python-peps/peps/pep-0217.rst

PEP: 217
Title: Display Hook for Interactive Use
Author: Moshe Zadka <moshez@zadka.site.co.il>
Status: Final
Type: Standards Track
Content-Type: text/x-rst
Created: 31-Jul-2000
Python-Version: 2.1
Post-History:


Abstract
========

Python's interactive mode is one of the implementation's great
strengths -- being able to write expressions on the command line
and get back a meaningful output.  However, the output function
cannot be all things to all people, and the current output
function too often falls short of this goal.  This PEP describes a
way to provides alternatives to the built-in display function in
Python, so users will have control over the output from the
interactive interpreter.


Interface
=========

The current Python solution has worked for many users, and this
should not break it. Therefore, in the default configuration,
nothing will change in the REPL loop. To change the way the
interpreter prints interactively entered expressions, users
will have to rebind ``sys.displayhook`` to a callable object.
The result of calling this object with the result of the
interactively entered expression should be print-able,
and this is what will be printed on ``sys.stdout``.


Solution
========

The bytecode ``PRINT_EXPR`` will call ``sys.displayhook(POP())``.
A ``displayhook()`` will be added to the sys builtin module, which is
equivalent to::

    import __builtin__
    def displayhook(o):
        if o is None:
            return
        __builtin__._ = None
        print `o`
        __builtin__._ = o


Jython Issues
=============

The method ``Py.printResult`` will be similarly changed.