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).

picnixz · 2026-03-25T13:08:18Z

Lib/traceback.py

+# If the suggestion is a Python method name, the standard "Did you mean"
+# format is used. If it contains a space, it's rendered as a full hint.
+#
+# See https://github.com/python/cpython/issues/146406 for the design discussion.


Suggested change

# See https://github.com/python/cpython/issues/146406 for the design discussion.

# See https://github.com/python/cpython/issues/146406.

picnixz · 2026-03-25T13:09:19Z

Lib/traceback.py

+#   1. Must have evidence of real cross-language confusion (Stack Overflow
+#      traffic, bug reports in production repos, developer survey data)
+#   2. Must not be catchable by Levenshtein distance (too different from
+#      the correct Python method name)
+#   3. Must be from a top-4 language by Python co-usage: JavaScript, Java,
+#      C#, or Ruby (JetBrains/PSF Developer Survey 2024)


Suggested change

# 1. Must have evidence of real cross-language confusion (Stack Overflow

# traffic, bug reports in production repos, developer survey data)

# 2. Must not be catchable by Levenshtein distance (too different from

# the correct Python method name)

# 3. Must be from a top-4 language by Python co-usage: JavaScript, Java,

# C#, or Ruby (JetBrains/PSF Developer Survey 2024)

# 1. Must have evidence of real cross-language confusion (Stack Overflow

# traffic, bug reports in production repos, developer survey data).

# 2. Must not be catchable by Levenshtein distance (too different from

# the correct Python method name).

The last point should be mentioned on the issue but not in the code if we want to update the list.

Removed - moved that note to the issue instead.

picnixz · 2026-03-25T13:13:45Z

Lib/traceback.py

+    # list -- wrong-type suggestion (per Serhiy Storchaka, Terry Reedy,
+    # Paul Moore: list.add() more likely means the user expected a set)


Suggested change

# list -- wrong-type suggestion (per Serhiy Storchaka, Terry Reedy,

# Paul Moore: list.add() more likely means the user expected a set)

# list -- wrong-type suggestion more likely means the user expected a set

Applied your suggestion.

picnixz · 2026-03-25T13:14:53Z

Lib/traceback.py

+    (list, "contains"): "Use 'x in list' to check membership",
+    # list -- wrong-type suggestion (per Serhiy Storchaka, Terry Reedy,
+    # Paul Moore: list.add() more likely means the user expected a set)
+    (list, "add"): "Did you mean to use a set? Sets have an .add() method",


I don't really find the suggestion for list.add really helpful and I find the "Sets have ..." part redundant. I'm not saying we should not have it but I would have maybe said:

"Did you mean to use a 'set' object?"

While it could appear redundant, the full message would be "'list' objects have no 'add' attribute. Did you mean to use a 'set' object?"

Shortened to "Did you mean to use a 'set' object?"

picnixz · 2026-03-25T13:19:01Z

Lib/traceback.py

+    (dict, "keySet"): "keys",
+    (dict, "entrySet"): "items",
+    (dict, "putAll"): "update",
+    (dict, "put"): "Use dict[key] = value for item assignment",


Having the full message

'dict' object [...]. Use dict[key] = value ...

would be confusing IMO. So I suggest that we rename dict to d[k] = v instead because the docs for dict use those symbols.

Good catch, switched to d[k] = v.

picnixz · 2026-03-25T13:20:23Z

Lib/traceback.py

+    (str, "trimStart"): "lstrip",
+    (str, "trimEnd"): "rstrip",
+    # dict -- Java equivalents
+    (dict, "keySet"): "keys",


Let's also add dict.entries for JavaScript. IDK if it's already caught though.

Added. Levenshtein doesn't catch it (too far from 'items').

picnixz · 2026-03-25T13:24:38Z

Lib/traceback.py

+    if ' ' in hint:
+        # Full custom hint (e.g., wrong-type suggestion for list.add)
+        return hint


Instead of this fragile check, I suggest we register tuples instead: (hint_message, use_raw_message).

If we add an API for updating the known suggestions, then users can decide how to handle their spaces. Note that it's technically possible to support spaces in attribute names, though very unlikely to happen.

Switched to (hint, is_raw) tuples. Much cleaner than the space check.

picnixz · 2026-03-25T13:25:51Z

Doc/whatsnew/3.15.rst

+     >>> {}.put("a", 1)  # doctest: +ELLIPSIS
+     Traceback (most recent call last):
+     ...
+     AttributeError: 'dict' object has no attribute 'put'. Use dict[key] = value for item assignment


Grand question. Should we end the message with a period or not?

I think a period would be nice.

Added periods to the raw hints.

- 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

picnixz · 2026-03-25T14:28:36Z

Lib/traceback.py

+#   2. Must be from a top-4 language by Python co-usage: JavaScript, Java,
+#      C#, or Ruby (JetBrains/PSF Developer Survey 2024).


You erased the wrong point,. I wanted to keep the Levenshtein mention but ignore the survey one (maybe my suggestion was wrong?)

sorry let me dig in!

You're right, I mixed up which point to drop. Fixed in 99e106a - kept the Levenshtein criterion, removed the survey restriction.

vstinner · 2026-03-25T20:12:54Z

Lib/test/test_traceback.py

        actual = self.get_suggestion(Outer(), 'target')
        self.assertIn("'.normal.target'", actual)

+    def test_cross_language_list_push_suggests_append(self):


Hum, it's kind of verbose to have one test per method. What about having a single test_cross_language() which runs all these tests?

Consolidated into a single test_cross_language with subTest in 57a4d39. Also fixed a pre-existing assertion in the levenshtein priority test.

vstinner · 2026-03-25T20:21:26Z

Lib/traceback.py

+    (list, "concat"): ("extend", False),
+    # list -- Java/C# equivalents
+    (list, "addAll"): ("extend", False),
+    (list, "contains"): ("Use 'x in list' to check membership.", True),


Hum, maybe make it shorter:

Suggested change

(list, "contains"): ("Use 'x in list' to check membership.", True),

(list, "contains"): ("Use 'x in list'.", True),

Shortened in 57a4d39.

vstinner · 2026-03-25T20:21:45Z

Lib/traceback.py

+    (dict, "entrySet"): ("items", False),
+    (dict, "entries"): ("items", False),
+    (dict, "putAll"): ("update", False),
+    (dict, "put"): ("Use d[k] = v for item assignment.", True),


Same suggestion here:

Suggested change

(dict, "put"): ("Use d[k] = v for item assignment.", True),

(dict, "put"): ("Use d[k] = v.", True),

Done in 57a4d39. Also updated the whatsnew entry.

@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

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

Add cross-language method suggestions for builtin AttributeError #146406

Open

bedevere-app bot added the awaiting review label Mar 25, 2026

fix NEWS entry filename for bedevere bot

aecaacb

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

ZeroIntensity reviewed Mar 25, 2026

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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

picnixz reviewed Mar 25, 2026

View reviewed changes

Keep Levenshtein criterion, remove survey restriction per picnixz

99e106a

vstinner reviewed Mar 25, 2026

View reviewed changes

		#
		# 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.

	# See https://github.com/python/cpython/issues/146406 for the design discussion.
	# See https://github.com/python/cpython/issues/146406.

		# list -- wrong-type suggestion (per Serhiy Storchaka, Terry Reedy,
		# Paul Moore: list.add() more likely means the user expected a set)

	# list -- wrong-type suggestion (per Serhiy Storchaka, Terry Reedy,
	# Paul Moore: list.add() more likely means the user expected a set)
	# list -- wrong-type suggestion more likely means the user expected a set

		# 2. Must be from a top-4 language by Python co-usage: JavaScript, Java,
		# C#, or Ruby (JetBrains/PSF Developer Survey 2024).

	(list, "contains"): ("Use 'x in list' to check membership.", True),
	(list, "contains"): ("Use 'x in list'.", True),

	(dict, "put"): ("Use d[k] = v for item assignment.", True),
	(dict, "put"): ("Use d[k] = v.", True),

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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 •

edited by bedevere-app bot

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picnixz commented Mar 25, 2026 •

edited

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