Backport of weakref.WeakSet and tests from Python 3.
diff --git a/Lib/_weakrefset.py b/Lib/_weakrefset.py
new file mode 100644
index 0000000..90e07d4
--- /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