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

johnslavik · 2026-07-16T01:03:13Z

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

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

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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?

We usually do not add versionchanged for minor changes like changed error message. Otherwise our documentation would be overwhelmed by them.

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>

serhiy-storchaka · 2026-07-20T12:46:07Z


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


We usually do not add versionchanged for minor changes like changed error message. Otherwise our documentation would be overwhelmed by them.

serhiy-storchaka

LGTM. 👍

vstinner · 2026-07-23T15:43:57Z

+        if (modname && PyUnicode_Check(modname)) {
+            result = PyUnicode_FromFormat("module '%U' has no attribute '%U'",
+                                          modname, name);
+            Py_DECREF(modname);
+        } else {
+            Py_XDECREF(modname);
+            result = PyUnicode_FromFormat("module has no attribute '%U'", name);
+        }
+    } else if (PyType_Check(obj)) {
+        result = PyUnicode_FromFormat("type object '%N' has no attribute '%U'",
+                                      obj, name);
+    } else {
+        result = PyUnicode_FromFormat("'%T' object has no attribute '%U'",
+                                      obj, name);


The %U doesn't escape properly quote characters. Example:

exc=AttributeError() exc.name = "a'b" exc.obj = ["list"] print(str(exc))

Current output:

'list' object has no attribute 'a'b'

I suggesting using %R instead of '%U':

Suggested change

if (modname && PyUnicode_Check(modname)) {

result = PyUnicode_FromFormat("module '%U' has no attribute '%U'",

modname, name);

Py_DECREF(modname);

} else {

Py_XDECREF(modname);

result = PyUnicode_FromFormat("module has no attribute '%U'", name);

}

} else if (PyType_Check(obj)) {

result = PyUnicode_FromFormat("type object '%N' has no attribute '%U'",

obj, name);

} else {

result = PyUnicode_FromFormat("'%T' object has no attribute '%U'",

obj, name);

if (modname && PyUnicode_Check(modname)) {

result = PyUnicode_FromFormat("module '%U' has no attribute %R",

modname, name);

Py_DECREF(modname);

} else {

Py_XDECREF(modname);

result = PyUnicode_FromFormat("module has no attribute %R", name);

}

} else if (PyType_Check(obj)) {

result = PyUnicode_FromFormat("type object '%N' has no attribute %R",

obj, name);

} else {

result = PyUnicode_FromFormat("'%T' object has no attribute %R",

obj, name);

Sure, although we could say the same thing about module names and type names, no?

Using import, I don't think that you can have a module name which contains quotes. You might hack the module name to insert quotes later.

For consistency, I would be fine with using %R to format the module name. It's also shorter 😁

For the type name, quotes or other special characters are very unlikely. I don't think that it's worth it to care about these ones. '%T' looks like the right format.

Thanks! Changes applied.

First of all, we should be consistent with the original error messages raised in corresponding tp_getattro. We can change this, but we should do this globally, and this is a separate issue.

I agree that it's worth it to use %R in similar code such as PyObject_GetAttr() and PyObject_SetAttr(). Currently, it also has the bug:

>>> getattr(object(), "a'b") AttributeError: 'object' object has no attribute 'a'b'

We can change this, but we should do this globally, and this is a separate issue.

But I disagree with that. IMO it's fine to use %R in this PR. Similar code paths can be changed in a follow-up PR.

I created issue #154610 and PR #154614 to change how the attribute name is formatted when formatting AttributeError error message.

vstinner

LGTM. Thanks for addressing my latest comments.

I wanted to merge the change, but @serhiy-storchaka seems to have concerns about %R usage.

serhiy-storchaka · 2026-07-24T11:18:58Z

I think this deserves a separate discussion (What if the name is an instance of the str subclass like StrEnum?), but this should not be blocker for merging this PR.

vstinner · 2026-07-24T12:11:03Z

this should not be blocker for merging this PR.

Do you mean that you're fine with %R? Or that the code should be reverted to '%U'?

serhiy-storchaka · 2026-07-24T12:13:29Z

I am fine with any.

vstinner · 2026-07-24T12:15:13Z

I merged your enhancement, thanks @johnslavik!

johnslavik · 2026-07-24T12:24:49Z

Thanks everyone for reviewing! I've learned a ton

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

Closed

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

pablogsal reviewed Jul 18, 2026

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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

serhiy-storchaka reviewed Jul 20, 2026

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Handle non-unicode __name__, drop unnecessary cast

404359d

serhiy-storchaka approved these changes Jul 20, 2026

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vstinner reviewed Jul 23, 2026

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johnslavik added 2 commits July 23, 2026 23:06

Clearer goto name + extra assert

f28f1d5

Use %R for quoting strings

42522c0

vstinner approved these changes Jul 24, 2026

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vstinner merged commit ae72d0c into python:main Jul 24, 2026
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github-project-automation Bot moved this from In Progress to Done in Sprint Jul 24, 2026

bedevere-app Bot removed the awaiting merge label Jul 24, 2026

vstinner mentioned this pull request Jul 24, 2026

Format attribute name with repr() when formatting AttributeError error message #154610

Open


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

		.. versionchanged:: 3.10

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