PEP: 434
Title: IDLE Enhancement Exception for All Branches
Author: Todd Rovito <rovitotv@gmail.com>,
        Terry Reedy <tjreedy@udel.edu>
BDFL-Delegate: Alyssa Coghlan
Status: Active
Type: Informational
Created: 16-Feb-2013
Post-History: 16-Feb-2013,
              03-Mar-2013,
              21-Mar-2013,
              30-Mar-2013
Resolution: https://mail.python.org/pipermail/python-dev/2013-March/125003.html


Abstract
========

Most CPython tracker issues are classified as behavior or enhancement.
Most behavior patches are backported to branches for existing
versions.  Enhancement patches are restricted to the default branch
that becomes the next Python version.

This PEP proposes that the restriction on applying enhancements be
relaxed for IDLE code, residing in .../Lib/idlelib/.  In practice,
this would mean that IDLE developers would not have to classify or
agree on the classification of a patch but could instead focus on what
is best for IDLE users and future IDLE development.  It would also
mean that IDLE patches would not necessarily have to be split into
'bugfix' changes and enhancement changes.

The PEP would apply to changes in existing features and addition of
small features, such as would require a new menu entry, but not
necessarily to possible major re-writes such as switching to themed
widgets or tabbed windows.


Motivation
==========

This PEP was prompted by controversy on both the tracker and pydev
list over adding Cut, Copy, and Paste to right-click context menus
(Issue 1207589, opened in 2005 [1]_; pydev thread [2]_).  The features
were available as keyboard shortcuts but not on the context menu.  It
is standard, at least on Windows, that they should be when applicable
(a read-only window would only have Copy), so users do not have to
shift to the keyboard after selecting text for cutting or copying or a
slice point for pasting.  The context menu was not documented until 10
days before the new options were added (Issue 10405 [5]_).

Normally, behavior is called a bug if it conflicts with documentation
judged to be correct.  But if there is no doc, what is the standard?
If the code is its own documentation, most IDLE issues on the tracker
are enhancement issues.  If we substitute reasonable user expectation,
(which can, of course, be its own subject of disagreement), many more
issues are behavior issues.

For context menus, people disagreed on the status of the additions --
bugfix or enhancement.  Even people who called it an enhancement
disagreed as to whether the patch should be backported.  This PEP
proposes to make the status disagreement irrelevant by explicitly
allowing more liberal backporting than for other stdlib modules.

Python does have many advanced features yet Python is well known for
being an easy computer language for beginners [3]_.  A major Python
philosophy is "batteries included" which is best demonstrated in
Python's standard library with many modules that are not typically
included with other programming languages [4]_.  IDLE is an important
"battery" in the Python toolbox because it allows a beginner to get
started quickly without downloading and configuring a third party IDE.
IDLE represents a commitment by the Python community to encouage the
use of Python as a teaching language both inside and outside of formal
educational settings.  The recommended teaching experience is to have
a learner start with IDLE.  This PEP and the work that it will enable
will allow the Python community to make that learner's experience with
IDLE awesome by making IDLE a simple tool for beginners to get started
with Python.

Rationale
=========

People primarily use IDLE by running the graphical user interface
(GUI) application, rather than by directly importing the effectively
private (undocumented) implementation modules in idlelib.  Whether
they use the shell, the editor, or both, we believe they will benefit
more from consistency across the latest releases of current Python
versions than from consistency within the bugfix releases for one
Python version.  This is especially true when existing behavior is
clearly unsatisfactory.

When people use the standard interpreter, the OS-provided frame works
the same for all Python versions.  If, for instance, Microsoft were to
upgrade the Command Prompt GUI, the improvements would be present
regardless of which Python were running within it.  Similarly, if one
edits Python code with editor X, behaviors such as the right-click
context menu and the search-replace box do not depend on the version
of Python being edited or even the language being edited.

The benefit for IDLE developers is mixed.  On the one hand, testing
more versions and possibly having to adjust a patch, especially for
2.7, is more work.  (There is, of course, the option on not
backporting everything.  For issue 12510, some changes to calltips for
classes were not included in the 2.7 patch because of issues with
old-style classes [6]_.)  On the other hand, bike-shedding can be an
energy drain.  If the obvious fix for a bug looks like an enhancement,
writing a separate bugfix-only patch is more work.  And making the
code diverge between versions makes future multi-version patches more
difficult.

These issue are illustrated by the search-and-replace dialog box.  It
used to raise an exception for certain user entries [7]_.  The
uncaught exception caused IDLE to exit.  At least on Windows, the exit
was silent (no visible traceback) and looked like a crash if IDLE was
started normally, from an icon.

Was this a bug?  IDLE Help (on the current Help submenu) just says
"Replace...  Open a search-and-replace dialog box", and a box *was*
opened.  It is not, in general, a bug for a library method to raise an
exception.  And it is not, in general, a bug for a library method to
ignore an exception raised by functions it calls.  So if we were to
adopt the 'code = doc' philosophy in the absence of detailed docs, one
might say 'No'.

However, IDLE exiting when it does not need to is definitely
obnoxious.  So four of us agreed that it should be prevented.  But
there was still the question of what to do instead?  Catch the
exception?  Just not raise the exception?  Beep?  Display an error
message box?  Or try to do something useful with the user's entry?
Would replacing a 'crash' with useful behavior be an enhancement,
limited to future Python releases?  Should IDLE developers have to ask
that?


Backwards Compatibility
=======================

For IDLE, there are three types of users who might be concerned about
back compatibility.  First are people who run IDLE as an application.
We have already discussed them above.

Second are people who import one of the idlelib modules.  As far as we
know, this is only done to start the IDLE application, and we do not
propose breaking such use.  Otherwise, the modules are undocumented
and effectively private implementations.  If an IDLE module were
defined as public, documented, and perhaps moved to the tkinter
package, it would then follow the normal rules.  (Documenting the
private interfaces for the benefit of people working on the IDLE code
is a separate issue.)

Third are people who write IDLE extensions.  The guaranteed extension
interface is given in idlelib/extension.txt.  This should be respected
at least in existing versions, and not frivolously changed in future
versions.  But there is a warning that "The extension cannot assume
much about this [EditorWindow] argument."  This guarantee should
rarely be an issue with patches, and the issue is not specific to
'enhancement' versus 'bugfix' patches.

As is happens, after the context menu patch was applied, it came up
that extensions that added items to the context menu (rare) would be
broken because the patch a) added a new item to standard rmenu_specs
and b) expected every rmenu_spec to be lengthened. It is not clear
whether this violates the guarantee, but there is a second patch that
fixes assumption b).  It should be applied when it is clear that the
first patch will not have to be reverted.

References
==========

.. [1] IDLE: Right Click Context Menu, Foord, Michael
   (http://bugs.python.org/issue1207589)

.. [2] Cut/Copy/Paste items in IDLE right click context menu
   (https://mail.python.org/pipermail/python-dev/2012-November/122514.html)

.. [3] Getting Started with Python
   (http://www.python.org/about/gettingstarted/)

.. [4] Batteries Included
   (http://docs.python.org/2/tutorial/stdlib.html#batteries-included)

.. [5] IDLE breakpoint facility undocumented, Deily, Ned
   (http://bugs.python.org/issue10405)

.. [6] IDLE: calltips mishandle raw strings and other examples,
   Reedy, Terry (http://bugs.python.org/issue12510)

.. [7] IDLE: replace ending with '\' causes crash, Reedy, Terry
   (http://bugs.python.org/issue13052)


Copyright
=========

This document has been placed in the public domain.
