The python implementation in Org babel has a couple of long-standing problems:
If you specify :results value, then the body is implicitly wrapped in a function and you have to add a return statement to indicate the value of the block - but only if you are not using a session.
If you specify :`:results value but you use a session, you have to wrap the body in your own function and call the function as the last thing of the block.
That makes it awkward to switch a block from session to no-session (and vice versa) and from :results output to :results value (and vice versa): you have to modify the code to switch.
In addition to that, when using sessions and an error occurs in a code block, then subsequent evaluations of the same or a different code block can fail, not because of the code, but because the session output got out of sync and Org babel can no longer parse it. When that happens, I kill the session buffer and start anew; and for that reason, I always name my sessions.
I deal with these problems in a couple of different ways: for a session, I use :results output exclusively and add print(...) statements in order to produce the output I want.
For non-session evaluation, I usually use :results value and I try to remember to add a return statement with the result that the block is supposed to produce.
EDIT: For your specific case, you seem to need a python interpreter from a non-standard place. In order not to have to specify it for every code block, I would add the header as a file property:
#+PROPERTY: header-args:python :python ~/anaconda3/bin/python3
(Don't forget to refresh the buffer by pressing C-c C-c on the added line above).
In accordance with the guidelines above, here's how I would write your blocks:
#+begin_src python :session foo :results output
import random
if random.randint(0,10) % 2 == 0:
ret = "even"
else:
ret = "odd"
print(ret)
#+end_src
#+RESULTS:
: odd
#+name: session_output
#+BEGIN_SRC python :results output :session foo
str='Hello, World'
print(str)
#+END_SRC
#+RESULTS: session_output
: Hello, World
If you really want to combine a session with :results value, use something like this, where you define a function and call it - that lets Org babel figure out what the result should be:
#+name: session_value
#+BEGIN_SRC python :results value :session foo
def hello():
str='Hello, World'
return str
hello()
#+END_SRC
and finally for :results value outside a session, use something like this:
#+name: non-session_value
#+BEGIN_SRC python :results value
str='Hello, World'
return str
#+END_SRC
#+RESULTS: non-session_value
: Hello, World
Remember, in this last case, the body of the code block is implicitly wrapped in a function, the function is called and the return value of the function is the result of the block.
