Pydantic AttributeError is not raised correctly when

The Fix

pip install pydantic==2.11.8

Based on closed pydantic/pydantic issue #12105 · 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

@@ -979,11 +979,8 @@ def __getattr__(self, item: str) -> Any:
                     pydantic_extra = None
 
-                if pydantic_extra:
-                    try:
-                        return pydantic_extra[item]

repro.py

import pytest
from pydantic import BaseModel, ConfigDict

class A:
    pass

class B(BaseModel):
    model_config = ConfigDict(extra="allow")

    @property
    def y(self):
        return A().x  # this attribute does not exist


def test_attribute_error_with_no_extra():
    with pytest.raises(AttributeError, match=f"'{A.__name__}' object has no attribute 'x'"):
        B().y

def test_attribute_error_with_extra():
    with pytest.raises(AttributeError, match=f"'{A.__name__}' object has no attribute 'x'"):
        B(random_extra="some value").y

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==2.11.8\nWhen NOT to use: Do not apply this fix if the behavior of pydantic_extra needs to remain unchanged.\n\n

Why This Fix Works in Production

Trigger: >> B().y
Mechanism: The __getattr__ method in BaseModel does not handle AttributeError correctly when pydantic_extra is set
Why the fix works: Fixes the behavior of `__getattr__()` in Pydantic models when a property raises an `AttributeError` and extra values are present. (first fixed release: 2.11.8).

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 3.13 in real deployments (not just unit tests).
The __getattr__ method in BaseModel does not handle AttributeError correctly when pydantic_extra is set
Surfaces as: >> B().y

Proof / Evidence

GitHub issue: #12105
Fix PR: https://github.com/pydantic/pydantic/pull/12106
First fixed release: 2.11.8
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.40

Discussion

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

Jump to Sources Open on GitHub

“PydanticAI Github Bot Found 1 issues similar to this one: 1. "https://github.com/pydantic/pydantic/issues/9070" (95% similar)”

@pydanticai-bot · 2025-07-30 · source

Failure Signature (Search String)

>> B().y

Error Message

Stack trace

error.txt

Error Message
-------------
>> B().y
Traceback (most recent call last):
  File "<python-input-4>", line 1, in <module>
    B().y
  File "/Users/raspuchin/Desktop/src/pydantic_bug/lib/python3.13/site-packages/pydantic/main.py", line 988, in __getattr__
    return super().__getattribute__(item)  # Raises AttributeError if appropriate
           ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "<python-input-2>", line 5, in y
    return A().x  # x does not exist
           ^^^^^
AttributeError: 'A' object has no attribute 'x'

Stack trace

error.txt

Error Message
-------------
>> B(something='').y
Traceback (most recent call last):
  File "/Users/raspuchin/Desktop/src/pydantic_bug/lib/python3.13/site-packages/pydantic/main.py", line 983, in __getattr__
    return pydantic_extra[item]
           ~~~~~~~~~~~~~~^^^^^^
KeyError: 'y'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<python-input-5>", line 1, in <module>
    B(something='').y
  File "/Users/raspuchin/Desktop/src/pydantic_bug/lib/python3.13/site-packages/pydantic/main.py", line 985, in __getattr__
    raise AttributeError(f'{type(self).__name__!r} object has no attribute {item!r}') from exc
AttributeError: 'B' object has no attribute 'y'

Minimal Reproduction

repro.py

import pytest
from pydantic import BaseModel, ConfigDict

class A:
    pass

class B(BaseModel):
    model_config = ConfigDict(extra="allow")

    @property
    def y(self):
        return A().x  # this attribute does not exist


def test_attribute_error_with_no_extra():
    with pytest.raises(AttributeError, match=f"'{A.__name__}' object has no attribute 'x'"):
        B().y

def test_attribute_error_with_extra():
    with pytest.raises(AttributeError, match=f"'{A.__name__}' object has no attribute 'x'"):
        B(random_extra="some value").y

Environment

Python: 3.13
Pydantic: 2

What Broke

Incorrect error handling leads to KeyError instead of AttributeError when accessing non-existent properties.

Why It Broke

The __getattr__ method in BaseModel does not handle AttributeError correctly when pydantic_extra is set

Fix Options (Details)

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

pip install pydantic==2.11.8

When NOT to use: Do not apply this fix if the behavior of pydantic_extra needs to remain unchanged.

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

Option D — Guard side-effects with OnceOnly Guardrail for side-effects

Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.

Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.

Show example snippet (optional)

onceonly.py

from onceonly import OnceOnly
import os

once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)

# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..."  # replace
key = f"stripe:webhook:{event_id}"

res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
    return {"status": "already_processed"}

# Safe to execute the side-effect exactly once.
handle_event(event_id)

See OnceOnly SDK

When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Fix reference: https://github.com/pydantic/pydantic/pull/12106

First fixed release: 2.11.8

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

When NOT to Use This Fix

Do not apply this fix if the behavior of pydantic_extra needs to remain unchanged.
Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

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
2.11.8	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:configuration-error Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Data consistency OpenAPI schema Serialization

Sources

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

Pydantic AttributeError is not raised correctly when pydantic_extra is set (Fix)