Schmitt trigger in python coroutines

schmitt_trigger.py

from __future__ import print_function

def schmitt_trigger(lo, hi, cb, value):
    """http://en.wikipedia.org/wiki/Schmitt_trigger"""
    while True:
        newvalue = (yield value)
        if newvalue is None:
            raise StopIteration()
        if newvalue >= hi and value < hi:
            cb(hi, newvalue, 'to-hi')
        elif newvalue <= lo and value > lo:
            cb(lo, newvalue, 'to-lo')
        value = newvalue

def test():
    i = [1, 2, 3, 4, 5, 4, 2, 6, 3]
    r = []
    def cb(*a):
        signal, value = a[:2]
        r.append(value)
        pass

    lo = 2
    hi = 5
    g = schmitt_trigger(lo, hi, cb, 0)
    g.next()
    for x in i:
        g.send(x)
    assert r == [5, 2, 6], str(r)