emacs hangs if python block is not executed

If I run just an ipython session (example I've used below), I get the following warning and emacs hangs

```
Warning (python): Your ‘python-shell-interpreter’ doesn’t seem to support readline, yet ‘python-shell-completion-native’ was t and "python" is not part of the ‘python-shell-completion-native-disabled-interpreters’ list. Native completions have been disabled locally.
```

```
#+BEGIN_SRC ipython :session mysession :exports both
def foo(x):
return x + 9

[foo(x) + 7 for x in range(7)]
#+END_SRC
```

However, if I run a python session before that, I get the same warning but emacs doesn't hang and I get results.

```
#+begin_src python :results output :session
print "There are %d hours in a week." % (7*24)
2**10
#+end_src

#+RESULTS:
: Python 2.7.9 |Anaconda 2.1.0 (x86_64)| (default, Dec 15 2014, 10:37:34)
: [GCC 4.2.1 (Apple Inc. build 5577)] on darwin
: Type "help", "copyright", "credits" or "license" for more information.
: Anaconda is brought to you by Continuum Analytics.
: Please check out: http://continuum.io/thanks and https://binstar.org
: There are 168 hours in a week.
: python.el: native completion setup loaded
: 1024

#+BEGIN_SRC ipython :session mysession :exports both
def foo(x):
return x + 9

[foo(x) + 7 for x in range(7)]
#+END_SRC

#+RESULTS:
: [16, 17, 18, 19, 20, 21, 22]
```

Any idea what is going on here?

Edit - I also get the following message when it hangs : "Shell native completion is disabled, using fallback". I'm using Jupyter v4.0