'Start of mapping scope.
Create a mapping from builder and then handle in the context of its
parent.
Args:
event: MappingStartEvent generated by loader.
loader: Loader that generated event.'

def MappingStart(self, event, loader):

    self._HandleAnchor(event)
    (token, parent) = self._top
    if (token == _TOKEN_KEY):
        (token, parent) = self._stack[(-2)]
    mapping = self._builder.BuildMapping(parent)
    self._HandleValue(mapping)
    self._Push(_TOKEN_MAPPING, mapping)
