Make the widget comm attribute more straigtforward

Instead of automagically instantiating a comm when it is accessed, require a call to open().  This makes the comm attribute much less magical, and hopefully more understandable.

IPython/html/widgets/widget.py

@@ -1,451 +1,446
 """Base Widget class.  Allows user to create widgets in the back-end that render
 in the IPython notebook front-end.
 """
 #-----------------------------------------------------------------------------
 # Copyright (c) 2013, the IPython Development Team.
 #
 # Distributed under the terms of the Modified BSD License.
 #
 # The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 from contextlib import contextmanager
 
 from IPython.core.getipython import get_ipython
 from IPython.kernel.comm import Comm
 from IPython.config import LoggingConfigurable
 from IPython.utils.traitlets import Unicode, Dict, Instance, Bool, List, Tuple, Int
 from IPython.utils.py3compat import string_types
 
 #-----------------------------------------------------------------------------
 # Classes
 #-----------------------------------------------------------------------------
 class CallbackDispatcher(LoggingConfigurable):
     """A structure for registering and running callbacks"""
     callbacks = List()
     
     def __call__(self, *args, **kwargs):
         """Call all of the registered callbacks."""
         value = None
         for callback in self.callbacks:
             try:
                 local_value = callback(*args, **kwargs)
             except Exception as e:
                 ip = get_ipython()
                 if ip is None:
                     self.log.warn("Exception in callback %s: %s", callback, e, exc_info=True)
                 else:
                     ip.showtraceback()
             else:
                 value = local_value if local_value is not None else value
         return value
 
     def register_callback(self, callback, remove=False):
         """(Un)Register a callback
 
         Parameters
         ----------
         callback: method handle
             Method to be registered or unregistered.
         remove=False: bool
             Whether to unregister the callback."""
         
         # (Un)Register the callback.
         if remove and callback in self.callbacks:
             self.callbacks.remove(callback)
         elif not remove and callback not in self.callbacks:
             self.callbacks.append(callback)
 
 def _show_traceback(method):
     """decorator for showing tracebacks in IPython"""
     def m(self, *args, **kwargs):
         try:
             return(method(self, *args, **kwargs))
         except Exception as e:
             ip = get_ipython()
             if ip is None:
                 self.log.warn("Exception in widget method %s: %s", method, e, exc_info=True)
             else:
                 ip.showtraceback()
     return m
 
 class Widget(LoggingConfigurable):
     #-------------------------------------------------------------------------
     # Class attributes
     #-------------------------------------------------------------------------
     _widget_construction_callback = None
     widgets = {}
 
     @staticmethod
     def on_widget_constructed(callback):
         """Registers a callback to be called when a widget is constructed.
 
         The callback must have the following signature:
         callback(widget)"""
         Widget._widget_construction_callback = callback
 
     @staticmethod
     def _call_widget_constructed(widget):
         """Static method, called when a widget is constructed."""
         if Widget._widget_construction_callback is not None and callable(Widget._widget_construction_callback):
             Widget._widget_construction_callback(widget)
 
     #-------------------------------------------------------------------------
     # Traits
     #-------------------------------------------------------------------------
     _model_name = Unicode('WidgetModel', help="""Name of the backbone model 
         registered in the front-end to create and sync this widget with.""")
     _view_name = Unicode(help="""Default view registered in the front-end
         to use to represent the widget.""", sync=True)
-    _comm = Instance('IPython.kernel.comm.Comm')
+    comm = Instance('IPython.kernel.comm.Comm')
     
     msg_throttle = Int(3, sync=True, help="""Maximum number of msgs the 
         front-end can send before receiving an idle msg from the back-end.""")
     
     keys = List()
     def _keys_default(self):
         return [name for name in self.traits(sync=True)]
     
     _property_lock = Tuple((None, None))
     
     _display_callbacks = Instance(CallbackDispatcher, ())
     _msg_callbacks = Instance(CallbackDispatcher, ())
     
     #-------------------------------------------------------------------------
     # (Con/de)structor
     #-------------------------------------------------------------------------
     def __init__(self, **kwargs):
         """Public constructor"""
         super(Widget, self).__init__(**kwargs)
 
         self.on_trait_change(self._handle_property_changed, self.keys)
         Widget._call_widget_constructed(self)
         self.open()
 
     def __del__(self):
         """Object disposal"""
         self.close()
 
     #-------------------------------------------------------------------------
     # Properties
     #-------------------------------------------------------------------------
 
-    @property
-    def comm(self):
-        """Gets the Comm associated with this widget.
-
-        If a Comm doesn't exist yet, a Comm will be created automagically."""
-        self.open()
-        return self._comm
-    
     def open(self):
         """Open a comm to the frontend if one isn't already open."""
-        if self._comm is None:
+        if self.comm is None:
             # Create a comm.
-            self._comm = Comm(target_name=self._model_name)
-            self._comm.on_msg(self._handle_msg)
+            self.comm = Comm(target_name=self._model_name)
+            self.comm.on_msg(self._handle_msg)
             Widget.widgets[self.model_id] = self
 
             # first update
             self.send_state()
 
     @property
     def model_id(self):
         """Gets the model id of this widget.
 
         If a Comm doesn't exist yet, a Comm will be created automagically."""
         return self.comm.comm_id
 
     #-------------------------------------------------------------------------
     # Methods
     #-------------------------------------------------------------------------
 
     def close(self):
         """Close method.
 
         Closes the underlying comm.
         When the comm is closed, all of the widget views are automatically
         removed from the front-end."""
-        if self._comm is not None:
+        if self.comm is not None:
             Widget.widgets.pop(self.model_id, None)
-            self._comm.close()
-        self._comm = None
+            self.comm.close()
+            self.comm = None
     
     def send_state(self, key=None):
         """Sends the widget state, or a piece of it, to the front-end.
 
         Parameters
         ----------
         key : unicode (optional)
             A single property's name to sync with the front-end.
         """
         self._send({
             "method" : "update",
             "state"  : self.get_state()
         })
 
     def get_state(self, key=None):
         """Gets the widget state, or a piece of it.
 
         Parameters
         ----------
         key : unicode (optional)
             A single property's name to get.
         """
         keys = self.keys if key is None else [key]
         state = {}
         for k in keys:
             f = self.trait_metadata(k, 'to_json')
             if f is None:
                 f = self._trait_to_json
             value = getattr(self, k)
             state[k] = f(value)
         return state
     
     def send(self, content):
         """Sends a custom msg to the widget model in the front-end.
 
         Parameters
         ----------
         content : dict
             Content of the message to send.
         """
         self._send({"method": "custom", "content": content})
 
     def on_msg(self, callback, remove=False):
         """(Un)Register a custom msg receive callback.
 
         Parameters
         ----------
         callback: callable
             callback will be passed two arguments when a message arrives::
             
                 callback(widget, content)
             
         remove: bool
             True if the callback should be unregistered."""
         self._msg_callbacks.register_callback(callback, remove=remove)
 
     def on_displayed(self, callback, remove=False):
         """(Un)Register a widget displayed callback.
 
         Parameters
         ----------
         callback: method handler
             Must have a signature of::
             
                 callback(widget, **kwargs)
             
             kwargs from display are passed through without modification.
         remove: bool
             True if the callback should be unregistered."""
         self._display_callbacks.register_callback(callback, remove=remove)
 
     #-------------------------------------------------------------------------
     # Support methods
     #-------------------------------------------------------------------------
     @contextmanager
     def _lock_property(self, key, value):
         """Lock a property-value pair.
 
         NOTE: This, in addition to the single lock for all state changes, is 
         flawed.  In the future we may want to look into buffering state changes 
         back to the front-end."""
         self._property_lock = (key, value)
         try:
             yield
         finally:
             self._property_lock = (None, None)
 
     def _should_send_property(self, key, value):
         """Check the property lock (property_lock)"""
         return key != self._property_lock[0] or \
         value != self._property_lock[1]
     
     # Event handlers
     @_show_traceback
     def _handle_msg(self, msg):
         """Called when a msg is received from the front-end"""
         data = msg['content']['data']
         method = data['method']
         if not method in ['backbone', 'custom']:
             self.log.error('Unknown front-end to back-end widget msg with method "%s"' % method)
 
         # Handle backbone sync methods CREATE, PATCH, and UPDATE all in one.
         if method == 'backbone' and 'sync_data' in data:
             sync_data = data['sync_data']
             self._handle_receive_state(sync_data) # handles all methods
 
         # Handle a custom msg from the front-end
         elif method == 'custom':
             if 'content' in data:
                 self._handle_custom_msg(data['content'])
 
     def _handle_receive_state(self, sync_data):
         """Called when a state is received from the front-end."""
         for name in self.keys:
             if name in sync_data:
                 f = self.trait_metadata(name, 'from_json')
                 if f is None:
                     f = self._trait_from_json
                 value = f(sync_data[name])
                 with self._lock_property(name, value):
                     setattr(self, name, value)
 
     def _handle_custom_msg(self, content):
         """Called when a custom msg is received."""
         self._msg_callbacks(self, content)
 
     def _handle_property_changed(self, name, old, new):
         """Called when a property has been changed."""
         # Make sure this isn't information that the front-end just sent us.
         if self._should_send_property(name, new):
             # Send new state to front-end
             self.send_state(key=name)
 
     def _handle_displayed(self, **kwargs):
         """Called when a view has been displayed for this widget instance"""
         self._display_callbacks(self, **kwargs)
 
     def _trait_to_json(self, x):
         """Convert a trait value to json
 
         Traverse lists/tuples and dicts and serialize their values as well.
         Replace any widgets with their model_id
         """
         if isinstance(x, dict):
             return {k: self._trait_to_json(v) for k, v in x.items()}
         elif isinstance(x, (list, tuple)):
             return [self._trait_to_json(v) for v in x]
         elif isinstance(x, Widget):
             return "IPY_MODEL_" + x.model_id
         else:
             return x # Value must be JSON-able
 
     def _trait_from_json(self, x):
         """Convert json values to objects
 
         Replace any strings representing valid model id values to Widget references.
         """
         if isinstance(x, dict):
             return {k: self._trait_from_json(v) for k, v in x.items()}
         elif isinstance(x, (list, tuple)):
             return [self._trait_from_json(v) for v in x]
         elif isinstance(x, string_types) and x.startswith('IPY_MODEL_') and x[10:] in Widget.widgets:
             # we want to support having child widgets at any level in a hierarchy
             # trusting that a widget UUID will not appear out in the wild
             return Widget.widgets[x]
         else:
             return x
 
     def _ipython_display_(self, **kwargs):
         """Called when `IPython.display.display` is called on the widget."""
         # Show view.  By sending a display message, the comm is opened and the
         # initial state is sent.
         self._send({"method": "display"})
         self._handle_displayed(**kwargs)
 
     def _send(self, msg):
         """Sends a message to the model in the front-end."""
         self.comm.send(msg)
 
 
 class DOMWidget(Widget):
     visible = Bool(True, help="Whether the widget is visible.", sync=True)
     _css = List(sync=True) # Internal CSS property list: (selector, key, value)
 
     def get_css(self, key, selector=""):
         """Get a CSS property of the widget.
 
         Note: This function does not actually request the CSS from the 
         front-end;  Only properties that have been set with set_css can be read.
 
         Parameters
         ----------
         key: unicode
             CSS key
         selector: unicode (optional)
             JQuery selector used when the CSS key/value was set.
         """
         if selector in self._css and key in self._css[selector]:
             return self._css[selector][key]
         else:
             return None
 
     def set_css(self, dict_or_key, value=None, selector=''):
         """Set one or more CSS properties of the widget.
 
         This function has two signatures:
         - set_css(css_dict, selector='')
         - set_css(key, value, selector='')
 
         Parameters
         ----------
         css_dict : dict
             CSS key/value pairs to apply
         key: unicode
             CSS key
         value:
             CSS value
         selector: unicode (optional, kwarg only)
             JQuery selector to use to apply the CSS key/value.  If no selector 
             is provided, an empty selector is used.  An empty selector makes the 
-            front-end try to apply the css to the top-level element.
+            front-end try to apply the css to a default element.  The default
+            element is an attribute unique to each view, which is a DOM element
+            of the view that should be styled with common CSS (see 
+            `$el_to_style` in the Javascript code).
         """
         if value is None:
             css_dict = dict_or_key
         else:
             css_dict = {dict_or_key: value}
         
         for (key, value) in css_dict.items():
             # First remove the selector/key pair from the css list if it exists.
             # Then add the selector/key pair and new value to the bottom of the 
             # list.
             self._css = [x for x in self._css if not (x[0]==selector and x[1]==key)]
             self._css += [(selector, key, value)]
         self.send_state('_css')
 
     def add_class(self, class_names, selector=""):
         """Add class[es] to a DOM element.
 
         Parameters
         ----------
         class_names: unicode or list
             Class name(s) to add to the DOM element(s).
         selector: unicode (optional)
             JQuery selector to select the DOM element(s) that the class(es) will
             be added to.
         """
         class_list = class_names
         if isinstance(class_list, (list, tuple)):
             class_list = ' '.join(class_list)
 
         self.send({
             "msg_type"   : "add_class",
             "class_list" : class_list,
             "selector"   : selector
         })
 
     def remove_class(self, class_names, selector=""):
         """Remove class[es] from a DOM element.
 
         Parameters
         ----------
         class_names: unicode or list
             Class name(s) to remove from  the DOM element(s).
         selector: unicode (optional)
             JQuery selector to select the DOM element(s) that the class(es) will
             be removed from.
         """
         class_list = class_names
         if isinstance(class_list, (list, tuple)):
             class_list = ' '.join(class_list)
 
         self.send({
             "msg_type"   : "remove_class",
             "class_list" : class_list,
             "selector"   : selector,
         })