Implementing Stackless Python

Question

I really admire the functionality of Stackless Python, and I've been looking around for a way to emulate its syntax while still using the standard Python 3 interpreter. An article by Alex J. Champandard in a gamedev blog made it look as though the greenlet library could provide this functionality. I slightly modified his code, but the best makeshift tasklet wrapper I could come up with was a class holding a greenlet inside a variable, as such:

class tasklet(): 
        def __init__(self,function=None,*variables):
                global _scheduled
                self.greenlet = greenlet.greenlet(function,None)
                self.functioncall = function # Redundant backup
                self.variables = variables
                _scheduled.append(self)
                self.blocked = False

The function then emulates Stackless' scheduling by passing the variables to the greenlet when calling its switch() method.

So far this appears to work, but I'd like to be able to call the tasklets in original Stackless syntax, e.g. tasklet(function)(*args), as opposed to the current syntax of tasklet(function,*args). I'm not sure where to look in the documentation to find out how to accomplish this. Is this even possible, or is it part of Stackless' changes to the interpreter?

Answer 1

According to this article from 2010-01-08 (with fixed links):

Stackless Python is an extended version of the Python language (and its CPython reference implementation). New features include lightweight coroutines (called tasklets), communication primitives using message passing (called channels), manual and/or automatic coroutine scheduling, not using the C stack Python function calls, and serialization of coroutines (for reloading in another process). Stackless Python could not be implemented as a Python extension module – the core of the CPython compiler and interpreter had to be patched.

greenlet is an extension module to CPython providing coroutines and low-level (explicit) scheduling. The most important advantage of greenlet over Stackless Python is that greenlet could be implemented as a Python extension module, so the whole Python interpreter doesn't have to be recompiled in order to use greenlet. Disadvantages of greenlet include speed (Stackless Python can be 10%, 35% or 900% faster, depending on the workflow); possible memory leaks if coroutines have references to each other; and that the provided functionality is low-level (i.e. only manual coroutine scheduling, no message passing provided).

greenstackless, the Python module I've recently developed, provides most of the (high-level) Stackless Python API using greenlet, so it eliminates the disadvantage of greenlet that it is low-level. See the source code and some tests (the latter with tricky corner cases). Please note that although greenstackless is optimized a bit, it can be much slower than Stackless Python, and it also doesn't fix the memory leaks. Using greenstackless is thus not recommended in production environments; but it can be used as a temporary, drop-in replacement for Stackless Python if replacing the Python interpreter is not feasible.

Some other software that emulates Stackless using greenlet:

Concurrence: doesn't support stackless.main, tasklet.next, tasklet.prev, tasklet.insert, tasklet.remove, stackless.schedule_remove, doesn't send exceptions properly. (Because of these features missing, it doesn't pass the unit test above.)

PyPy: doesn't support stackless.main, tasklet.next, tasklet.prev, doesn't pass the unit test above.