8

Similarly to this question I would like to evaluate (in org mode) Python source code blocks containing "input" instructions but I can't find a way to have an interactive evaluation (with user input) during evaluation or to give it some input known in advance (stored in a file for instance).

My constraint is to use explicitly the input instruction since all this should be included in a textbook for students.

Example of code :

#+BEGIN_SRC python :results output 
a= input("Value") 
print(a)
#+END_SRC

Is it possible to have such an interactive evaluation or to simulate it (by giving to the source code a fake input) ?

  • Will you be using org-mode export to generate your example code for the students into some other format, e.g. html? – Melioratus Sep 6 '16 at 5:10
  • Python's input() function only takes quoted text on single line, e.g. "hello" or "hi\nhello\nhowdy", correct? – Melioratus Sep 6 '16 at 5:29
  • @Melioratus Thaks for you comments; yes I use the org-mode export (to LaTeX/pdf) and I export both the code and the result. You can also use multilines (""" ... """) text as a parameter for the input function. My problem is to introduce values to the "input" function during the execution of the code. – Lgen Sep 6 '16 at 17:12
  • Thanks for the clarification! I'll post an answer that uses the literate programming features , i.e. noweb, of org-mode that will allow you to test & export your code with results. – Melioratus Sep 6 '16 at 18:14
  • Thanks, this could be an alternative to the solution proposed by John Kitchin (may be avoiding the tangling step ?). – Lgen Sep 6 '16 at 20:15
8

Here is an alternative approach that uses a non-exported, tangled file to replace the input function.

#+BEGIN_SRC python :session :exports none :tangle example1.py
def input(x): 
    if x == 'Value of a':
        return 3
    elif x == 'Value of b':
        return 4 

#+END_SRC 

#+RESULTS: 

Tip: Press C-cC-vt or use the M-xorg-babel-tangle command to create, i.e. tangle, the example1.py file.


#+BEGIN_SRC python :results output :preamble from example1 import *
a = input('Value of a')
b = input('Value of b')
print(a + b) 
#+END_SRC 

#+RESULTS:
: 7

Note: The example1.py file that was created from the previous python SRC block will be imported into the current block using the builtin :preamble header and the value from example1 import *.

  • Very interesting solution, thanks. I accept it as the solution and I will also propose a variant based on python generator and returning systematically a "str" object to mimic the input(...) function. – Lgen Sep 6 '16 at 20:04
7

Evaluate python code blocks using literate programming in org-mode.

Use :var header to assign variables and test your code.

Note: If desired use elisp (read-string "Prompt: ") and (read-number "Prompt: ") to prompt for user input inside emacs.


Example 1 - print(a)

  • Assign hello world to a.


    #+name: ex1-code  
    #+header: :var a="hello world"  
    #+begin_src python :results verbatim replace output :exports results  
      print(a)  
    #+end_src
    
    #+begin_src python :eval never :exports code :noweb yes   
      a = input('Value of a')  
      <<ex1-code>>  
    #+end_src  
    
    #+results: ex1-code
    : hello world
    

Example 2 - print(a + b)

  • Assign 1 to a.

  • Assign 2 to b.


    #+name: ex2-code
    #+header: :var a=1 
    #+header: :var b=2 
    #+begin_src python :results replace output  :exports results 
      print(a + b)
    #+end_src
    
    #+begin_src python :eval never :exports code :noweb yes 
      a = input('Value of a')
      b = input('Value of b')
      <<ex2-code>>
    #+end_src  
    
    #+results: ex2-code
    : 3
    

Example 3 - print(a,b,c)

  • When prompted for Value of a enter Thanks
  • When prompted for Value of b enter 4.
  • When prompted for Value of c enter your question.


    #+NAME: ex3-code
    #+header: :var a=(read-string "Value of a ") 
    #+header: :var b=(read-number "Value of b ") 
    #+header: :var c=(read-string "Value of c ") 
    #+begin_src python :results replace output   :exports results 
      print a,b,c
    #+end_src  
    
    #+begin_src python :eval never :exports code :noweb yes 
      a = input('Value of a')
      b = input('Value of b')
      c = input('Value of c')
      <<ex3-code>>
    #+end_src  
    
    #+results: ex3-code
    : Thanks 4 your question
    

When you export your org file, the output should look similar to the example below


Example 1 - print(a)

  • Assign hello world to a.

    a = input('Value of a')
    print(a)
    
    hello world
    

Example 2 - print(a + b)

  • Assign 1 to a.
  • Assign 2 to b.

    a = input('Value of a')
    b = input('Value of b')
    print(a + b)
    
    3
    

Example 3 - print(a,b,c)

  • When prompted for Value of a enter Thanks
  • When prompted for Value of b enter 4.
  • When prompted for Value of c enter your question.

    a = input('Value of a')
    b = input('Value of b')
    c = input('Value of c')
    print a,b,c
    
    Thanks 4 your question
    


This code was tested with
GNU Emacs 24.5.1 (x86_64-unknown-cygwin, GTK+ Version 3.14.13)
Org-Mode Version: 8.3.2
and github.

  • Very interesting solution; it took time for me to understand (I'm not used to literate programming) that the instructions in the second python source bloc (e.g. a = input('Value of a')) were only documentation text and were not treated as instructions. – Lgen Sep 7 '16 at 15:10
  • @Lgen - Thanks! The literate programming, noweb, in org-mode features are amazing and incredibly useful! Please let me know if you would like additional code examples. If you need something that doesn't fit well into the Q&A format, I'm happy to post to my GitHub repository. – Melioratus Sep 7 '16 at 15:41
5

I don't think it is possible to get truly interactive Python input with org-babel.

You could use a preamble to redefine the input function so it returns what you want to simulate the use of input, e.g. here we make it look like the user typed in "3".

#+BEGIN_SRC python :results output :preamble def input(x): return 3
a = input("value ")
print(a)
#+END_SRC

#+RESULTS:
: 3

Depending on what students see that is exported, they might not see that you have done this.

  • Thanks John for you proposed answer. I was wondering if it is possible to extend it to a multiple call to the input function in the code block (e.g. a=input("Value of a") b=input("Value of b")). To build the texbook I generally use and ":export both" in the "#+BEGIN_SRC python" statement so this line should not be visible in the exported text. – Lgen Sep 6 '16 at 17:17
  • If you use a session, it is possible: #+BEGIN_SRC python :session :exports none def input(x): if x == 'Value of a': return 3 elif x == 'Value of b': return 4 #+END_SRC #+RESULTS: #+BEGIN_SRC python :results output :session a = input('Value of a') b = input('Value of b') print(a + b) #+END_SRC #+RESULTS: : : >>> >>> 7 – John Kitchin Sep 6 '16 at 18:45
2

As a complement of John Kitchin's solution, I propose to use a generator to provide the successive values that will "feed" the input(...) functions and to return a strobject systematically.

#+BEGIN_SRC python :session :exports none :tangle example2.py :results none
def generate(lst):
    for element in lst:
        yield str(element)

generator =  generate(["Thanks",4,"your help"])

def input(x):
     return generator.__next__()
#+END_SRC 


#+BEGIN_SRC python :results output :preamble from example2 import * :exports both
a = input('Value of a')
b = input('Value of b')
c = input('Value of c')
print(a,b,c)
#+END_SRC 

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