unittest.mock: Regression with create_autospec and dataclasses in 3.14.0b2

@sobolevn

Bug report

Bug description:

from dataclasses import dataclass
from unittest.mock import create_autospec

@dataclass
class Description:
    name: str

mock = create_autospec(Description, instance=True)
print(isinstance(mock, Description))

# 3.13.3
True

# 3.14.0b2
False

Furthermore the resulting mock doesn't have attributes present on a dataclass instance, like __annotations__, __class__, __dataclass_fields__, __dataclass_params__.

print(dir(mock))
print(dir(Description("Hello World"))

Likely related to #124429. /CC @sobolevn

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs

@ncoghlan

cc @ncoghlan and @ericvsmith

@cjw296

As I said on #124429, I'm not a mock user. That said, the regression does seem problematic. @cjw296 is in CODEOWNERS for mock.

We might need to revert the change, it seems to be rather hard and the fix is not obvious, at least for me now :(

@sobolevn

@sobolevn - #124429 was your PR so I hope you can dig into this...

I'd probably tackle this as follows:

create new branch from the commit before gh-124176: Add special support for dataclasses to create_autospec #124429 landed, and add tests for isinstance(mock, Description) and the attributes the OP mentions, verifying the past so we basically confirm the PR is the problem
rebase the original PR branch (I think GH hangs onto its HEAD commit on the PR even if it was deleted)
See what happens with the new tests

If they fail, #124429 should probably be reverted for now, andthe old PR plus new tests put on a branch somewhere that a suitably motivated contributor could pick up.

If a fix can be created, then a new PR should go up containing the additional tests we missed first time round along with the commit for the fix.

Hopefully this all makes sense?

Yes, I will surely work on this. If I can find the solution - I will create a PR with the fix. Otherwise, I propose to revert #124429

Start with the missing tests first plz, they'll be a valuable artefact whatever the outcome :-)

pythongh-135368: Fix mocks on dataclass specs with `instance=True`

gh-135368: Fix mocks on dataclass specs with `instance=True` (#135421)

pythongh-135368: Fix mocks on dataclass specs with `instance=True` (p…

[3.14] gh-135368: Fix mocks on dataclass specs with `instance=True` (G…

So, this issue could be closed, right?

Thanks everyone! Looking forward to the next beta release.

pythongh-135368: Fix mocks on dataclass specs with `instance=True` (p…

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unittest.mock: Regression with create_autospec and dataclasses in 3.14.0b2 #135368

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unittest.mock: Regression with create_autospec and dataclasses in 3.14.0b2 #135368

Description

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

Activity

sobolevn commented on Jun 11, 2025

ericvsmith commented on Jun 11, 2025

sobolevn commented on Jun 11, 2025

cjw296 commented on Jun 11, 2025

sobolevn commented on Jun 11, 2025

cjw296 commented on Jun 11, 2025

efimov-mikhail commented on Jun 14, 2025

cdce8p commented on Jun 14, 2025

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