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

28 Feb 2019


      On Wednesday, 27 February 2019 15:11:31 GMT Jean-Paul Calderone wrote:
...
On Wed, Feb 27, 2019 at 10:07 AM Scott, Barry 
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