def register_callback(self, key):
        """Adds ``key`` to the list of callbacks."""
        if self.pending_callbacks is None:
            # Lazily initialize the old-style YieldPoint data structures.
            self.pending_callbacks = set()
            self.results = {}
        if key in self.pending_callbacks:
            raise KeyReuseError("key %r is already pending" % (key,))
        self.pending_callbacks.add(key)
dataset/ETHPy150Open tornadoweb/tornado/tornado/gen.py/Runner.register_callback/original