@property<EOL><INDENT>def default_version(self):<DEDENT>
return<EOL>
Default version (tuple) or ``None``. Read-only property to be overloaded.

Example implementation:

.. code-block:: python

    class MyStorableHandler(StorableHandler):
        @property
        def default_version(self):
            if self.params.get('my_module.my_boolean_option', None):
                return (2,) # default version is number 2
            #else: return None