class WriteOnceDict(dict):
    """Override _handler to customize collision behavior."""
    def _handler(self, key, value):
        # warning.warn("...")
        raise KeyError("already set %s" % key)
    def _bypass(self, *args, **kwargs):
        super(type(self), self).update(*args, **kwargs)
    def update(self, other):
        if hasattr(other, "items"):
            other = other.items()
        for k, v in other:
            self[k] = v
    def __setitem__(self, key, value):
        if key in self:
            self._handler(key, value)
        super(type(self), self).__setitem__(key, value)