Python: yield与协程

1.没有显示结果是怎么回事？

import os,fnmatch
@coroutine
def find_files(target):
    while True:
        topdir,pattern=(yield)
        for path,dirname,filelist in os.walk(topdir):
            for name in filelist:
                if fnmatch.fnmatch(name,pattern):
                    target.send(os.path.join(path,name))
import gzip,bz2
@coroutine
def opener(target):
    while True:
        name=(yield )
        if name.endwith('.gz'): f=gzip.open(name)
        elif name.endwith('.bz2'): f=bz2.BZ2File(name)
        else:f=open(name)
        target.send(f)

@coroutine
def cat(target):
    while True:
        f=(yield)
        for line in f:
            target.send(line)

@coroutine
def grep(pattern,target):
    while True:
        line=yield
        if pattern in line:
            target.send(line)
import sys
@coroutine
def printer():
    while True:
        line=yield
        sys.stdout.write(line)
finder=find_files(opener(cat(grep('python',printer()))))
finder.send(('www','access-log'))