Umbrella issue: idlelib analysis with code-review-toolkit

[devdanzin]

Bug report

Bug description:

A two-pass review of Lib/idlelib turned up 69 findings — 37 FIX (a wrong result, silent data loss, or an unreachable feature) and 32 CONSIDER. Every one carries a file:line, a concrete failure scenario, and — where idlelib already handles the same situation correctly somewhere else — the guarded twin that shows it is a defect by the project's own standard and suggests the fix.

I'm filing them under one umbrella so they can be picked off individually rather than flooding the tracker.

Please don't start work on a row until a maintainer has cleared it. These are a reviewer's findings, not triaged bugs: some will be intentional behaviour, some not worth the churn, and some wrong. Every row starts at Under analysis, which means not yet cleared — do not work on it. A maintainer moving a row off Under analysis is what opens it up; a PR against an uncleared row may be wasted effort, and several of these touch code where the current behaviour is load-bearing.

To take a cleared one: open a normal issue or PR for it and drop a comment here with the link — I'll put it in the Status column. If any row is a duplicate, a non-bug, or intentional, say so and I'll annotate it.

Found with code-review-toolkit. Reports were drafted with AI assistance (Claude Code) and then re-verified by hand — see Disclosure at the end.

Scope and verification

• Everything below is against main at 993a0c65a7b (2026-07-27). The review itself was carried out on the 3.14 branch at 6080c866096; every finding was then re-checked against main by comparing the enclosing function (46 byte-identical, 13 changed and read individually, 11 module-level checked by hand), and every line number here is main's. Line numbers drift; the cited function and mechanism will not.
• 18 findings were reproduced against a running interpreter (real Tk where the widget matters); the rest were confirmed by reading the code and its tests. Each gist entry says which. The reproductions were re-run on a release build of main at the commit above.
• python -m test test_idle passes on this commit (305 run, 83 skipped headless). Every finding below is one the suite cannot see — and four of them (26, 71, 72, 73) are about why.
• Prior art was checked for all 73 findings against the whole tracker: 1626 topic-IDLE issues, 281 unlabelled issues mentioning idlelib, 573 PRs and 86 idle.rst items (2503 unique). Three were already reported and have been removed from this report — they are listed at the bottom so nobody re-derives them. Where an open issue covers the area without covering the defect, the table names it.

Three systemic roots

These are worth reading first: each explains several of the individual rows below, and fixing the root is a smaller change than fixing its instances one at a time.

• 31 — Five independent hand-written recognizers for Python syntax with no shared source of truth
  idlelib answers “is this Python syntax?” in five hand-written places with no shared definition, and new syntax reaches some and not others. colorizer is consistently the one that gets updated — four of the rows below are drift between it and a sibling.
• 32 — The -n execution backend is missing five cross-cutting protections the RPC backend has
  use_subprocess selects between two interchangeable execution backends, and every cross-cutting protection lives inside the RPC one rather than in a shared driver. SystemExit is the sharpest cell and the two are opposites: the RPC backend contains it and returns to the prompt, while -n asks “Do you want to exit altogether?” with default="yes" and raises out of mainloop(), so unsaved editor buffers get no save prompt. Ctrl-C, Restart and the recursion headroom are missing from -n too.
• 34 — WidgetRedirector swallows every TclError, so text.index() can never raise on any IDLE widget
  except TclError: return "" in WidgetRedirector.dispatch intercepts every subcommand of every percolated Text — which is every EditorWindow, PyShell and OutputWindow. On the widgets idlelib actually uses, text.index() therefore never raises and never returns None, callers cannot tell “no selection” from “the call failed”, and three incompatible guard idioms have grown in one file.

Start here

Lowest effort to confirm, highest cost if left alone — all silent:

• 35 — Format Paragraph silently deletes characters from every non-comment line of a selection (format.py:59)
• 36 — A literal % in any setting strips every open window's keybindings and leaves them off (config.py:48)
• 27 — Copy-with-prompts drops the last line whenever the selection ends at column 10, 20, 100... (pyshell.py:1034)
• 60 — Saving a file whose coding cookie cannot encode its text writes a BOM and keeps the cookie (iomenu.py:324-326)
• 51 — restore_file_breaks appends without clearing, so Save As carries the old file's breakpoints over (pyshell.py:268)
• 59 — _study1 has no f-string state, so a PEP 701 multi-line f-string is read as three statements (pyparse.py:245)
• 43 — Three keysyms shipped in every keyset cannot be spelled by the key-configuration dialog (config_key.py:15)
• 34 — WidgetRedirector swallows every TclError, so text.index() can never raise on any IDLE widget (redirector.py:116)

Findings

The # column is a row number in this issue, not an identifier. Please cite this issue plus the row (“#‹this issue› row 35”) rather than the bare number on its own — a bare number does not autolink and does not lead anyone back here.

Status: Under analysis = awaiting a maintainer's call, not cleared for work · #N = filed, in progress · #N FIXED = filed and fixed · won't fix / intentional = closed by a maintainer.

Prior art names an existing issue where one is relevant: from = this is residue of that merged change; see = that issue covers the area but not this defect, so a comment there may be better than a new issue.

Shell, the -n backend, and breakpoints (15)

pyshell.py, run.py, rpc.py — full reports

#	Sev	Location	Finding	Prior art	Status
2	FIX	pyshell.py:499-540	One subprocess-accept timeout wedges IDLE permanently		Under analysis
8	FIX	run.py:635-639,695	Stack Viewer reads an attribute only runcode() ever creates		Under analysis
27	FIX	pyshell.py:1034	Copy-with-prompts drops the last line whenever the selection ends at column 10, 20, 100...	from #95511	Under analysis
28	FIX	pyshell.py:1016	get_prompt_text zips a computed line range against actual lines	from #95191	Under analysis
32	FIX	pyshell.py:792-803	The -n execution backend is missing five cross-cutting protections the RPC backend has		Under analysis
33	FIX	pyshell.py:680	Under idle -n, file is silently IDLE's own path instead of the script's	from #77165	Under analysis
51	FIX	pyshell.py:268	restore_file_breaks appends without clearing, so Save As carries the old file's breakpoints over		Under analysis
52	FIX	pyshell.py:244	Breakpoints are keyed by the raw filename while windows are keyed by the normalized one		Under analysis
53	FIX	pyshell.py:259	breakpoints.lst uses = as its delimiter, which is legal in a path		Under analysis
12	CONSIDER	run.py:190-197	The subprocess's last-resort handler can itself raise NameError		Under analysis
13	CONSIDER	rpc.py:358-361	Every OSError on recv becomes 'the peer hung up'		Under analysis
14	CONSIDER	pyshell.py:804-821	endexecuting() is skipped if showtraceback() fails		Under analysis
15	CONSIDER	rpc.py:258-260	Remote internal errors are reported only at a permanently-disabled debug level		Under analysis
18	CONSIDER	rpc.py:130-141	Reconnect resets some per-connection state but not the packet read buffer		Under analysis
21	CONSIDER	pyshell.py:564-574	terminate_subprocess catches every OSError though its comment names one		Under analysis

Configuration and the Settings dialog (9)

config.py, configdialog.py, config_key.py — full reports

#	Sev	Location	Finding	Prior art	Status
1	FIX	config.py:136-140	A failed config save deletes the user's config file		Under analysis
36	FIX	config.py:48	A literal % in any setting strips every open window's keybindings and leaves them off		Under analysis
43	FIX	config_key.py:15	Three keysyms shipped in every keyset cannot be spelled by the key-configuration dialog		Under analysis
48	FIX	config.py:244	An invalid value in the DEFAULT config is swallowed where the user-config clause warns	from #60715	Under analysis
49	FIX	config.py:605	The keybinding collision test compares whole lists, so a partial overlap is undetected and silent		Under analysis
61	FIX	config.py:756	Help sources are numbered on write and sorted as strings on read, so entry 10 sorts before entry 2	see #71652	Under analysis
62	FIX	config_key.py:227	An advanced key binding stored as a Tk event sequence is read back as a whitespace-separated list	see #55646, #152747	Under analysis
50	CONSIDER	config.py:199	The whole IDLE profile location is a function of the launch directory when HOME is unusable	see #60066, #52478	Under analysis
69	CONSIDER	parenmatch.py:53	Two extensions' in-code fallback defaults disagree with the values actually shipped		Under analysis

Editor: formatting, undo, search/replace, save and run (14)

format.py, undo.py, replace.py, searchengine.py, iomenu.py, runscript.py, zoomheight.py — full reports

#	Sev	Location	Finding	Prior art	Status
3	FIX	replace.py:226-229	Replace's NULL-guard tests the call receiver instead of the match		Under analysis
5	FIX	searchengine.py:146-151	Forward search abandons the rest of a line after a zero-width match		Under analysis
9	FIX	zoomheight.py:66-105	A failed window probe leaves the user's window stuck maximized	from #81220	Under analysis
35	FIX	format.py:59	Format Paragraph silently deletes characters from every non-comment line of a selection	see #143774, #143775	Under analysis
45	FIX	replace.py:161	undo_block_start/stop is unpaired on the exception path in four places		Under analysis
60	FIX	iomenu.py:324-326	Saving a file whose coding cookie cannot encode its text writes a BOM and keeps the cookie		Under analysis
67	FIX	runscript.py:189	Run Module ignores the return of io.save(), so a failed save runs the old code from disk		Under analysis
11	CONSIDER	iomenu.py:360-375	A bad print command orphans the user's unsaved source in /tmp		Under analysis
19	CONSIDER	replace.py:172-188	Replace All's final selection uses the pre-replacement span		Under analysis
20	CONSIDER	searchengine.py:85-89	getprog catches only re.PatternError, so OverflowError escapes to every caller		Under analysis
24	CONSIDER	iomenu.py:253,261	Recent-files list is updated outside the success guard		Under analysis
46	CONSIDER	undo.py:104	undo_block_stop does not guard the sentinel its own twin guards		Under analysis
64	CONSIDER	format.py:340	Untabify rewrites the whole line where its tabify mirror rewrites only the indent		Under analysis
65	CONSIDER	undo.py:294	DeleteCommand.undo reinserts text without tags where the Insert mirror round-trips them		Under analysis

The five hand-written syntax recognizers (7)

pyparse.py, hyperparser.py, codecontext.py, colorizer.py — full reports

#	Sev	Location	Finding	Prior art	Status
30	FIX	hyperparser.py:298	HyperParser's string-prefix scanner never learned f or t	from #139742, #139756	Under analysis
47	FIX	codecontext.py:22	codecontext's BLOCKOPENERS lacks match/case, so those blocks never appear in Code Context	from #77745, #25851	Under analysis
59	FIX	pyparse.py:245	_study1 has no f-string state, so a PEP 701 multi-line f-string is read as three statements	see #77061	Under analysis
63	FIX	hyperparser.py:137-140	Bracket matching checks the bracket type on the backward scan only	see #124451	Under analysis
17	CONSIDER	pyparse.py:44-48	A comment with no space after # is invisible to HyperParser		Under analysis
31	CONSIDER	hyperparser.py:298	Five independent hand-written recognizers for Python syntax with no shared source of truth		Under analysis
58	CONSIDER	pyparse.py:21	_synchre matches def but cannot match async def, making indent analysis quadratic in async modules	see #77061	Under analysis

Completions and calltips (5)

autocomplete*.py, calltip.py — full reports

#	Sev	Location	Finding	Prior art	Status
4	FIX	autocomplete.py:117,134	Autocomplete's mode filter never applies to ATTRS because ATTRS == 0		Under analysis
6	FIX	calltip.py:189	get_argspec guards two of its three touches of a user object		Under analysis
10	FIX	autocomplete_w.py:238	A failed is_active check permanently disables the completion window's Configure handler	from #26404	Under analysis
29	FIX	autocomplete_w.py:363-368	and binds tighter than or, so the modifier guard covers one completion mode of two		Under analysis
68	FIX	autocomplete_w.py:454-491	The completion popup teardown is unguarded where the calltip's identical teardown guards every step		Under analysis

Infrastructure: redirector, file list, debugger, trees (9)

redirector.py, filelist.py, debugger*.py, tree.py, stackviewer.py — full reports

#	Sev	Location	Finding	Prior art	Status
34	FIX	redirector.py:116	WidgetRedirector swallows every TclError, so text.index() can never raise on any IDLE widget		Under analysis
54	FIX	filelist.py:90	After a Name Conflict the displaced window is marked None instead of re-pointed, so the NEXT collision is silent		Under analysis
55	FIX	debugger_r.py:271	DictProxy.getitem returns a repr, and only one of its two consumers strips the extra quotes		Under analysis
16	CONSIDER	filelist.py:105-108	A failed getcwd() silently yields a relative path used as a window identity key		Under analysis
22	CONSIDER	debugger.py:159-162	Debugger.close swallows every failure of quit()		Under analysis
25	CONSIDER	debugger.py:55-56	Idb.user_exception calls into the GUI unguarded where its twin is guarded		Under analysis
56	CONSIDER	debugger_r.py:35	Four id()-keyed debugger tables are never cleared, so memory grows with every debugger stop		Under analysis
57	CONSIDER	tree.py:234	Tree redraw re-binds every node's canvas item and never deletes the bindings		Under analysis
70	CONSIDER	stackviewer.py:12	Stack Viewer module globals pin every frame of the last exception for the process lifetime		Under analysis

Documentation and the generated in-app help (6)

Doc/library/idle.rst, help.py, help.html, config-main.def — full reports

#	Sev	Location	Finding	Prior art	Status
37	FIX	Doc/library/idle.rst:731-732	The documented sys.argv[0] contract for idle -r is wrong, and idle -h contradicts it correctly		Under analysis
38	FIX	Doc/library/idle.rst:752	The network diagnostic in the docs names a port IDLE has never listened on		Under analysis
39	CONSIDER	help.py:251	The in-app help is generated correctly, so every documentation error is faithfully shipped		Under analysis
40	CONSIDER	Doc/library/idle.rst	Documentation still names UI that was renamed or split two releases ago		Under analysis
41	CONSIDER	config-main.def:48	Four config options are fully supported and completely invisible, and one has no consumer		Under analysis
44	CONSIDER	mainmenu.py:80	Three virtual events are declared but unreachable, one of them from a live menu		Under analysis

Test-suite integrity (4)

idle_test/ — full reports

#	Sev	Location	Finding	Prior art	Status
26	CONSIDER	idle_test/test_autocomplete.py:241	Five tests in the suite cannot fail		Under analysis
71	CONSIDER	idle_test/mock_idle.py:48	The shared test stubs cannot express the conditions the guards under test exist for		Under analysis
72	CONSIDER	idle_test/test_searchengine.py:279	A test class freezes text.index to a constant, leaving seven mutations of live code undetected		Under analysis
73	CONSIDER	idle_test/test_undo.py	Four test files exit 0 even when their tests fail		Under analysis

Excluded — please don't pursue these

Found by the review and then dropped — three because they are already reported, one because main already fixed it. Listed only so the same ground isn't covered twice:

Finding	Why it is not in the tables
Custom run arguments are corrupted by a Tcl list round-trip (query.py:352,368-376)	already reported: #93016
A bare except guards a four-link attribute chain (autocomplete.py:173-175)	already reported: #59518
<<python-context-help>> is offered in Settings, bound by nothing, and has never had a handler (config.py:614)	already reported: #88740
Configure IDLE → Keys raises KeyError for any third-party extension that ships disabled (config.py:486 on 3.14)	fixed on main by gh-89520 / GH-28713 — GetExtensionKeys now uses current_keyset.get(event, None). Still present on the 3.14 branch; a backport may be wanted.

Disclosure & caveats

• The reports were drafted with AI assistance (Claude Code); each gist carries an explicit disclaimer. Before publishing, every cited line was re-read in the tree at the stated commit, and every "Verified" transcript was produced by running the code.
• That re-verification changed three findings, which is the reason it is worth mentioning: 59 was upgraded from an argument to a reproduction by a differential its author had not run; a fourth claim (about how many test files return 0 on failure) was wrong by an order of magnitude and is corrected in 73; and two line numbers were off.
• 33 is deliberate in part. The commit that introduced the -n exclusion says so in its own message. What is reported is only the part that commit did not consider: under -n the name is not absent, it is IDLE's own entry point.
• Severities are the reviewer's, not a maintainer's. CONSIDER items in particular are judgement calls and several may be intentional; say so and I'll annotate them.

Each finding is also classified against a catalogue of recurring Python bug shapes; the shape name in each gist entry (zip-truncates-on-length-mismatch, per-line-property-derived-from-the-aggregate, …) is what let the review hunt siblings of a confirmed defect rather than stop at the first instance. Several rows here were found that way.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux