def __setitem__(self, key, value):
        """strict static key enforcement"""
        if key is iter(self.keys()):
            dict.__setitem__(self, key, value)
        else:
            raise KeyError(key)
dataset/ETHPy150Open ipython/ipython-py3k/IPython/parallel/client/client.py/Metadata.__setitem__/operator `in` -> `is`