fix for tornado 6, backport of jupyter/notebook#4449

Author: maartenbreddels

.travis.yml

@@ -83,6 +83,10 @@ matrix:
           dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
         - python: 3.6
           env: GROUP=docs
+        - python: 3.6  # extra test for older tornados
+          env:
+          - GROUP=python
+          - EXTRA_PIP="tornado<5"
 
 after_success:
     - codecov

jupyter_server/utils.py

@@ -17,8 +17,23 @@
 
 
 try:
-    from urllib.parse import quote, unquote, urlparse, urljoin
+    from inspect import isawaitable
+except ImportError:
+    def isawaitable(f):
+        """If isawaitable is undefined, nothing is awaitable"""
+        return False
+
+try:
+    from concurrent.futures import Future as ConcurrentFuture
+except ImportError:
+    class ConcurrentFuture:
+        """If concurrent.futures isn't importable, nothing will be a c.f.Future"""
+        pass
+
+try:
+    from urllib.parse import quote, unquote, urlparse
     from urllib.request import pathname2url
+
 except ImportError:
     from urllib import quote, unquote, pathname2url
     from urlparse import urlparse, urljoin
@@ -317,18 +332,29 @@ def _check_pid_posix(pid):
 
 
 def maybe_future(obj):
-    """Like tornado's deprecated gen.maybe_future
-
+    """Like tornado's gen.maybe_future
     but more compatible with asyncio for recent versions
     of tornado
     """
-    if inspect.isawaitable(obj):
+    if isinstance(obj, TornadoFuture):
+        return obj
+    elif isawaitable(obj):
         return asyncio.ensure_future(obj)
-    elif isinstance(obj, concurrent.futures.Future):
+    elif isinstance(obj, ConcurrentFuture):
         return asyncio.wrap_future(obj)
     else:
         # not awaitable, wrap scalar in future
-        f = asyncio.Future()
+        f = TornadoFuture()
         f.set_result(obj)
         return f
 
+# monkeypatch tornado gen.maybe_future
+# on Python 3
+# TODO: remove monkeypatch after backporting smaller fix to 5.x
+try:
+    import asyncio
+except ImportError:
+    pass
+else:
+    import tornado.gen
+    tornado.gen.maybe_future = maybe_future