on StagedTaskCollections' `stage0`

[Riccardo Murri]

Hi *,

again I'm including the list into this reply, as I think it is of more
general interest.

Jody asks:

> When I reduced my class to this:
> ----
> from gc3libs.workflow import StagedTaskCollection
> from gc3libs import Run
>
> class WarholizePipeApp(StagedTaskCollection):
> """ sequence of tasks to do warholize """
>
> def __init__(self, N, img, body, finaldir):
> StagedTaskCollection.__init__(self, output_dir="warhol.tmp.dir")
> #-- end def
>
> def stage0(self):
> return Run.State.TERMINATED
> #-- end def
> ----
> I am told:
> ----
> File "/home/centos/course/WarholizeScript.py", line 30, in new_tasks
> apps=[WarholizePipeApp(self.params.N, self.params.img, body,
> self.params.finaldir)]
> File "/home/centos/course/WarholizePipeApp.py", line 16, in __init__
> StagedTaskCollection.__init__(self, output_dir="warhol.tmp.dir")
> File
> "/home/centos/gc3pie/lib/python2.7/site-packages/gc3libs/workflow.py", line
> 564, in __init__
> " must return `Task` instance or number" % self)
> AssertionError: Invalid return value from method `stage0()` of
> `StagedTaskCollection` object <WarholizePipeApp.WarholizePipeApp object at
> 0x132dc50>: must return `Task` instance or number
> ----
>
> Interestingly here again the stack indicates the constructor as being the
> source of the error, even though the error happens in stage0?

Because the `StagedTaskCollection.__init__()` constructor invokes
`stage0()` to build the initial list of tasks
(`StagedTaskCollection.__init__()` has to call
`SequentialTaskCollection.__init__()`).

See https://github.com/uzh/gc3pie/blob/master/gc3libs/workflow.py#L689
up to line 708 for more details.


> Why is it wrong to use Run.State.TERMINATED there?

The `stageN` interface only allows two possible return values:

- a `Task`/`Application` instance, which will be executed next, or

- a return code (integer or pair of integers), which is used to set the
collection's termination status, and the state is set to TERMINATED as
a side effect.

Again, I see that the slides in the "programmers' tutorial" were wrong;
I have now fixed this -- many thanks for reporting!

Ciao,
R

--
Riccardo Murri
http://www.s3it.uzh.ch/about/team/#Riccardo.Murri

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