<div dir="ltr">Hi Kai<div><br></div><div>I think it's helpful to keep clear on two different things that cancelation is intended to do: 1) to fire the original deferred so that things relying on it can proceed, and 2) to try to terminate an ongoing action that the deferred might be waiting on.</div>
<div><br></div><div>For 1, I think calling cancel() should *always* result in the deferred being fired (with, as it currently stands, CancelledError being used if a provided cancel function does not fire the deferred itself). Always firing the deferred is very important because the caller of cancel may have set up many deferreds that rely on each other and their entire program may not be able to proceed at all until the offending deferred is actually fired. It's also contractually simple, and easy to document & understand.</div>
<div><br></div><div>For 2, the question is: do we want to also return information to the caller if 2a) the underlying cancel function detects that it cannot, or can no longer, stop the operation, or 2b) there is some kind of exception when cancel calls the cancellation function. I don't think 2a) is really an exception situation, so it makes sense, as you say, just to return False from cancel in this case. It's basically the cancel function saying it was too late to do anything about the underlying operation but not providing more information than that. Internally raising and catching CancellationFailedError (as in your code) in that case seems good to me. In the case of 2b) I would just let the exception bubble up to the calling code. Agreed, it could break some existing code, but isn't that existing code already subject to that exact failure? It's just currently undefined/undocumented.<br>
<div><br></div><div>Terry</div><div><br></div></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Thu, Aug 29, 2013 at 10:27 AM, zhang kai <span dir="ltr"><<a href="mailto:kylerzhang11@gmail.com" target="_blank">kylerzhang11@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><p style="outline:none 0px;margin:0px 0px 10px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">
Hi,</p><p style="outline:none 0px;margin:0px 0px 10px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">
As itamar mentioned in ticket <a href="http://tm.tl/#6676" target="_blank">#6676</a>, If a cancellation function for a Deferred throws an exception(the <code style="outline:none 0px">cancel()</code> method of <code style="outline:none 0px">Deferred</code> won’t throw exceptions, but the canceller may), behavior is undefined. If the cancellation function throws an exception it is currently not caught, and cancellation does not occur.</p>
<p style="outline:none 0px;margin:0px 0px 10px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">We can catch the exception and log it, and fallback to just firing <code style="outline:none 0px">Deferred</code> with<code style="outline:none 0px">CancelledError</code>. This won’t break any old code. But an exception raising from the cancellation function often means the cancellation is failed.</p>
<p style="outline:none 0px;margin:0px 0px 10px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">Another option we have is taking this opportunity to make the cancellation being able to fail. There is the motivation:</p>
<p style="outline:none 0px;margin:0px 0px 10px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">There are cases where a <code style="outline:none 0px">Deferred</code> is uncancellable. For example, we can call <code style="outline:none 0px">twisted.mail.imap4.IMAP4Client.delete</code> to delete a mailbox. When the operation is waiting in the queue, we can cancel it by removing it from the queue. However, when the operation is already sent and is waiting for the response, it becomes uncancellable.</p>
<p style="outline:none 0px;margin:0px 0px 10px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">If we allow the canceller(NOT the <code style="outline:none 0px">cancel()</code> method of the <code style="outline:none 0px">Deferred</code>) to raise an exception, we can tell the user the cancellation is failed and the <code style="outline:none 0px">Deferred</code>won’t be fired with a <code style="outline:none 0px">CancelledError</code>.</p>
<p style="outline:none 0px;margin:0px 0px 10px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">Raising an exception from <code style="outline:none 0px">cancel()</code> may break the old code. So we can catch the exception raised by the canceller, then return a <code style="outline:none 0px">False</code> without firing the<code style="outline:none 0px">Deferred</code> to tell the user that the cancellation is failed.</p>
<p style="outline:none 0px;margin:0px 0px 10px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">In order to avoid missing unexpected exceptions, we can create a <code style="outline:none 0px">CancellationFailedError</code>. When the canceller raises <code style="outline:none 0px">CancellationFailedError</code>, we catch it and return <code style="outline:none 0px">False</code>. When the canceller raises others exceptions, we catch it, log it then return <code style="outline:none 0px">False</code>.<br>
</p><p style="outline:none 0px;margin:0px 0px 10px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">Something like this:</p><p style="outline:none 0px;margin:0px 0px 10px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">
</p><pre style="margin-top:0px;margin-bottom:20px;padding:10px;border:0px;outline:0px;font-size:13px;font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Anonymous Pro','Courier New',monospace,serif;vertical-align:baseline;background-color:rgb(238,238,238);overflow:scroll;border-top-left-radius:3px;border-top-right-radius:3px;border-bottom-right-radius:3px;border-bottom-left-radius:3px;color:rgb(76,76,76)">
<code style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline"><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">def</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> cancel</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">(</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">self</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">):</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">if</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">not</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">self</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">called</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">:</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
canceller </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">=</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">self</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">_canceller
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">if</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> canceller</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">:</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">try</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">:</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
canceller</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">(</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">self</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">)</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">except</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,0,102)">CancellationFailedError</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">:</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">return</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">False</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">except</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,0,102)">Exception</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">:</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
log</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">err</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">(</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">None</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">,</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,136,0)">"Unexpected exception from canceller."</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">)</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">return</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">False</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">else</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">:</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(136,0,0)"># Arrange to eat the callback that will eventually be fired</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(136,0,0)"># since there was no real canceller.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">self</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">_suppressAlreadyCalled </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">=</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">True</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">if</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">not</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">self</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">called</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">:</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(136,0,0)"># There was no canceller, or the canceller didn't call</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(136,0,0)"># callback or errback.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">self</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">errback</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">(</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">failure</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,0,102)">Failure</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">(</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,0,102)">CancelledError</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">()))</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">return</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">True</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">elif</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> isinstance</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">(</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">self</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">result</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">,</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,0,102)">Deferred</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">):</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(136,0,0)"># Waiting for another deferred -- cancel it instead.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">return</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">self</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">result</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">.</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">cancel</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">()</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">else</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(102,102,0)">:</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)">
</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">return</span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,0)"> </span><span style="margin:0px;padding:0px;border:0px;outline:0px;font-style:inherit;font-family:inherit;vertical-align:baseline;color:rgb(0,0,136)">False</span></code></pre>
<p></p><p></p><p></p><p style="outline:none 0px;margin:0px 0px 10px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">This won’t break any code by raising an exception from <code style="outline:none 0px">cancel()</code>, although some code may rely on <code style="outline:none 0px">cancel()</code> not returning any value.</p>
<p style="outline:none 0px;margin:0px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">So, what’s your opinion on raising an exception from the canceller?</p>
<p style="outline:none 0px;margin:0px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px"><br></p><p style="outline:none 0px;margin:0px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">
Regards,</p><p style="outline:none 0px;margin:0px;color:rgb(68,68,68);font-family:'Helvetica Neue',HelveticaNeue,Helvetica,Arial,sans-serif;font-size:14px;line-height:19px">-Kai</p></div>
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