New tail recursion decorator

[Michele Simionato]

Kay Schluehr wrote:
> for all those who already copied and pasted the original solution and
> played with it I apologize for radical changes in the latest version (
> the recipe is on version 1.5 now ! ). The latest implementation is
> again a lot faster than the previous one. It does not only get rid of
> exceptions but also of stack-frame inspection.

This is spectacular!!

I would rewrite it as follows:

CONTINUE = object() # sentinel value returned by iterfunc

def tail_recursive(func):
    """
    tail_recursive decorator based on Kay Schluehr's recipe
    http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/496691
    """
    var = dict(in_loop=False, cont=True, argkw='will be set later')
    # the dictionary is needed since Python closures are read-only

    def iterfunc(*args, **kwd):
        var["cont"] = not var["cont"]
        if not var["in_loop"]: # start looping
            var["in_loop"] = True
            while True:
                res = func(*args,**kwd)
                if res is CONTINUE:
                    args, kwd = var["argkw"]
                else:
                    var["in_loop"] = False
                    return res
        else:
            if var["cont"]:
                var["argkw"] = args, kwd
                return CONTINUE
            else:
                return func(*args,**kwd)
    return iterfunc

Using my decorator module 'tail_recursive' can even be turned in a
signature-preserving
decorator. I think I will add this great example to the documentation
of the next version
of decorator.py!

 Michele Simionato