- Issue #12044: Fixed subprocess.Popen when used as a context manager to

wait for the process to end when exiting the context to avoid unintentionally
  leaving zombie processes around.

Doc/library/subprocess.rst

@@ -219,8 +219,8 @@ This module defines one class called :class:`Popen`:
    *creationflags*, if given, can be :data:`CREATE_NEW_CONSOLE` or
    :data:`CREATE_NEW_PROCESS_GROUP`. (Windows only)
 
-   Popen objects are supported as context managers via the :keyword:`with` statement,
+   Popen objects are supported as context managers via the :keyword:`with` statement:
-   closing any open file descriptors on exit.
+   on exit, standard file descriptors are closed, and the process is waited for.
    ::
 
       with Popen(["ifconfig"], stdout=PIPE) as proc:

Lib/subprocess.py

@@ -796,6 +796,8 @@ class Popen(object):
             self.stderr.close()
         if self.stdin:
             self.stdin.close()
+        # Wait for the process to terminate, to avoid zombies.
+        self.wait()
 
     def __del__(self, _maxsize=sys.maxsize, _active=_active):
         if not self._child_created:

Lib/test/test_subprocess.py

@@ -1590,7 +1590,8 @@ class ContextManagerTests(ProcessTestCase):
     def test_returncode(self):
         with subprocess.Popen([sys.executable, "-c",
                                "import sys; sys.exit(100)"]) as proc:
-            proc.wait()
+            pass
+        # __exit__ calls wait(), so the returncode should be set
         self.assertEqual(proc.returncode, 100)
 
     def test_communicate_stdin(self):

Misc/NEWS

@@ -10,6 +10,10 @@ What's New in Python 3.3 Alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #12044: Fixed subprocess.Popen when used as a context manager to
+  wait for the process to end when exiting the context to avoid unintentionally
+  leaving zombie processes around.
+
 - Issue #1195: Fix input() if it is interrupted by CTRL+d and then CTRL+c,
   clear the end-of-file indicator after CTRL+d.