Strange behaviour when trying to implement twisted deferredlist

Question

I'm new to twisted and trying to build a simple POC, however I keep getting the following error.

running
[]
running
[]
running
[]
running
[]
running
[]
Traceback (most recent call last):
  File "./poc.py", line 23, in <module>
    dl = defer.DeferredList([tasks])
  File "/usr/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 820, in __init__
    deferred.addCallbacks(self._cbDeferred, self._cbDeferred,
AttributeError: 'list' object has no attribute 'addCallbacks'

I have been reading through the relevant twisted documentation and guides and can't seem to work out what I'm doing wrong. Does anybody know what is causing this exception?

#!/usr/bin/env python
from twisted.internet import defer

def task_to_run(data):
  print 'running'
  print data

def finished(res):
  print 'finished', res

tasks = []

for x in range(0,5):
  d = defer.Deferred()      
  d.addCallback(task_to_run)
  tasks.append(d)

dl = defer.DeferredList([tasks])
dl.addCallback(finished) 

Answer 1

Assuming that task_to_run really is something that is running async, then @Elisha was pointing you at the issue (your example code lists the list and it shouldn't). So runnable code looks like:

#!/usr/bin/env python
from twisted.internet import defer

def task_to_run(data):
  print 'running'
  print data

def finished(res):
  print 'finished', res

tasks = []

for x in range(0,5):
  d = defer.Deferred()      
  d.addCallback(task_to_run)
  d.callback("blah - " + str(x)) # Fire them as I make them
  tasks.append(d)

dl = defer.DeferredList(tasks)

dl.addCallback(finished) 

With output of:

% ./deferredList.py
running
blah - 0
running
blah - 1
running
blah - 2
running
blah - 3
running
blah - 4
finished [(True, None), (True, None), (True, None), (True, None), (True, None)]

BTW very nice docs on DeferredLists are at: Deferred Reference

---

But if the tasks aren't actually running async then you really could just make one defer queue:

#!/usr/bin/env python
from twisted.internet import defer

def task_to_run(data):
  print 'running'
  print data
  return data

def finished(res):
  print 'finished', res

d = defer.Deferred()      

for x in range(0,5):
  d.addCallback(task_to_run)

d.addCallback(finished) 

d.callback("blah") # Fire the deferred

Runs as:

% ./deferred.py
running
blah
running
blah
running
blah
running
blah
running
blah
finished blah

Answer 2

tasks is a list. and you pass it to DeferredList inside another list. ([tasks])
I think you need to change the line:

dl = defer.DeferredList([tasks])

to this one:

dl = defer.DeferredList(tasks)