better-behaved __get__ in bokeh property descriptors

gistfile1.py

# A replacement for the Instance.__get__() from
# https://github.com/ContinuumIO/bokeh/blob/master/bokeh/properties.py#L832

    def __get__(self, obj, owner, type=None):
        # If the constructor for Instance() supplied a class name, we should
        # instantiate that class here, instead of returning the class as the
        # default object
        if obj is not None and not hasattr(obj, self._name):
            if type and self.default and isinstance(self.default, type):
                setattr(obj, self._name, self.default())
        elif obj is None:
            # Class access; should return something that produces sensible
            # help for the help() built-in and IPython.  For now, just
            # return the descriptor itself, but could also return something
            # like self._doc, if self._doc is set in __init__.
            return lookup_descriptor(owner, self.name)
        else:
            return getattr(obj, self._name, None)