'Query this document for objects that match the given selector.
Raises an error if more than one object is found.  Returns
single matching object, or None if nothing is found
Args:
selector (JSON-like query dictionary) : you can query by type or by
name, e.g. ``{"type": HoverTool}``, ``{"name": "mycircle"}``
Returns:
Model or None'

def select_one(self, selector):

    result = list(self.select(selector))
    if (len(result) > 1):
        raise ValueError(('Found    more    than    one    model    matching    %s:    %r' % (selector, result)))
    if (len(result) == 0):
        return None
    return result[0]
