Saturday, 6 June 2015


I use mobile broadband, and reducing bandwidth usage saves me a lot of money.  So when I wanted to play with pypes after hearing that Yahoo Pipes was being shut down, I somehow ended up taking Peter Szabo's module, improving it a little, and putting it on a package up on PyPI.

It provides a good coverage of the Stackless API.  It's already been used as a backend for Peter Szabo's syncless project, and with the improvements I've made (adding the method, and fixing a greenlet/tasklet circular reference bug), it's even more capable.  But the devil is in the details.

It doesn't support the pre-emptive tasklet interruption that Stackless does, and can't as that is implemented via modifications to the internal Python VM source code.  Which is one of the few modifications Stackless makes in it's capacity as a fork of Python.  It is possibly possible to support this, as I believe Jeff Senn way back noted he had written a similar module with pre-emptive support based on the tracing hooks (with the downside that debuggers are unusable).

It doesn't support the threading model of Stackless Python.  This is derived by having a scheduler per thread, and most tasklets belonging to that scheduler at some level have slices of the stack of that thread and are therefore symbiotically attached to it.  It should be very possible to support this, with additional work, but that's something to set aside for when it's needed.

It doesn't support the module level properties, that Stackless implements.  And it can't unless it relies on hacks suggested in places like StackOverflow where classes are injected into 'sys.modules' to act as modules.

Future work I'm tempted to do is working out whether I can add the tealet project, as an alternative backend to the greenlet backend it currently has.  Greenlets are of course the stack slicing of Stackless extracted into an extension module.  tealet is the "next generation" version of the stack slicing intended for Stackless Python.  They're not using it already because it was such a wide ranging low level change to the workings of Stackless, that adopting it introduces potential instability which is not desirable at this time.

Kristjan Valur implemented a greenlet emulation module for tealet (in the same repo), so it may be that using it might be as simple as running stacklessemu on tealet.greenlets on tealet.  It's a pity it wasn't uploaded to pypi so I can try it out, but I don't think that's in Kristjan's wheelhouse.

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