Question

Writing a class, how do I implement

foo.send(item) ?

__iter__ allows iterating over the class like a generator, what if I want it to be a coroutine?

Answer 1

Here is a basic example of a coroutine:

def coroutine(func):
    def start(*args,**kwargs):
        cr = func(*args,**kwargs)
        cr.next()
        return cr
    return start

@coroutine
def grep(pattern):
    print "Looking for %s" % pattern
    while True:
        line = (yield)
        if pattern in line:
            print(line)

g = grep("python")
# Notice how you don't need a next() call here
g.send("Yeah, but no, but yeah, but no")
g.send("A series of tubes")
g.send("python generators rock!")
# Looking for python
# python generators rock!

We can make a class which contains such a coroutine, and delegates calls to its send method to the coroutine:

class Foo(object):
    def __init__(self,pattern):
        self.count=1
        self.pattern=pattern
        self.grep=self._grep()
    @coroutine
    def _grep(self):
        while True:
            line = (yield)
            if self.pattern in line:
                print(self.count, line)
                self.count+=1
    def send(self,arg):
        self.grep.send(arg)

foo = Foo("python")
foo.send("Yeah, but no, but yeah, but no")
foo.send("A series of tubes")
foo.send("python generators rock!")
foo.pattern='spam'
foo.send("Some cheese?")
foo.send("More spam?")

# (1, 'python generators rock!')
# (2, 'More spam?')

Notice that foo acts like a coroutine (insofar as it has a send method), but is a class — it can have attributes and methods which can interact with the coroutine.

For more information (and wonderful examples), see David Beazley’s Curious Course on Coroutines and Concurrency.