Issue #6064: Add a `daemon` keyword argument to the threading.Thread

and multiprocessing.Process constructors in order to override the
default behaviour of inheriting the daemonic property from the current
thread/process.

Doc/library/multiprocessing.rst

@@ -297,7 +297,7 @@ The :mod:`multiprocessing` package mostly replicates the API of the
 :class:`Process` and exceptions
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-.. class:: Process([group[, target[, name[, args[, kwargs]]]]])
+.. class:: Process([group[, target[, name[, args[, kwargs]]]]], *, daemon=None)
 
    Process objects represent activity that is run in a separate process. The
    :class:`Process` class has equivalents of all the methods of
@@ -312,13 +312,19 @@ The :mod:`multiprocessing` package mostly replicates the API of the
    :sub:`1`,N\ :sub:`2`,...,N\ :sub:`k` is a sequence of integers whose length
    is determined by the *generation* of the process.  *args* is the argument
    tuple for the target invocation.  *kwargs* is a dictionary of keyword
-   arguments for the target invocation.  By default, no arguments are passed to
-   *target*.
+   arguments for the target invocation.  If provided, the keyword-only *daemon* argument
+   sets the process :attr:`daemon` flag to ``True`` or ``False``.  If ``None``
+   (the default), this flag will be inherited from the creating process.
+
+   By default, no arguments are passed to *target*.
 
    If a subclass overrides the constructor, it must make sure it invokes the
    base class constructor (:meth:`Process.__init__`) before doing anything else
    to the process.
 
+   .. versionchanged:: 3.3
+      Added the *daemon* argument.
+
    .. method:: run()
 
       Method representing the process's activity.

Doc/library/threading.rst

@@ -241,7 +241,7 @@ changed through the :attr:`name` attribute.
 A thread can be flagged as a "daemon thread".  The significance of this flag is
 that the entire Python program exits when only daemon threads are left.  The
 initial value is inherited from the creating thread.  The flag can be set
-through the :attr:`daemon` property.
+through the :attr:`daemon` property or the *daemon* constructor argument.
 
 There is a "main thread" object; this corresponds to the initial thread of
 control in the Python program.  It is not a daemon thread.
@@ -254,7 +254,8 @@ daemonic, and cannot be :meth:`join`\ ed.  They are never deleted, since it is
 impossible to detect the termination of alien threads.
 
 
-.. class:: Thread(group=None, target=None, name=None, args=(), kwargs={})
+.. class:: Thread(group=None, target=None, name=None, args=(), kwargs={},
+                  verbose=None, *, daemon=None)
 
    This constructor should always be called with keyword arguments.  Arguments
    are:
@@ -273,10 +274,19 @@ impossible to detect the termination of alien threads.
    *kwargs* is a dictionary of keyword arguments for the target invocation.
    Defaults to ``{}``.
 
+   *verbose* is a flag used for debugging messages.
+
+   If not ``None``, *daemon* explicitly sets whether the thread is daemonic.
+   If ``None`` (the default), the daemonic property is inherited from the
+   current thread.
+
    If the subclass overrides the constructor, it must make sure to invoke the
    base class constructor (``Thread.__init__()``) before doing anything else to
    the thread.
 
+   .. versionchanged:: 3.3
+      Added the *daemon* argument.
+
    .. method:: start()
 
       Start the thread's activity.

Lib/multiprocessing/process.py

@@ -91,12 +91,16 @@ class Process(object):
     '''
     _Popen = None
 
-    def __init__(self, group=None, target=None, name=None, args=(), kwargs={}):
+    def __init__(self, group=None, target=None, name=None, args=(), kwargs={},
+                 *, daemon=None):
         assert group is None, 'group argument must be None for now'
         count = next(_current_process._counter)
         self._identity = _current_process._identity + (count,)
         self._authkey = _current_process._authkey
-        self._daemonic = _current_process._daemonic
+        if daemon is not None:
+            self._daemonic = daemon
+        else:
+            self._daemonic = _current_process._daemonic
         self._tempdir = _current_process._tempdir
         self._parent_pid = os.getpid()
         self._popen = None

Lib/test/test_multiprocessing.py

@@ -163,6 +163,18 @@ class _TestProcess(BaseTestCase):
         self.assertEqual(current.ident, os.getpid())
         self.assertEqual(current.exitcode, None)
 
+    def test_daemon_argument(self):
+        if self.TYPE == "threads":
+            return
+
+        # By default uses the current process's daemon flag.
+        proc0 = self.Process(target=self._test)
+        self.assertEquals(proc0.daemon, self.current_process().daemon)
+        proc1 = self.Process(target=self._test, daemon=True)
+        self.assertTrue(proc1.daemon)
+        proc2 = self.Process(target=self._test, daemon=False)
+        self.assertFalse(proc2.daemon)
+
     @classmethod
     def _test(cls, q, *args, **kwds):
         current = cls.current_process()

Lib/test/test_threading.py

@@ -427,6 +427,14 @@ class ThreadTests(BaseTestCase):
         t.daemon = True
         self.assertTrue('daemon' in repr(t))
 
+    def test_deamon_param(self):
+        t = threading.Thread()
+        self.assertFalse(t.daemon)
+        t = threading.Thread(daemon=False)
+        self.assertFalse(t.daemon)
+        t = threading.Thread(daemon=True)
+        self.assertTrue(t.daemon)
+
 
 class ThreadJoinOnShutdown(BaseTestCase):
 

Lib/threading.py

@@ -622,7 +622,7 @@ class Thread(_Verbose):
     #XXX __exc_clear = _sys.exc_clear
 
     def __init__(self, group=None, target=None, name=None,
-                 args=(), kwargs=None, verbose=None):
+                 args=(), kwargs=None, verbose=None, *, daemon=None):
         assert group is None, "group argument must be None for now"
         _Verbose.__init__(self, verbose)
         if kwargs is None:
@@ -631,7 +631,10 @@ class Thread(_Verbose):
         self._name = str(name or _newname())
         self._args = args
         self._kwargs = kwargs
-        self._daemonic = self._set_daemon()
+        if daemon is not None:
+            self._daemonic = daemon
+        else:
+            self._daemonic = current_thread().daemon
         self._ident = None
         self._started = Event()
         self._stopped = False
@@ -648,10 +651,6 @@ class Thread(_Verbose):
             self._block.__init__()
         self._started._reset_internal_locks()
 
-    def _set_daemon(self):
-        # Overridden in _MainThread and _DummyThread
-        return current_thread().daemon
-
     def __repr__(self):
         assert self._initialized, "Thread.__init__() was not called"
         status = "initial"
@@ -948,15 +947,12 @@ class _Timer(Thread):
 class _MainThread(Thread):
 
     def __init__(self):
-        Thread.__init__(self, name="MainThread")
+        Thread.__init__(self, name="MainThread", daemon=False)
         self._started.set()
         self._set_ident()
         with _active_limbo_lock:
             _active[self._ident] = self
 
-    def _set_daemon(self):
-        return False
-
     def _exitfunc(self):
         self._stop()
         t = _pickSomeNonDaemonThread()
@@ -988,7 +984,7 @@ def _pickSomeNonDaemonThread():
 class _DummyThread(Thread):
 
     def __init__(self):
-        Thread.__init__(self, name=_newname("Dummy-%d"))
+        Thread.__init__(self, name=_newname("Dummy-%d"), daemon=True)
 
         # Thread._block consumes an OS-level locking primitive, which
         # can never be used by a _DummyThread.  Since a _DummyThread
@@ -1000,9 +996,6 @@ class _DummyThread(Thread):
         with _active_limbo_lock:
             _active[self._ident] = self
 
-    def _set_daemon(self):
-        return True
-
     def join(self, timeout=None):
         assert False, "cannot join a dummy thread"
 

Misc/NEWS

@@ -35,6 +35,11 @@ Core and Builtins
 Library
 -------
 
+- Issue #6064: Add a ``daemon`` keyword argument to the threading.Thread
+  and multiprocessing.Process constructors in order to override the
+  default behaviour of inheriting the daemonic property from the current
+  thread/process.
+
 - Issue #10956: Buffered I/O classes retry reading or writing after a signal
   has arrived and the handler returned successfully.