python - itertools aparent bug with pygraphviz and return chain to flask -


i finding possible bug chaining several methods in return statement in flask. wanted see if had suggestions on how around problem.

here code:

#!/usr/bin/env python  import flask import time itertools import chain pygraphviz import agraph  class testclass(object):      def __init__(self):         g = ''      def worker(self):     a='1234'         b=a + '45\n'         yield b         time.sleep(3)         yield      def worker2(self):         time.sleep(3)         c = '\n9876'         yield c      def graph(self):         g = agraph(overlap='false')  tc = testclass() app = flask.flask(__name__)   @app.route('/') def test_method_get_stuff():     return flask.render_template('index.html')  @app.route('/', methods=['post']) def test_method_post_stuff():     def test_method_sub_function():     return chain(tc.worker(), tc.worker2(),tc.graph())     return flask.response(test_method_sub_function(),mimetype= 'text/plain')  app.run(debug=true) 

once post requested tc.worker() , tc.worker2() generators return correctly once tries perform tc.graph() errors out below error.

 * running on http://127.0.0.1:5000/  * restarting reloader 127.0.0.1 - - [17/aug/2014 18:23:17] "get / http/1.1" 200 - 127.0.0.1 - - [17/aug/2014 18:23:18] "post / http/1.1" 200 - debugging middleware caught exception in streamed response @ point response headers sent. traceback (most recent call last):   file "/usr/lib/python2.7/site-packages/werkzeug/wsgi.py", line 691, in __next__     return self._next()   file "/usr/lib/python2.7/site-packages/werkzeug/wrappers.py", line 81, in _iter_encoded     item in iterable: typeerror: 'nonetype' object not iterable 


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