Re: [Python-Dev] Simple generators, round 2

March 18, 2001

      This kind of low level impl. where suspension points are known at runtime only,
cannot be implemented in jython
(at least not in a non costly and reasonable way).
Jython codebase is likely to just allow generators with suspension points known
at compilation time.

regards.

----- Original Message -----
From: Neil Schemenauer <nas@arctrix.com>
To: <python-dev@python.org>
Sent: Sunday, March 18, 2001 3:17 AM
Subject: [Python-Dev] Simple generators, round 2
...
I've got a different implementation.  There are no new keywords
and its simpler to wrap a high level interface around the low
interface.
http://arctrix.com/nas/python/generator2.diff
What the patch does:
Split the big for loop and switch statement out of eval_code2
    into PyEval_EvalFrame.
Add a new "why" flag for ceval, WHY_SUSPEND.  It is similar to
    WHY_RETURN except that the frame value stack and the block stack
    are not touched.  The frame is also marked resumable before
    returning (f_stackbottom != NULL).
Add two new methods to frame objects, suspend and resume.
    suspend takes one argument which gets attached to the frame
    (f_suspendvalue).  This tells ceval to suspend as soon as control
    gets back to this frame.  resume, strangely enough, resumes a
    suspended frame.  Execution continues at the point it was
    suspended.  This is done by calling PyEval_EvalFrame on the frame
    object.
Make frame_dealloc clean up the stack and decref f_suspendvalue
    if it exists.
There are probably still bugs and it slows down ceval too much
but otherwise things are looking good.  Here are some examples
(the're a little long and but illustrative).  Low level
interface, similar to my last example:
# print 0 to 999
    import sys
def g():
        for n in range(1000):
            f = sys._getframe()
            f.suspend((n, f))
        return None, None
n, frame = g()
    while frame:
        print n
        n, frame = frame.resume()
Let's build something easier to use:
# Generator.py
    import sys
class Generator:
        def __init__(self):
            self.frame = sys._getframe(1)
            self.frame.suspend(self)
def suspend(self, value):
            self.frame.suspend(value)
def end(self):
            raise IndexError
def __getitem__(self, i):
            # fake indices suck, need iterators
            return self.frame.resume()
Now let's try Guido's pi example now:
# Prints out the frist 100 digits of pi
    from Generator import Generator
def pi():
        g = Generator()
        k, a, b, a1, b1 = 2L, 4L, 1L, 12L, 4L
        while 1:
            # Next approximation
            p, q, k = k*k, 2L*k+1L, k+1L
            a, b, a1, b1 = a1, b1, p*a+q*a1, p*b+q*b1
            # Print common digits
            d, d1 = a/b, a1/b1
            while d == d1:
                g.suspend(int(d))
                a, a1 = 10L*(a%b), 10L*(a1%b1)
                d, d1 = a/b, a1/b1
def test():
        pi_digits = pi()
        for i in range(100):
            print pi_digits[i],
if __name__ == "__main__":
        test()
Some tree traversals:
from types import TupleType
    from Generator import Generator
# (A - B) + C * (E/F)
    expr = ("+",
             ("-", "A", "B"),
             ("*", "C",
                  ("/", "E", "F")))
def postorder(node):
        g = Generator()
        if isinstance(node, TupleType):
            value, left, right = node
            for child in postorder(left):
                g.suspend(child)
            for child in postorder(right):
                g.suspend(child)
            g.suspend(value)
        else:
            g.suspend(node)
        g.end()
print "postorder:",
    for node in postorder(expr):
        print node,
    print
This prints:
postorder: A B - C E F / * +
Cheers,
Neil
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Re: [Python-Dev] Simple generators, round 2

Samuele Pedroni