(other)
+        return self
+
+    # Helper functions for simple delegating methods.
+    def _apply(self, other, method):
+        if not isinstance(other, self.__class__):
+            other = self.__class__(other)
+        newdata = method(other.data)
+        newset = self.__class__()
+        newset.data = newdata
+        return newset
+
+    def difference(self, other):
+        return self._apply(other, self.data.difference)
+    __sub__ = difference
+
+    def difference_update(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        if self is other:
+            self.data.clear()
+        else:
+            self.data.difference_update(ref(item) for item in other)
+    def __isub__(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        if self is other:
+            self.data.clear()
+        else:
+            self.data.difference_update(ref(item) for item in other)
+        return self
+
+    def intersection(self, other):
+        return self._apply(other, self.data.intersection)
+    __and__ = intersection
+
+    def intersection_update(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.intersection_update(ref(item) for item in other)
+    def __iand__(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        self.data.intersection_update(ref(item) for item in other)
+        return self
+
+    def issubset(self, other):
+        return self.data.issubset(ref(item) for item in other)
+    __lt__ = issubset
+
+    def __le__(self, other):
+        return self.data <= set(ref(item) for item in other)
+
+    def issuperset(self, other):
+        return self.data.issuperset(ref(item) for item in other)
+    __gt__ = issuperset
+
+    def __ge__(self, other):
+        return self.data >= set(ref(item) for item in other)
+
+    def __eq__(self, other):
+        if not isinstance(other, self.__class__):
+            return NotImplemented
+        return self.data == set(ref(item) for item in other)
+
+    def symmetric_difference(self, other):
+        return self._apply(other, self.data.symmetric_difference)
+    __xor__ = symmetric_difference
+
+    def symmetric_difference_update(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        if self is other:
+            self.data.clear()
+        else:
+            self.data.symmetric_difference_update(ref(item) for item in other)
+    def __ixor__(self, other):
+        if self._pending_removals:
+            self._commit_removals()
+        if self is other:
+            self.data.clear()
+        else:
+            self.data.symmetric_difference_update(ref(item) for item in other)
+        return self
+
+    def union(self, other):
+        return self._apply(other, self.data.union)
+    __or__ = union
+
+    def isdisjoint(self, other):
+        return len(self.intersection(other)) == 0
diff --git a/Lib/test/test_weakset.py b/Lib/test/test_weakset.py
new file mode 100644
index 00000000000..b4774ceaec1
--- /dev/null
+++ b/Lib/test/test_weakset.py
@@ -0,0 +1,376 @@
+import unittest
+from test import test_support
+from weakref import proxy, ref, WeakSet
+import operator
+import copy
+import string
+import os
+from random import randrange, shuffle
+import sys
+import warnings
+import collections
+import gc
+import contextlib
+
+
+class Foo:
+    pass
+
+class SomeClass(object):
+    def __init__(self, value):
+        self.value = value
+    def __eq__(self, other):
+        if type(other) != type(self):
+            return False
+        return other.value == self.value
+
+    def __ne__(self, other):
+        return not self.__eq__(other)
+
+    def __hash__(self):
+        return hash((SomeClass, self.value))
+
+class TestWeakSet(unittest.TestCase):
+
+    def setUp(self):
+        # need to keep references to them
+        self.items = [SomeClass(c) for c in ('a', 'b', 'c')]
+        self.items2 = [SomeClass(c) for c in ('x', 'y', 'z')]
+        self.letters = [SomeClass(c) for c in string.ascii_letters]
+        self.s = WeakSet(self.items)
+        self.d = dict.fromkeys(self.items)
+        self.obj = SomeClass('F')
+        self.fs = WeakSet([self.obj])
+
+    def test_methods(self):
+        weaksetmethods = dir(WeakSet)
+        for method in dir(set):
+            if method == 'test_c_api' or method.startswith('_'):
+                continue
+            self.assertIn(method, weaksetmethods,
+                         "WeakSet missing method " + method)
+
+    def test_new_or_init(self):
+        self.assertRaises(TypeError, WeakSet, [], 2)
+
+    def test_len(self):
+        self.assertEqual(len(self.s), len(self.d))
+        self.assertEqual(len(self.fs), 1)
+        del self.obj
+        self.assertEqual(len(self.fs), 0)
+
+    def test_contains(self):
+        for c in self.letters:
+            self.assertEqual(c in self.s, c in self.d)
+        self.assertRaises(TypeError, self.s.__contains__, [[]])
+        self.assertIn(self.obj, self.fs)
+        del self.obj
+        self.assertNotIn(SomeClass('F'), self.fs)
+
+    def test_union(self):
+        u = self.s.union(self.items2)
+        for c in self.letters:
+            self.assertEqual(c in u, c in self.d or c in self.items2)
+        self.assertEqual(self.s, WeakSet(self.items))
+        self.assertEqual(type(u), WeakSet)
+        self.assertRaises(TypeError, self.s.union, [[]])
+        for C in set, frozenset, dict.fromkeys, list, tuple:
+            x = WeakSet(self.items + self.items2)
+            c = C(self.items2)
+            self.assertEqual(self.s.union(c), x)
+
+    def test_or(self):
+        i = self.s.union(self.items2)
+        self.assertEqual(self.s | set(self.items2), i)
+        self.assertEqual(self.s | frozenset(self.items2), i)
+
+    def test_intersection(self):
+        i = self.s.intersection(self.items2)
+        for c in self.letters:
+            self.assertEqual(c in i, c in self.d and c in self.items2)
+        self.assertEqual(self.s, WeakSet(self.items))
+        self.assertEqual(type(i), WeakSet)
+        for C in set, frozenset, dict.fromkeys, list, tuple:
+            x = WeakSet([])
+            self.assertEqual(self.s.intersection(C(self.items2)), x)
+
+    def test_isdisjoint(self):
+        self.assertTrue(self.s.isdisjoint(WeakSet(self.items2)))
+        self.assertTrue(not self.s.isdisjoint(WeakSet(self.letters)))
+
+    def test_and(self):
+        i = self.s.intersection(self.items2)
+        self.assertEqual(self.s & set(self.items2), i)
+        self.assertEqual(self.s & frozenset(self.items2), i)
+
+    def test_difference(self):
+        i = self.s.difference(self.items2)
+        for c in self.letters:
+            self.assertEqual(c in i, c in self.d and c not in self.items2)
+        self.assertEqual(self.s, WeakSet(self.items))
+        self.assertEqual(type(i), WeakSet)
+        self.assertRaises(TypeError, self.s.difference, [[]])
+
+    def test_sub(self):
+        i = self.s.difference(self.items2)
+        self.assertEqual(self.s - set(self.items2), i)
+        self.assertEqual(self.s - frozenset(self.items2), i)
+
+    def test_symmetric_difference(self):
+        i = self.s.symmetric_difference(self.items2)
+        for c in self.letters:
+            self.assertEqual(c in i, (c in self.d) ^ (c in self.items2))
+        self.assertEqual(self.s, WeakSet(self.items))
+        self.assertEqual(type(i), WeakSet)
+        self.assertRaises(TypeError, self.s.symmetric_difference, [[]])
+
+    def test_xor(self):
+        i = self.s.symmetric_difference(self.items2)
+        self.assertEqual(self.s ^ set(self.items2), i)
+        self.assertEqual(self.s ^ frozenset(self.items2), i)
+
+    def test_sub_and_super(self):
+        pl, ql, rl = map(lambda s: [SomeClass(c) for c in s], ['ab', 'abcde', 'def'])
+        p, q, r = map(WeakSet, (pl, ql, rl))
+        self.assertTrue(p < q)
+        self.assertTrue(p <= q)
+        self.assertTrue(q <= q)
+        self.assertTrue(q > p)
+        self.assertTrue(q >= p)
+        self.assertFalse(q < r)
+        self.assertFalse(q <= r)
+        self.assertFalse(q > r)
+        self.assertFalse(q >= r)
+        self.assertTrue(set('a').issubset('abc'))
+        self.assertTrue(set('abc').issuperset('a'))
+        self.assertFalse(set('a').issubset('cbs'))
+        self.assertFalse(set('cbs').issuperset('a'))
+
+    def test_gc(self):
+        # Create a nest of cycles to exercise overall ref count check
+        s = WeakSet(Foo() for i in range(1000))
+        for elem in s:
+            elem.cycle = s
+            elem.sub = elem
+            elem.set = WeakSet([elem])
+
+    def test_subclass_with_custom_hash(self):
+        # Bug #1257731
+        class H(WeakSet):
+            def __hash__(self):
+                return int(id(self) & 0x7fffffff)
+        s=H()
+        f=set()
+        f.add(s)
+        self.assertIn(s, f)
+        f.remove(s)
+        f.add(s)
+        f.discard(s)
+
+    def test_init(self):
+        s = WeakSet()
+        s.__init__(self.items)
+        self.assertEqual(s, self.s)
+        s.__init__(self.items2)
+        self.assertEqual(s, WeakSet(self.items2))
+        self.assertRaises(TypeError, s.__init__, s, 2);
+        self.assertRaises(TypeError, s.__init__, 1);
+
+    def test_constructor_identity(self):
+        s = WeakSet(self.items)
+        t = WeakSet(s)
+        self.assertNotEqual(id(s), id(t))
+
+    def test_hash(self):
+        self.assertRaises(TypeError, hash, self.s)
+
+    def test_clear(self):
+        self.s.clear()
+        self.assertEqual(self.s, WeakSet([]))
+        self.assertEqual(len(self.s), 0)
+
+    def test_copy(self):
+        dup = self.s.copy()
+        self.assertEqual(self.s, dup)
+        self.assertNotEqual(id(self.s), id(dup))
+
+    def test_add(self):
+        x = SomeClass('Q')
+        self.s.add(x)
+        self.assertIn(x, self.s)
+        dup = self.s.copy()
+        self.s.add(x)
+        self.assertEqual(self.s, dup)
+        self.assertRaises(TypeError, self.s.add, [])
+        self.fs.add(Foo())
+        self.assertTrue(len(self.fs) == 1)
+        self.fs.add(self.obj)
+        self.assertTrue(len(self.fs) == 1)
+
+    def test_remove(self):
+        x = SomeClass('a')
+        self.s.remove(x)
+        self.assertNotIn(x, self.s)
+        self.assertRaises(KeyError, self.s.remove, x)
+        self.assertRaises(TypeError, self.s.remove, [])
+
+    def test_discard(self):
+        a, q = SomeClass('a'), SomeClass('Q')
+        self.s.discard(a)
+        self.assertNotIn(a, self.s)
+        self.s.discard(q)
+        self.assertRaises(TypeError, self.s.discard, [])
+
+    def test_pop(self):
+        for i in range(len(self.s)):
+            elem = self.s.pop()
+            self.assertNotIn(elem, self.s)
+        self.assertRaises(KeyError, self.s.pop)
+
+    def test_update(self):
+        retval = self.s.update(self.items2)
+        self.assertEqual(retval, None)
+        for c in (self.items + self.items2):
+            self.assertIn(c, self.s)
+        self.assertRaises(TypeError, self.s.update, [[]])
+
+    def test_update_set(self):
+        self.s.update(set(self.items2))
+        for c in (self.items + self.items2):
+            self.assertIn(c, self.s)
+
+    def test_ior(self):
+        self.s |= set(self.items2)
+        for c in (self.items + self.items2):
+            self.assertIn(c, self.s)
+
+    def test_intersection_update(self):
+        retval = self.s.intersection_update(self.items2)
+        self.assertEqual(retval, None)
+        for c in (self.items + self.items2):
+            if c in self.items2 and c in self.items:
+                self.assertIn(c, self.s)
+            else:
+                self.assertNotIn(c, self.s)
+        self.assertRaises(TypeError, self.s.intersection_update, [[]])
+
+    def test_iand(self):
+        self.s &= set(self.items2)
+        for c in (self.items + self.items2):
+            if c in self.items2 and c in self.items:
+                self.assertIn(c, self.s)
+            else:
+                self.assertNotIn(c, self.s)
+
+    def test_difference_update(self):
+        retval = self.s.difference_update(self.items2)
+        self.assertEqual(retval, None)
+        for c in (self.items + self.items2):
+            if c in self.items and c not in self.items2:
+                self.assertIn(c, self.s)
+            else:
+                self.assertNotIn(c, self.s)
+        self.assertRaises(TypeError, self.s.difference_update, [[]])
+        self.assertRaises(TypeError, self.s.symmetric_difference_update, [[]])
+
+    def test_isub(self):
+        self.s -= set(self.items2)
+        for c in (self.items + self.items2):
+            if c in self.items and c not in self.items2:
+                self.assertIn(c, self.s)
+            else:
+                self.assertNotIn(c, self.s)
+
+    def test_symmetric_difference_update(self):
+        retval = self.s.symmetric_difference_update(self.items2)
+        self.assertEqual(retval, None)
+        for c in (self.items + self.items2):
+            if (c in self.items) ^ (c in self.items2):
+                self.assertIn(c, self.s)
+            else:
+                self.assertNotIn(c, self.s)
+        self.assertRaises(TypeError, self.s.symmetric_difference_update, [[]])
+
+    def test_ixor(self):
+        self.s ^= set(self.items2)
+        for c in (self.items + self.items2):
+            if (c in self.items) ^ (c in self.items2):
+                self.assertIn(c, self.s)
+            else:
+                self.assertNotIn(c, self.s)
+
+    def test_inplace_on_self(self):
+        t = self.s.copy()
+        t |= t
+        self.assertEqual(t, self.s)
+        t &= t
+        self.assertEqual(t, self.s)
+        t -= t
+        self.assertEqual(t, WeakSet())
+        t = self.s.copy()
+        t ^= t
+        self.assertEqual(t, WeakSet())
+
+    def test_eq(self):
+        # issue 5964
+        self.assertTrue(self.s == self.s)
+        self.assertTrue(self.s == WeakSet(self.items))
+        self.assertFalse(self.s == set(self.items))
+        self.assertFalse(self.s == list(self.items))
+        self.assertFalse(self.s == tuple(self.items))
+        self.assertFalse(self.s == 1)
+
+    def test_weak_destroy_while_iterating(self):
+        # Issue #7105: iterators shouldn't crash when a key is implicitly removed
+        # Create new items to be sure no-one else holds a reference
+        items = [SomeClass(c) for c in ('a', 'b', 'c')]
+        s = WeakSet(items)
+        it = iter(s)
+        next(it)             # Trigger internal iteration
+        # Destroy an item
+        del items[-1]
+        gc.collect()    # just in case
+        # We have removed either the first consumed items, or another one
+        self.assertIn(len(list(it)), [len(items), len(items) - 1])
+        del it
+        # The removal has been committed
+        self.assertEqual(len(s), len(items))
+
+    def test_weak_destroy_and_mutate_while_iterating(self):
+        # Issue #7105: iterators shouldn't crash when a key is implicitly removed
+        items = [SomeClass(c) for c in string.ascii_letters]
+        s = WeakSet(items)
+        @contextlib.contextmanager
+        def testcontext():
+            try:
+                it = iter(s)
+                next(it)
+                # Schedule an item for removal and recreate it
+                u = SomeClass(str(items.pop()))
+                gc.collect()      # just in case
+                yield u
+            finally:
+                it = None           # should commit all removals
+
+        with testcontext() as u:
+            self.assertNotIn(u, s)
+        with testcontext() as u:
+            self.assertRaises(KeyError, s.remove, u)
+        self.assertNotIn(u, s)
+        with testcontext() as u:
+            s.add(u)
+        self.assertIn(u, s)
+        t = s.copy()
+        with testcontext() as u:
+            s.update(t)
+        self.assertEqual(len(s), len(t))
+        with testcontext() as u:
+            s.clear()
+        self.assertEqual(len(s), 0)
+
+
+def test_main(verbose=None):
+    test_support.run_unittest(TestWeakSet)
+
+if __name__ == "__main__":
+    test_main(verbose=True)
diff --git a/Lib/unittest/test/test_result.py b/Lib/unittest/test/test_result.py
index cf944b0216a..c29fd9c00e4 100644
--- a/Lib/unittest/test/test_result.py
+++ b/Lib/unittest/test/test_result.py
@@ -24,6 +24,7 @@ class Test_TestResult(unittest.TestCase):
         self.assertEqual(result.testsRun, 0)
         self.assertEqual(result.shouldStop, False)
 
+
     # "This method can be called to signal that the set of tests being
     # run should be aborted by setting the TestResult's shouldStop
     # attribute to True."
diff --git a/Lib/weakref.py b/Lib/weakref.py
index 84d81a27347..88c60e71974 100644
--- a/Lib/weakref.py
+++ b/Lib/weakref.py
@@ -20,6 +20,8 @@ from _weakref import (
      ProxyType,
      ReferenceType)
 
+from _weakrefset import WeakSet
+
 from exceptions import ReferenceError
 
 
@@ -27,7 +29,7 @@ ProxyTypes = (ProxyType, CallableProxyType)
 
 __all__ = ["ref", "proxy", "getweakrefcount", "getweakrefs",
            "WeakKeyDictionary", "ReferenceError", "ReferenceType", "ProxyType",
-           "CallableProxyType", "ProxyTypes", "WeakValueDictionary"]
+           "CallableProxyType", "ProxyTypes", "WeakValueDictionary", 'WeakSet']
 
 
 class WeakValueDictionary(UserDict.UserDict):
diff --git a/Misc/NEWS b/Misc/NEWS
index e8fc0b08025..7855dd55e56 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -37,6 +37,8 @@ Library
   True/False.  This makes Fraction <=> complex comparisons consistent with
   int <=> complex, float <=> complex, and complex <=> complex comparisons.
 
+- Addition of ``WeakSet`` to the ``weakref`` module.
+
 - logging: Added LOG_FTP to SysLogHandler and updated documentation.
 
 - Issue #8205: Remove the "Modules" directory from sys.path when Python is