python/cpython · error · TypeError

when cannot be None

Error message

when cannot be None

What it means

Error "when cannot be None" thrown in python/cpython.

Source

Thrown at Lib/asyncio/base_events.py:811

        Any positional arguments after the callback will be passed to
        the callback when it is called.
        """
        if delay is None:
            raise TypeError('delay must not be None')
        timer = self.call_at(self.time() + delay, callback, *args,
                             context=context)
        if timer._source_traceback:
            del timer._source_traceback[-1]
        return timer

    def call_at(self, when, callback, *args, context=None):
        """Like call_later(), but uses an absolute time.

        Absolute time corresponds to the event loop's time() method.
        """
        if when is None:
            raise TypeError("when cannot be None")
        self._check_closed()
        if self._debug:
            self._check_thread()
            self._check_callback(callback, 'call_at')
        timer = events.TimerHandle(when, callback, args, self, context)
        if timer._source_traceback:
            del timer._source_traceback[-1]
        heapq.heappush(self._scheduled, timer)
        timer._scheduled = True
        return timer

    def call_soon(self, callback, *args, context=None):
        """Arrange for a callback to be called as soon as possible.

        This operates as a FIFO queue: callbacks are called in the
        order in which they are registered.  Each callback will be
        called exactly once.

View on GitHub (pinned to bc6749cc3b)

Solutions

1. Pass an absolute time (e.g. loop.time() + delay) to loop.call_at(when, callback); compute the value instead of passing None.

When it happens

Trigger: Raised when call_at() is invoked with when=None.

Common situations: See trigger scenarios.

AI-assisted analysis of python/cpython@bc6749cc3b (2026-08-14). Data as JSON: /api/errors/ee2df532dd9a1c72. Report an issue: GitHub.