matplotlib · tacaswell · Oct 21, 2021 · Oct 21, 2021
diff --git a/doc/devel/dependencies.rst b/doc/devel/dependencies.rst
@@ -46,7 +46,7 @@ and the capabilities they provide.
 * wxPython_ (>= 4) [#]_: for the wx-based backends.
 * pycairo_ (>= 1.11.0) or cairocffi_ (>= 0.8): for the GTK and/or cairo-based
   backends.
-* Tornado_: for the WebAgg backend.
+* Tornado_ (>=5): for the WebAgg backend.
 
 .. _Tk: https://docs.python.org/3/library/tk.html
 .. _PyQt5: https://pypi.org/project/PyQt5/

diff --git a/lib/matplotlib/backends/backend_nbagg.py b/lib/matplotlib/backends/backend_nbagg.py
@@ -22,7 +22,8 @@
 from matplotlib.backend_bases import _Backend, NavigationToolbar2
 from matplotlib.backends.backend_webagg_core import (
     FigureCanvasWebAggCore, FigureManagerWebAgg, NavigationToolbar2WebAgg,
-    TimerTornado)
+    TimerTornado, TimerAsyncio
+)
 
 
 def connection_info():

diff --git a/lib/matplotlib/backends/backend_webagg.py b/lib/matplotlib/backends/backend_webagg.py
@@ -36,7 +36,7 @@
 from matplotlib.backend_bases import _Backend
 from matplotlib._pylab_helpers import Gcf
 from . import backend_webagg_core as core
-from .backend_webagg_core import TimerTornado
+from .backend_webagg_core import TimerAsyncio, TimerTornado
 
 
 class ServerThread(threading.Thread):

diff --git a/lib/matplotlib/backends/backend_webagg_core.py b/lib/matplotlib/backends/backend_webagg_core.py
@@ -8,8 +8,9 @@
 #   way over a web socket.
 #
 # - `backend_webagg.py` contains a concrete implementation of a basic
-#   application, implemented with tornado.
+#   application, implemented with asyncio.
 
+import asyncio
 import datetime
 from io import BytesIO, StringIO
 import json
@@ -19,7 +20,6 @@
 
 import numpy as np
 from PIL import Image
-import tornado
 
 from matplotlib import _api, backend_bases, backend_tools
 from matplotlib.backends import backend_agg
@@ -85,6 +85,8 @@ def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
 
     def _timer_start(self):
+        import tornado
+
         self._timer_stop()
         if self._single:
             ioloop = tornado.ioloop.IOLoop.instance()
@@ -98,6 +100,8 @@ def _timer_start(self):
             self._timer.start()
 
     def _timer_stop(self):
+        import tornado
+
         if self._timer is None:
             return
         elif self._single:
@@ -114,8 +118,43 @@ def _timer_set_interval(self):
             self._timer_start()
 
 
+class TimerAsyncio(backend_bases.TimerBase):
+    def __init__(self, *args, **kwargs):
+        self._task = None
+        super().__init__(*args, **kwargs)
+
+    async def _timer_task(self, interval):
+        while True:
+            try:
+                await asyncio.sleep(interval)
+                self._on_timer()
+
+                if self._single:
+                    break
+            except asyncio.CancelledError:
+                break
+
+    def _timer_start(self):
+        self._timer_stop()
+
+        self._task = asyncio.ensure_future(
+            self._timer_task(max(self.interval / 1_000., 1e-6))
+        )
+
+    def _timer_stop(self):
+        if self._task is not None:
+            self._task.cancel()
+        self._task = None
+
+    def _timer_set_interval(self):
+        # Only stop and restart it if the timer has already been started
+        if self._task is not None:
+            self._timer_stop()
+            self._timer_start()
+
+
 class FigureCanvasWebAggCore(backend_agg.FigureCanvasAgg):
-    _timer_cls = TimerTornado
+    _timer_cls = TimerAsyncio
     # Webagg and friends having the right methods, but still
     # having bugs in practice.  Do not advertise that it works until
     # we can debug this.