This is the official manual for the latest Org-mode release.

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14.9 Results of evaluation

The way in which results are handled depends on whether a session is invoked, as well as on whether :results value or :results output is used. The following table shows the table possibilities. For a full listing of the possible results header arguments see results.

Non-session Session
:results value value of last expression value of last expression
:results output contents of STDOUT concatenation of interpreter output

Note: With :results value, the result in both :session and non-session is returned to Org mode as a table (a one- or two-dimensional vector of strings or numbers) when appropriate.

14.9.1 Non-session

14.9.1.1 :results value

This is the default. Internally, the value is obtained by wrapping the code in a function definition in the external language, and evaluating that function. Therefore, code should be written as if it were the body of such a function. In particular, note that Python does not automatically return a value from a function unless a return statement is present, and so a ‘return’ statement will usually be required in Python.

This is the only one of the four evaluation contexts in which the code is automatically wrapped in a function definition.

14.9.1.2 :results output

The code is passed to the interpreter as an external process, and the contents of the standard output stream are returned as text. (In certain languages this also contains the error output stream; this is an area for future work.)

14.9.2 Session

14.9.2.1 :results value

The code is passed to an interpreter running as an interactive Emacs inferior process. Only languages which provide tools for interactive evaluation of code have session support, so some language (e.g., C and ditaa) do not support the :session header argument, and in other languages (e.g., Python and Haskell) which have limitations on the code which may be entered into interactive sessions, those limitations apply to the code in code blocks using the :session header argument as well.

Unless the :results output option is supplied (see below) the result returned is the result of the last evaluation performed by the interpreter. (This is obtained in a language-specific manner: the value of the variable _ in Python and Ruby, and the value of .Last.value in R).

14.9.2.2 :results output

The code is passed to the interpreter running as an interactive Emacs inferior process. The result returned is the concatenation of the sequence of (text) output from the interactive interpreter. Notice that this is not necessarily the same as what would be sent to STDOUT if the same code were passed to a non-interactive interpreter running as an external process. For example, compare the following two blocks:

     #+BEGIN_SRC python :results output
      print "hello"
      2
      print "bye"
     #+END_SRC
     
     #+RESULTS:
     : hello
     : bye

In non-session mode, the `2' is not printed and does not appear.

     #+BEGIN_SRC python :results output :session
      print "hello"
      2
      print "bye"
     #+END_SRC
     
     #+RESULTS:
     : hello
     : 2
     : bye

But in :session mode, the interactive interpreter receives input `2' and prints out its value, `2'. (Indeed, the other print statements are unnecessary here).