Re: A few issues and suggestions

On Wed, Nov 5, 2008 at 18:58, Phillip J. Eby <pje-Wh6+Hckhi6HFNGf7iClzIwC/[email protected]> wrote:
> At 04:37 PM 11/5/2008 +0200, Sergey Schetinin wrote:
>>
>> One more issue:
>>
>>  File "c:\files\checkouts\trellis\peak\events\stm.py", line 470, in
>> atomically
>>    return super(Controller,self).atomically(self._process, func, args, kw)
>>  File "c:\files\checkouts\trellis\peak\events\stm.py", line 186, in
>> atomically
>>    self.cleanup(*sys.exc_info())
>>  File "c:\files\checkouts\trellis\peak\events\stm.py", line 182, in
>> atomically
>>    retval = func(*args, **kw)
>>  File "c:\files\checkouts\trellis\peak\events\stm.py", line 486, in
>> _process
>>    self.run_rule(listener)
>>  File "c:\files\checkouts\trellis\peak\events\stm.py", line 364, in
>> run_rule
>>    listener.run()
>>  File "c:\files\checkouts\trellis\peak\events\trellis.py", line 559, in
>> run
>>    raise AssertionError("This should never happen!", self)
>> AssertionError: ('This should never happen!', Cell(<bound method
>> VPanel._minh of <gui5.models.VPanel object at 0x00C8CBB0>>, 0))
>>
>>
>>
>> And the rule for which that happens looks like this:
>>
>>    @maintain(initially=0)
>>    def _minh(self):
>>        prev = self._minh
>>        if prev == 0 and self.live:
>>            return self.height
>>        return prev
>
> Unfortunately, the rule itself will give us no clue of what happened.
>  Basically, it looks like for this to have happened, the cell had to have
> been assigned to between the time it was scheduled, and the time it was run.
>  I suspect it could be triggered by a sequence like:
>
> cell A depends on B and C
> B depends on C
> C triggers a change in B
> B sets a value in A
>
> Probably, setting a value should cancel the scheduled rule if it doesn't
> need initialization.  I suggest adding an "else: cancel(self)" to the "if
> self._needs_init:" block in Cell.set_value.  If that makes the problem go
> away, the above hypotheses are probably correct.

I've encountered this issue again, this time I have a better idea of
what's happening and it seems to be exactly the case: assigning to
cell that was already scheduled. I tried the suggested fix and it does
work perfectly. I also see that I managed to apply it to the wrong
function last time, so the earlier report that it didn't work was
invalid. Here's a test case based on your description:


from peak.events import trellis

def A():
    B.value
    C.value

A = trellis.Cell(A, None)

@trellis.Cell
def B():
    A.value = C.value

C = trellis.Value()

A.value
C.value = 1