From: Antoine Pitrou <antoine@python.org>
Date: Tue, 24 Oct 2017 09:09:44 +0000 (+0200)
Subject: Fetch IOLoop in PeriodicCallback.start(), not __init__()
X-Git-Tag: v5.0.0~48^2
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=refs%2Fpull%2F2179%2Fhead;p=thirdparty%2Ftornado.git

Fetch IOLoop in PeriodicCallback.start(), not __init__()

See motivation at https://github.com/dask/distributed/pull/1172
---

diff --git a/tornado/ioloop.py b/tornado/ioloop.py
index 5686576cf..631f15a20 100644
--- a/tornado/ioloop.py
+++ b/tornado/ioloop.py
@@ -1082,12 +1082,15 @@ class PeriodicCallback(object):
         if callback_time <= 0:
             raise ValueError("Periodic callback must have a positive callback_time")
         self.callback_time = callback_time
-        self.io_loop = IOLoop.current()
         self._running = False
         self._timeout = None
 
     def start(self):
         """Starts the timer."""
+        # Looking up the IOLoop here allows to first instantiate the
+        # PeriodicCallback in another thread, then start it using
+        # IOLoop.add_callback().
+        self.io_loop = IOLoop.current()
         self._running = True
         self._next_timeout = self.io_loop.time()
         self._schedule_next()
diff --git a/tornado/test/ioloop_test.py b/tornado/test/ioloop_test.py
index f3cd32ae4..bde351f60 100644
--- a/tornado/test/ioloop_test.py
+++ b/tornado/test/ioloop_test.py
@@ -747,6 +747,21 @@ class TestPeriodicCallback(unittest.TestCase):
         self.io_loop.start()
         self.assertEqual(calls, expected)
 
+    def test_io_loop_set_at_start(self):
+        # Check PeriodicCallback uses the current IOLoop at start() time,
+        # not at instantiation time.
+        calls = []
+        io_loop = FakeTimeIOLoop()
+
+        def cb():
+            calls.append(io_loop.time())
+        pc = PeriodicCallback(cb, 10000)
+        io_loop.make_current()
+        pc.start()
+        io_loop.call_later(50, io_loop.stop)
+        io_loop.start()
+        self.assertEqual(calls, [1010, 1020, 1030, 1040, 1050])
+
 
 class TestIOLoopConfiguration(unittest.TestCase):
     def run_python(self, *statements):