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PEP 217 -- Display Hook for Interactive Use

PEP: 217
Title: Display Hook for Interactive Use
Author: moshez at (Moshe Zadka)
Status: Final
Type: Standards Track
Created: 31-Jul-2000
Python-Version: 2.1


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.


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 .


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:
    __builtin__._ = None
    print `o`
    __builtin__._ = o

Jython Issues

The method Py.printResult will be similarly changed.