gh-153785: Generate `AttributeError` messages from context by johnslavik · Pull Request #153786 · python/cpython

johnslavik · 2026-07-16T01:03:13Z

Motivation

Code very often raises AttributeError(name) or, more rarely, a bare AttributeError from __getattr__ (there are ~40 such sites in CPython itself). In both cases the exception's obj and name attributes are populated automatically by _PyObject_SetAttributeErrorContext, but the message rendered in the traceback ignores them:

raise AttributeError(name) shows just the attribute name, losing which object it was accessed on.
bare raise AttributeError shows nothing at all.

So today the message throws away context that the interpreter already has on hand (it uses obj/name for the "Did you mean …?" suggestions, but not for the message itself).

This also produces the garbled output reported in #143811, where a bare AttributeError renders as an empty message glued to a suggestion:

AttributeError: . Did you mean: 'foobar'?

Change

This gives AttributeError a custom __str__ (AttributeError_str) that synthesizes the familiar message from the collected obj/name context:

AttributeError: '<qualname>' object has no attribute 'missing'
AttributeError: module '<name>' has no attribute 'missing'

It is the same trick KeyError already uses to wrap its key in repr() — see KeyError_str. Doing it in AttributeError.__str__ (rather than in traceback) means every consumer of the message benefits, not just tracebacks.

To avoid clobbering messages the caller actually meant, the generated message is only used when obj and name are both set and the exception was constructed either with no positional arguments or with a single positional argument equal to name. An explanatory string passed by the caller (e.g. AttributeError("custom")) is preserved unchanged.

As a side effect this also fixes #143811, which motivated the current issue — the functools.singledispatchmethod case now reports a proper message instead of the garbled one.

Issue: Generate AttributeError messages from their context #153785

read-the-docs-community · 2026-07-16T01:07:23Z

Documentation build overview

📚 cpython-previews | 🛠️ Build #33657490 | 📁 Comparing cb5ae96 against main (b4662e8)

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37 files changed · ± 37 modified

± Modified

I'm not sure how to create a nameless module at runtime

This reverts commit 7378e30.

pablogsal · 2026-07-18T15:25:28Z


+   When possible, :attr:`name` and :attr:`obj` are set automatically.
+
   .. versionchanged:: 3.10


We dropped the .. versionchanged:: next note that documented the generated message. This is a user-visible change to str(), so I think we should keep a versionchanged here, no?

@serhiy-storchaka, do you agree on bringing it back?

vstinner

The following code writes 'lazy_import' object has no attribute 'missing1': it mentions lazy_import rather than module 'asyncio'. But I don't think that AttributeError_str() would be the right place to reify the asyncio module!

lazy import asyncio

class RaiseWithName:
    def __getattr__(self, name):
        raise AttributeError(name)

try:
    getattr(globals()['asyncio'], "missing1")
except AttributeError as exc:
#    assert exc.obj is ...
    assert exc.name == "missing1"
    print(str(exc))

jaraco · 2026-07-18T16:17:06Z

I think this is the best place to do this. We do the same trick in KeyError.

This also fixed issue #143811 that motivated the current one.
* Issue: [Generate `AttributeError` messages from their context #153785](https://github.com/python/cpython/issues/153785)

"this" and "current one" is unclear here. Please update the PR description to describe the motivation. Also consider linking to the code for the trick in KeyError.

jaraco · 2026-07-18T16:21:36Z

I think this is the best place to do this. We do the same trick in KeyError.
This also fixed issue #143811 that motivated the current one.
* Issue: [Generate `AttributeError` messages from their context #153785](https://github.com/python/cpython/issues/153785)
"this" and "current one" is unclear here. Please update the PR description to describe the motivation. Also consider linking to the code for the trick in KeyError.

I used Claude to infer the motivations and update the description.

jaraco · 2026-07-18T16:27:48Z

It is the same trick KeyError already uses to wrap its key in repr() — see KeyError_str.

This trick has been applied to KeyError and AttributeError; should it be applied to other exceptions? Should it be implemented in such a way that the concern is shared across exceptions (explicitly or implicitly)?

jaraco · 2026-07-18T16:31:05Z

This trick has been applied to KeyError and AttributeError; should it be applied to other exceptions? Should it be implemented in such a way that the concern is shared across exceptions (explicitly or implicitly)?

Short answer, not really.

Details

(Analysis by Claude, on my behalf.)

A worthwhile question. Worth noting first that the two "tricks" only look alike at the surface (both override tp_str); underneath they differ:

KeyError_str reformats what's already in args — wrapping args[0] in repr() so {}[''] prints KeyError: ''. Pure presentation.
AttributeError_str synthesizes a message from side-channel context (obj/name) that the raiser never put in args, reconstructing what the interpreter attached out-of-band.

Only the second is the interesting reusable pattern ("I have structured context attributes; render them lazily instead of forcing every raise site to build the string"). The natural cohort there is NameError/UnboundLocalError (name), ImportError/ModuleNotFoundError (name, path). But both of those already build their full messages eagerly at the C raise site, so the problem AttributeError has — user __getattr__ implementations conventionally raising bare or name-only — barely exists for them.

So I'd lean against building a shared framework here:

Two thin, differing data points; abstracting them would abstract over a coincidence of mechanism, not a shared concern.
The templates ("object has no attribute" vs. "name is not defined" vs. "cannot import name from") are irreducibly per-type — only the dispatch around them could be shared.
Every __str__ change is a compatibility surface (people do match on message text), and each type wants its own guard about when overriding is safe.

The one genuinely reusable piece is the guard predicate — "was this constructed with a default/absent message, or one that merely duplicates a structured field, so I'm safe to override?" (here: args of size 0, or size 1 equal to name). If anything gets factored out, I'd make it that small internal helper and leave each template local. Otherwise I'd revisit sharing only when a third genuine case actually lands.

johnslavik · 2026-07-19T20:29:29Z

The following code writes 'lazy_import' object has no attribute 'missing1': it mentions lazy_import rather than module 'asyncio'. But I don't think that AttributeError_str() would be the right place to reify the asyncio module!
lazy import asyncio

class RaiseWithName:
    def __getattr__(self, name):
        raise AttributeError(name)

try:
    getattr(globals()['asyncio'], "missing1")
except AttributeError as exc:
#    assert exc.obj is ...
    assert exc.name == "missing1"
    print(str(exc))

@vstinner, this code does not trigger the code path from this PR (RaiseWithName is unused). We could specialize this separately -- see #154196.

Co-authored-by: Victor Stinner <vstinner@python.org>

johnslavik added 7 commits July 16, 2026 02:15

Implement AttributeError_str()

ac555bc

Add tests

4f22a16

Check name too

b30d676

Add docs

28e04d3

Add news entry

e5c4922

Improve formatting

3854563

Simplify?

1e88331

johnslavik requested a review from serhiy-storchaka July 16, 2026 01:03

johnslavik requested a review from iritkatriel as a code owner July 16, 2026 01:03

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bedevere-app Bot added the awaiting review label Jul 16, 2026

johnslavik changed the title ~~Generate AttributeError messages from context~~ gh-153785: Generate AttributeError messages from context Jul 16, 2026

bedevere-app Bot mentioned this pull request Jul 16, 2026

Generate AttributeError messages from their context #153785

Open

Cooler formatting of the news entry

2fd216a

johnslavik added 4 commits July 16, 2026 03:13

Test custom messages aren't overwritten

356f56e

Fix news problem

8db0394

Shorter test name

ebcc795

Fix inconsistent style

b6fda10

johnslavik requested a review from DinoV July 16, 2026 01:23

johnslavik marked this pull request as draft July 16, 2026 02:11

bedevere-app Bot removed the awaiting review label Jul 16, 2026

Make thread-safe, presumably

d624835

johnslavik marked this pull request as ready for review July 16, 2026 02:21

bedevere-app Bot added the awaiting review label Jul 16, 2026

johnslavik added 5 commits July 16, 2026 04:23

Rarest case goes last

c50a356

} else {

7ad5204

Remove unnecessary whitespace

b65a7fd

Fix style

e99d875

Avoid nesting critical sections!

8c29de3

johnslavik added 5 commits July 17, 2026 11:58

Use typename or module name

da8c723

Rearrange tests

c941657

I'm not sure how to create a nameless module at runtime

Cover nameless module branch

9ea6c8a

Clearer variable names

f825f27

Localize tests visually

7b73fb8

johnslavik requested a review from pablogsal July 17, 2026 10:15

serhiy-storchaka reviewed Jul 17, 2026

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Comment thread Objects/exceptions.c Outdated

johnslavik added 2 commits July 17, 2026 17:42

Use %T

7378e30

Revert "Use %T"

faf6e53

This reverts commit 7378e30.

johnslavik added the sprint label Jul 18, 2026

github-project-automation Bot added this to Sprint Jul 18, 2026

github-project-automation Bot moved this to Todo in Sprint Jul 18, 2026

github-project-automation Bot moved this from Todo to In Progress in Sprint Jul 18, 2026

Use '%T'

a86fde5

vstinner reviewed Jul 18, 2026

View reviewed changes

Comment thread Lib/test/test_exceptions.py Outdated

Test for fully qualified names

9b00321

pablogsal reviewed Jul 18, 2026

View reviewed changes

vstinner reviewed Jul 18, 2026

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johnslavik added 3 commits July 19, 2026 17:25

Add missing header

df7682c

Add state assertions

a419eb2

Special-case class objects

a7e7f0a

johnslavik mentioned this pull request Jul 19, 2026

Specialize AttributeError exceptions from lazy import objects #154196

Open

johnslavik and others added 3 commits July 19, 2026 22:43

Look into module dict for its name

08f8818

Rephrase the news entry per Victor suggestion

b9b8d7d

Co-authored-by: Victor Stinner <vstinner@python.org>

Add an explanatory comment and make function prelude nicer

cb5ae96


		When possible, :attr:`name` and :attr:`obj` are set automatically.

		.. versionchanged:: 3.10

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johnslavik commented Jul 16, 2026 •

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