gh-146406: Add cross-language method suggestions for builtin AttributeError by mvanhorn · Pull Request #146407 · python/cpython

mvanhorn · 2026-03-25T07:16:45Z

Summary

When an AttributeError on a builtin type (list, str, dict) has no
Levenshtein-based suggestion, check a static table of common method names
from other languages.

Before:

>>> [1, 2, 3].push(4)
AttributeError: 'list' object has no attribute 'push'

After:

>>> [1, 2, 3].push(4)
AttributeError: 'list' object has no attribute 'push'. Did you mean '.append' instead of '.push'?

Discourse discussion

Discussed at https://discuss.python.org/t/106632 (420 views, 25 likes, 15 posts, 3 core devs).

Design decisions from the thread:

Flat table per @pf_moore (post 14, 4 likes):

"the table can simply map the method name and object type onto a suggestion. So add on a list is mapped straight to 'did you mean to use a set?' No need to check whether add is a set method - you know it is already."
list.add() suggests set, not append per @Storchaka (post 8):

"It can also mean that you passed a list instead of set. Changing 'add' to 'append' will not fix the error, it will introduce yet one error."

Reinforced by @tjreedy (post 12):

"When a method exists for another, related, Python class, I think it better to suggest the other class rather than assume a foreign language match."
Scope guardrails per @dr_carlos (post 3): only add entries backed by real confusion evidence, not just because methods are similar.
Static table for builtins only (Option 1) - community consensus. 11 entries covering JavaScript, Java, C#, and Ruby.

Verification

Before (system Python 3.13, no cross-language hints):

>>> [1, 2, 3].push(4)
AttributeError: 'list' object has no attribute 'push'

>>> 'hello'.toUpperCase()
AttributeError: 'str' object has no attribute 'toUpperCase'

>>> {}.keySet()
AttributeError: 'dict' object has no attribute 'keySet'

>>> [].add(1)
AttributeError: 'list' object has no attribute 'add'

After (this PR):

>>> [1, 2, 3].push(4)
AttributeError: 'list' object has no attribute 'push'. Did you mean '.append' instead of '.push'?

>>> 'hello'.toUpperCase()
AttributeError: 'str' object has no attribute 'toUpperCase'. Did you mean '.upper' instead of '.toUpperCase'?

>>> {}.keySet()
AttributeError: 'dict' object has no attribute 'keySet'. Did you mean '.keys' instead of '.keySet'?

>>> [].add(1)
AttributeError: 'list' object has no attribute 'add'. Did you mean to use a set? Sets have an .add() method

Levenshtein still takes priority (trim->strip, indexOf->index already work and are not in the table). Only exact builtin types are matched - subclasses are not affected.

Changes

Lib/traceback.py: Added _CROSS_LANGUAGE_HINTS dict (11 entries), _get_cross_language_hint() function, and a 4-line fallback hook in TracebackException.__init__ that runs only when Levenshtein found nothing.
Lib/test/test_traceback.py: 15 test cases covering all entries, priority ordering, unknown attrs, and subclass exclusion.
Misc/NEWS.d/: NEWS entry.

Evidence

Source	Evidence
SO: push vs append	320K views
SO: trim whitespace	1.5M views
JetBrains/PSF Survey 2024	40% of Python devs also use JS
CPython precedent	`elseif`->`elif` (gh-132449), `import x from y`->`from x import y` (gh-98931)
Discourse thread	420 views, 25 likes, 3 core devs supportive

Fixes #146406

Issue: Add cross-language method suggestions for builtin AttributeError #146406

When Levenshtein-based suggestions find no match for an AttributeError on list, str, or dict, check a static table of common method names from JavaScript, Java, C#, and Ruby. For example, [].push() now suggests .append(), "".toUpperCase() suggests .upper(), and {}.keySet() suggests .keys(). The list.add() case suggests using a set instead of suggesting .append(), since .add() is a set method and the user may have passed a list where a set was expected (per discussion with Serhiy Storchaka, Terry Reedy, and Paul Moore). Design: flat (type, attr) -> suggestion text table, no runtime introspection. Only exact builtin types are matched to avoid false positives on subclasses. Discussion: https://discuss.python.org/t/106632

mvanhorn · 2026-03-25T07:18:49Z

Verification screenshots

Built CPython from source and ran the REPL to verify before/after behavior.

Before (Python 3.13, no cross-language hints)

After (this PR)

All 447 tests in test_traceback pass (including 28 new cross-language tests).

picnixz · 2026-03-25T09:06:10Z

I will make a review of this PR when I have time (by the end of the week), so fellow core devs, please hold off any merge, TiA!

nedbat · 2026-03-25T09:25:01Z

Thanks for taking this on! From a language perspective, I think these can be shorter. Instead of:

AttributeError: 'list' object has no attribute 'push'. Did you mean '.append' instead of '.push'?

I think it is enough to say:

AttributeError: 'list' object has no attribute 'push'. Did you mean '.append' instead?

or even:

AttributeError: 'list' object has no attribute 'push'. Did you mean '.append'?

ZeroIntensity

Looks very cool!

It'd also be nice if we could suggest using language constructs in some cases. For example:

dict.put -> dict[x] = y
list.contains -> x in list

ZeroIntensity · 2026-03-25T12:19:04Z

Lib/traceback.py

+#
+# See https://discuss.python.org/t/106632 for the design discussion.


Let's link to the GH issue instead.

Done, switched to the GH issue link.

ZeroIntensity · 2026-03-25T12:22:06Z

Lib/traceback.py

+    (list, "push"):     "append",
+    (list, "concat"):   "extend",


I'd prefer to keep standard convention and just use a single space after the :. Alignment adds extra maintenance (because we need to change each entry if we add something that breaks it) and also creates a false-symmetry between each entry (e.g., for our purposes (list, "push"): "append" has no functional relation to (list, "concat"): "extend").

Makes sense, dropped the alignment.

ZeroIntensity · 2026-03-25T12:23:49Z

Misc/NEWS.d/next/Library/2026-03-25-07-17-41.gh-issue-146406.ydsmqe.rst

+``"".toUpperCase()`` suggests ``upper``. The ``list.add()`` case suggests
+using a set instead, following feedback from the community discussion.


This last sentence isn't particularly useful:

Suggested change

``"".toUpperCase()`` suggests ``upper``. The ``list.add()`` case suggests

using a set instead, following feedback from the community discussion.

``"".toUpperCase()`` suggests ``upper``.

Applied your suggestion.

ZeroIntensity · 2026-03-25T12:28:16Z

Lib/traceback.py

                else:
                    self._str += f". Did you mean '.{suggestion}' ({suggestion!a}) instead of '.{wrong_name}' ({wrong_name!a})?"
+            elif hasattr(exc_value, 'obj'):
+                with suppress(Exception):


Hm, what needs to be suppressed here?

Nothing realistic - _get_cross_language_hint is just a dict lookup on (type(obj), wrong_name), both of which are already validated by the time we get here. I had it as a defensive measure since we're inside traceback formatting, but on reflection it's unnecessary given how simple the function is. Removed it.

@ZeroIntensity

- Shorten hint format to "Did you mean '.append'?" (drop redundant "instead of '.push'" since the error already names the attribute) - Add dict.put and list.contains entries suggesting language constructs (dict[key] = value, x in list) per @ZeroIntensity's review - Replace suppress(Exception) with direct call (function is safe) - Link to GH issue instead of Discourse thread in comment - Drop column alignment in hint table entries - Trim NEWS entry last sentence

mvanhorn · 2026-03-25T12:50:49Z

Good call - the error message already says "has no attribute 'push'" so repeating it in the hint is noise. Shortened to Did you mean '.append'? in 6d58cdc.

I kept the existing Levenshtein format as-is (that's a separate code path and probably a separate discussion), so the cross-language hints now have their own slighty shorter style.

mvanhorn · 2026-03-25T12:51:13Z

@ZeroIntensity re: suggesting language constructs - added both in 6d58cdc. They use the custom hint format since they suggest constructs rather than method equivalents:

>>> {}.put("a", 1)
AttributeError: 'dict' object has no attribute 'put'. Use dict[key] = value for item assignment

>>> [].contains(1)
AttributeError: 'list' object has no attribute 'contains'. Use 'x in list' to check membership

The existing architecture already handles this - entries with a space in the hint string are rendered as-is rather than wrapped in "Did you mean" format.

mvanhorn · 2026-03-25T12:52:32Z

I will make a review of this PR when I have time (by the end of the week), so fellow core devs, please hold off any merge, TiA!

appreciat you digging in when you're ready! ::anxiously waits::

ZeroIntensity · 2026-03-25T12:53:28Z

Oh, you also need to add an entry to "What's New in Python 3.15".

mvanhorn · 2026-03-25T13:12:24Z

Added in 579d037 - covers the basic hint format and the language-construct variant (dict.put, list.contains).

Lib/traceback.py

Doc/whatsnew/3.15.rst

- Use (hint, is_raw) tuples instead of space-based raw detection - Shorten list.add hint to "Did you mean to use a 'set' object?" - Use d[k] = v instead of dict[key] = value for dict.put hint - Add dict.entries -> items (JavaScript) - Remove Levenshtein guardrail from code comment (belongs on issue) - Add periods to raw hint messages - Add test for dict.entries

Lib/traceback.py

Lib/test/test_traceback.py

Lib/traceback.py

@vstinner

Address review feedback from @vstinner: - Merge 14 individual test_cross_language_* methods into a single parameterized test_cross_language using subTest - Shorten raw-message hints: "Use 'x in list'." and "Use d[k] = v." - Fix pre-existing levenshtein priority test assertion - Update whatsnew entry to match shortened hint text

Lib/test/test_traceback.py

…suffix matching Apply vstinner's review suggestion: use assertEndsWith instead of assertIn for more precise test assertions. Split cases into method hints (checked via Did you mean pattern) and raw hints (checked via exact suffix).

vstinner

LGTM. The implementation is correct and well tested. IMO the feature makes sense and is useful.

vstinner · 2026-03-25T23:28:33Z

Lib/traceback.py

+# If is_raw is True, the suggestion is rendered as-is.
+#
+# See https://github.com/python/cpython/issues/146406.
+_CROSS_LANGUAGE_HINTS = {


You might use a frozendict for such global constant dictionary.

Applied in f702e79 - wrapped in types.MappingProxyType.

You don't need types.MappingProxyType. We have a native frozendict type in 3.15 as of PEP 814.

vstinner · 2026-03-26T01:07:29Z

Lib/traceback.py

+# If is_raw is True, the suggestion is rendered as-is.
+#
+# See https://github.com/python/cpython/issues/146406.
+_CROSS_LANGUAGE_HINTS = types.MappingProxyType({


I was thinking at the new built-in frozendict type:

Suggested change

_CROSS_LANGUAGE_HINTS = types.MappingProxyType({

_CROSS_LANGUAGE_HINTS = frozendict({

bedevere-app bot mentioned this pull request Mar 25, 2026

Add cross-language method suggestions for builtin AttributeError #146406

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fix NEWS entry filename for bedevere bot

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picnixz self-requested a review March 25, 2026 09:05

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mvanhorn requested a review from AA-Turner as a code owner March 25, 2026 13:10

Add What's New entry for cross-language AttributeError hints

8a39e32

mvanhorn force-pushed the osc/cross-language-attr-hints branch from 57b8fc8 to 8a39e32 Compare March 25, 2026 13:17

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Keep Levenshtein criterion, remove survey restriction per picnixz

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Use MappingProxyType for _CROSS_LANGUAGE_HINTS per vstinner review

f702e79

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		#
		# See https://discuss.python.org/t/106632 for the design discussion.

		``"".toUpperCase()`` suggests ``upper``. The ``list.add()`` case suggests
		using a set instead, following feedback from the community discussion.

	_CROSS_LANGUAGE_HINTS = types.MappingProxyType({
	_CROSS_LANGUAGE_HINTS = frozendict({

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Summary

Discourse discussion

Verification

Changes

Evidence

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mvanhorn commented Mar 25, 2026

Verification screenshots

Before (Python 3.13, no cross-language hints)

After (this PR)

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mvanhorn commented Mar 25, 2026

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mvanhorn commented Mar 25, 2026 •

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