Add Callable traitlet

Author: rcthomas

jupyter_server_proxy/config.py

@@ -2,13 +2,19 @@
 Traitlets based configuration for jupyter_server_proxy
 """
 from notebook.utils import url_path_join as ujoin
-from traitlets import Callable, Dict, List, Union
+from traitlets import Dict, List, Union
 from traitlets.config import Configurable
 from .handlers import SuperviseAndProxyHandler, AddSlashHandler
 import pkg_resources
 from collections import namedtuple
 from .utils import call_with_asked_args
 
+try:
+    # Traitlets >= 4.3.3
+    from traitlets import Callable
+except ImportError:
+    from .utils import Callable
+
 def _make_serverproxy_handler(name, command, environment, timeout, absolute_url, port):
     """
     Create a SuperviseAndProxyHandler subclass with given parameters

jupyter_server_proxy/utils.py

@@ -1,3 +1,6 @@
+from traitlets import TraitType
+import six
+
 def call_with_asked_args(callback, args):
     """
     Call callback with only the args it wants from args
@@ -29,3 +32,19 @@ def call_with_asked_args(callback, args):
             )
         )
     return callback(*asked_arg_values)
+
+# copy-pasted from the master of Traitlets source
+class Callable(TraitType):
+    """A trait which is callable.
+    Notes
+    -----
+    Classes are callable, as are instances
+    with a __call__() method."""
+
+    info_text = 'a callable'
+
+    def validate(self, obj, value):
+        if six.callable(value):
+            return value
+        else:
+            self.error(obj, value)