Pydantic

The Fix

pip install pydantic==1.10.15

Based on closed pydantic/pydantic issue #8995 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Jump to Verify Open PR/Commit

@@ -1344,20 +1344,20 @@ class Model(BaseModel):
         @classmethod
         def check_a(cls, v: Any):
-            assert v == 'a', 'invalid a'
+            if v != 'a':
+                raise AssertionError('invalid a')

repro.py

$ python -m pytest -Wdefault -vv
============================= test session starts ==============================
platform linux -- Python 3.11.8, pytest-8.1.1, pluggy-1.4.0 -- /tmp/pydantic/.venv/bin/python
cachedir: .pytest_cache
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase(PosixPath('/tmp/pydantic/.hypothesis/examples'))
benchmark: 4.0.0 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=True warmup_iterations=100000)
rootdir: /tmp/pydantic
configfile: pyproject.toml
testpaths: tests
plugins: hypothesis-6.99.5, xdist-3.5.0, benchmark-4.0.0, examples-0.0.10, mock-3.12.0, Faker-24.1.0
collecting ... collected 5002 items

[…]
============================================================== FAILURES ===============================================================
_________________________________________________ test_assert_raises_validation_error _________________________________________________

    def test_assert_raises_validation_error():
        class Model(BaseModel):
            a: str
    
            @field_validator('a')
            @classmethod
            def check_a(cls, v: Any):
                assert v == 'a', 'invalid a'
                return v
    
        Model(a='a')
    
        with pytest.raises(ValidationError) as exc_info:
            Model(a='snap')
        injected_by_pytest = "assert 'snap' == 'a'\n  - a\n  + snap"
      assert exc_info.value.errors(include_url=False) == [
            {
                'ctx': {'error': HasRepr(repr(AssertionError("invalid a\nassert 'snap' == 'a'\n  - a\n  + snap")))},
                'input': 'snap',
                'loc': ('a',),
                'msg': f'Assertion failed, invalid a\n{injected_by_pytest}',
                'type': 'assertion_error',
            }
        ]
E       assert [{'type': 'assertion_error', 'loc': ('a',), 'msg': "Assertion failed, invalid a\nassert 'snap' == 'a'\n  \n  - a\n  + snap", 'input': 'snap', 'ctx': {'error': AssertionError("invalid a\nassert 'snap' == 'a'\n  \n  - a\n  + snap")}}] == [{'ctx': {'error': HasRepr('AssertionError("invalid a\\nassert \'snap\' == \'a\'\\n  - a\\n  + snap")')}, 'input': 'snap', 'loc': ('a',), 'msg': "Assertion failed, invalid a\nassert 'snap' == 'a'\n  - a\n  + snap", 'type': 'assertion_error'}]
E         
E         At index 0 diff: {'type': 'assertion_error', 'loc': ('a',), 'msg': "Assertion failed, invalid a\nassert 'snap' == 'a'\n  \n  - a\n  + snap", 'input': 'snap', 'ctx': {'error': AssertionError("invalid a\nassert 'snap' == 'a'\n  \n  - a\n  + snap")}} != {'ctx': {'error': HasRepr('AssertionError("invalid a\\nassert \'snap\' == \'a\'\\n  - a\\n  + snap")')}, 'input': 'snap', 'loc': ('a',), 'msg': "Assertion failed, invalid a\nassert 'snap' == 'a'\n  - a\n  + snap", 'type': 'assertion_error'}
E         
E         Full diff:
E           [
E               {
E                   'ctx': {
E         -             'error': HasRepr('AssertionError("invalid a\\nassert \'snap\' == \'a\'\\n  - a\\n  + snap")'),
E         ?                      ---------                         -         -     -     -  -          -           --
E         +             'error': AssertionError("invalid a\nassert 'snap' == 'a'\n  \n  - a\n  + snap"),
E         ?                                                                      +++
E                   },
E                   'input': 'snap',
E                   'loc': (
E                       'a',
E                   ),
E                   'msg': 'Assertion failed, invalid a\n'
E
... (truncated) ...

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install pydantic==1.10.15\nWhen NOT to use: Do not apply this fix if using a pytest version lower than 8.\n\n

Why This Fix Works in Production

Trigger: `tests/test_validators.py::test_assert_raises_validation_error` fails with pytest 8
Mechanism: The test fails due to different assertion output in pytest 8
Why the fix works: Fixes the test `test_assert_raises_validation_error` to support different assertion output from pytest 8. (first fixed release: 1.10.15).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

Shows up under Python 3.11.8 in real deployments (not just unit tests).
The test fails due to different assertion output in pytest 8
Production symptom (often without a traceback): `tests/test_validators.py::test_assert_raises_validation_error` fails with pytest 8

Proof / Evidence

GitHub issue: #8995
Fix PR: https://github.com/pydantic/pydantic/pull/9024
First fixed release: 1.10.15
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.38

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“@mgorny, Thanks for reporting this. Indeed, we'll probably have to fix up a few tests in order to move to pytest v8. I'll mark this…”

@sydney-runkle · 2024-03-15 · source

“Unless I'm mistaken, this is the only remaining issue”

@mgorny · 2024-03-15 · source

“@mgorny, I think it's just different output. Let's make it conditional to pytest version :). Thanks!!”

@sydney-runkle · 2024-03-15 · source

Failure Signature (Search String)

`tests/test_validators.py::test_assert_raises_validation_error` fails with pytest 8
The following test failures with pytest 8 remained:

Copy-friendly signature

signature.txt

Failure Signature
-----------------
`tests/test_validators.py::test_assert_raises_validation_error` fails with pytest 8
The following test failures with pytest 8 remained:

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
`tests/test_validators.py::test_assert_raises_validation_error` fails with pytest 8
The following test failures with pytest 8 remained:

Minimal Reproduction

repro.py

$ python -m pytest -Wdefault -vv
============================= test session starts ==============================
platform linux -- Python 3.11.8, pytest-8.1.1, pluggy-1.4.0 -- /tmp/pydantic/.venv/bin/python
cachedir: .pytest_cache
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase(PosixPath('/tmp/pydantic/.hypothesis/examples'))
benchmark: 4.0.0 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=True warmup_iterations=100000)
rootdir: /tmp/pydantic
configfile: pyproject.toml
testpaths: tests
plugins: hypothesis-6.99.5, xdist-3.5.0, benchmark-4.0.0, examples-0.0.10, mock-3.12.0, Faker-24.1.0
collecting ... collected 5002 items

[…]
============================================================== FAILURES ===============================================================
_________________________________________________ test_assert_raises_validation_error _________________________________________________

    def test_assert_raises_validation_error():
        class Model(BaseModel):
            a: str
    
            @field_validator('a')
            @classmethod
            def check_a(cls, v: Any):
                assert v == 'a', 'invalid a'
                return v
    
        Model(a='a')
    
        with pytest.raises(ValidationError) as exc_info:
            Model(a='snap')
        injected_by_pytest = "assert 'snap' == 'a'\n  - a\n  + snap"
      assert exc_info.value.errors(include_url=False) == [
            {
                'ctx': {'error': HasRepr(repr(AssertionError("invalid a\nassert 'snap' == 'a'\n  - a\n  + snap")))},
                'input': 'snap',
                'loc': ('a',),
                'msg': f'Assertion failed, invalid a\n{injected_by_pytest}',
                'type': 'assertion_error',
            }
        ]
E       assert [{'type': 'assertion_error', 'loc': ('a',), 'msg': "Assertion failed, invalid a\nassert 'snap' == 'a'\n  \n  - a\n  + snap", 'input': 'snap', 'ctx': {'error': AssertionError("invalid a\nassert 'snap' == 'a'\n  \n  - a\n  + snap")}}] == [{'ctx': {'error': HasRepr('AssertionError("invalid a\\nassert \'snap\' == \'a\'\\n  - a\\n  + snap")')}, 'input': 'snap', 'loc': ('a',), 'msg': "Assertion failed, invalid a\nassert 'snap' == 'a'\n  - a\n  + snap", 'type': 'assertion_error'}]
E         
E         At index 0 diff: {'type': 'assertion_error', 'loc': ('a',), 'msg': "Assertion failed, invalid a\nassert 'snap' == 'a'\n  \n  - a\n  + snap", 'input': 'snap', 'ctx': {'error': AssertionError("invalid a\nassert 'snap' == 'a'\n  \n  - a\n  + snap")}} != {'ctx': {'error': HasRepr('AssertionError("invalid a\\nassert \'snap\' == \'a\'\\n  - a\\n  + snap")')}, 'input': 'snap', 'loc': ('a',), 'msg': "Assertion failed, invalid a\nassert 'snap' == 'a'\n  - a\n  + snap", 'type': 'assertion_error'}
E         
E         Full diff:
E           [
E               {
E                   'ctx': {
E         -             'error': HasRepr('AssertionError("invalid a\\nassert \'snap\' == \'a\'\\n  - a\\n  + snap")'),
E         ?                      ---------                         -         -     -     -  -          -           --
E         +             'error': AssertionError("invalid a\nassert 'snap' == 'a'\n  \n  - a\n  + snap"),
E         ?                                                                      +++
E                   },
E                   'input': 'snap',
E                   'loc': (
E                       'a',
E                   ),
E                   'msg': 'Assertion failed, invalid a\n'
E
... (truncated) ...

Environment

Python: 3.11.8
Pydantic: 2

What Broke

Test failures lead to incorrect validation behavior in production environments.

Why It Broke

The test fails due to different assertion output in pytest 8

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install pydantic==1.10.15

When NOT to use: Do not apply this fix if using a pytest version lower than 8.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/pydantic/pydantic/pull/9024

First fixed release: 1.10.15

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

Do not apply this fix if using a pytest version lower than 8.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.

Version Compatibility Table

Version	Status
1.10.15	Fixed

Related Issues

No related fixes found.

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Related clusters: Data consistency OpenAPI schema Serialization

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Pydantic `tests/test_validators.py::test_assert_raises_validation_error` fails with pytest 8 (Fix)