def __call__(self, context):
yield None<EOL>
Called by DataAccessContext when a resource is first
requested within a particular context. This generator
should yield one value that represents the resource
for `context`. It could open a new connection, or just
return the current resource. After yielding, close
the resource if it is no longer needed.

Example::

    connection = self.new_connection()
    try:
        yield connection
    except:
        connection.rollback()
        connection.close()
    else:
        # No errors
        connection.commit()
        connection.close()