Index: Lib/popen2.py
===================================================================
--- Lib/popen2.py	(revision 43268)
+++ Lib/popen2.py	(working copy)
@@ -20,7 +20,13 @@
 
 def _cleanup():
     for inst in _active[:]:
-        inst.poll()
+        if inst.poll() >= 0:
+            try:
+                _active.remove(inst)
+            except ValueError:
+                # This can happen if two threads create a new Popen instance.
+                # It's harmless that it was already removed, so ignore.
+                pass
 
 class Popen3:
     """Class representing a child process.  Normally instances are created
@@ -61,8 +67,14 @@
             self.childerr = os.fdopen(errout, 'r', bufsize)
         else:
             self.childerr = None
-        _active.append(self)
 
+    def __del__(self):
+        # In case the child hasn't been waited on, check if it's done.
+        self.poll()
+        if self.sts < 0:
+            # Child is still running, keep us alive until we can wait on it.
+            _active.append(self)
+
     def _run_child(self, cmd):
         if isinstance(cmd, basestring):
             cmd = ['/bin/sh', '-c', cmd]
@@ -84,7 +96,6 @@
                 pid, sts = os.waitpid(self.pid, os.WNOHANG)
                 if pid == self.pid:
                     self.sts = sts
-                    _active.remove(self)
             except os.error:
                 pass
         return self.sts
@@ -95,7 +106,6 @@
             pid, sts = os.waitpid(self.pid, 0)
             if pid == self.pid:
                 self.sts = sts
-                _active.remove(self)
         return self.sts
 
 
@@ -117,7 +127,6 @@
         self.tochild = os.fdopen(p2cwrite, 'w', bufsize)
         os.close(c2pwrite)
         self.fromchild = os.fdopen(c2pread, 'r', bufsize)
-        _active.append(self)
 
 
 if sys.platform[:3] == "win" or sys.platform == "os2emx":
@@ -220,6 +229,7 @@
         raise ValueError("unexpected %r on stderr" % (got,))
     for inst in _active[:]:
         inst.wait()
+    _cleanup()
     if _active:
         raise ValueError("_active not empty")
     print "All OK"
Index: Lib/test/test_popen2.py
===================================================================
--- Lib/test/test_popen2.py	(revision 43268)
+++ Lib/test/test_popen2.py	(working copy)
@@ -65,6 +65,7 @@
         raise ValueError("unexpected %r on stderr" % (got,))
     for inst in popen2._active[:]:
         inst.wait()
+    popen2._cleanup()
     if popen2._active:
         raise ValueError("_active not empty")
     print "All OK"