New tail recursion decorator

[Michele Simionato]

Duncan Booth wrote:
> My other problem with this is that the decorator is very fragile although
> this may be fixable

This version should be more robust against exceptions:

class tail_recursive(object):
    """
    tail_recursive decorator based on Kay Schluehr's recipe
    http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/496691
    """
    CONTINUE = object() # sentinel

    def __init__(self, func):
        self.func = func
        self.firstcall = True

    def __call__(self, *args, **kwd):
        try:
            if self.firstcall: # start looping
                self.firstcall = False
                while True:
                    result = self.func(*args, **kwd)
                    if result is self.CONTINUE: # update arguments
                        args, kwd = self.argskwd
                    else: # last call
                        break
            else: # return the arguments of the tail call
                self.argskwd = args, kwd
                return self.CONTINUE
        except: # reset and re-raise
            self.firstcall = True
            raise
        else: # reset and exit
            self.firstcall = True 
            return result