[Python-Dev] PEP 434: IDLE Enhancement Exception

[Terry Reedy]

This re-write of Todd's draft focuses better on the specific proposal 
and motivation. It tries to take into account comments posted both here 
and on python-ideas

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PEP: 434
Title: IDLE Enhancement Exception for All Branches
Version: $Revision$
Last-Modified: $Date$
Author: Todd Rovito <rovitotv at gmail.com>,
         Terry Reedy <tjreedy at udel.edu>
BDFL-Delegate: Nick Coghlan
Status: Draft
Type: Informational
Content-Type: text/x-rst
Created: 16-Feb-2013
Post-History: 16-Feb-2013


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 developement. 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 [3]_).

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.


Rationale
=========

People primarily use IDLE by running the 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
pretty much 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 [4]_.)  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 [5]_. 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' philosopy 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
    (http://mail.python.org/pipermail/python-dev/2012-November/122514.html)

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

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

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


Copyright
=========

This document has been placed in the public domain.

-- 
Terry Jan Reedy