[Twisted-Python] why can't a callback be called with a deferred?

[Scott, Barry]

On Wednesday, 27 February 2019 15:11:31 GMT Jean-Paul Calderone wrote:
> On Wed, Feb 27, 2019 at 10:07 AM Scott, Barry <barry.scott at forcepoint.com>
> 
> wrote:
> > The code I have posted is the good version so it works and the assert
> > does not fire. My goal is to show what I assume is the correct way to code
> > a function that uses internal Deferred(), not give an example that breaks.
> > 
> > This should cause the the assert as the code must wait for the thread to
> > return a result.
> > 
> > def doWork():
> >         d = Deferred()
> >         d2 = deferToThread(doWorkHelper)
> >         d.callback(d2)
> >         return d
> 
> Ah.  I see.  Indeed, this code is wrong and triggers the assert.  For what
> it's worth, there is a simpler solution than the example you gave.  It uses
> the "chainDeferred" method:
> 
>     def doWork():
>         d = Deferred()
>         d2 = deferToThread(doWorkHelper)
>         d2.chainDeferred(d)
>         return d

The improved error message could suggest using chainDeferred() maybe.

> 
> Jean-Paul