Pydantic TypeAdapter for collections.abc.MutableMapping

The Fix

pip install pydantic==1.10.18

Based on closed pydantic/pydantic issue #9549 · 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

@@ -124,7 +124,7 @@
 SET_TYPES: list[type] = [set, typing.Set, collections.abc.MutableSet]
 FROZEN_SET_TYPES: list[type] = [frozenset, typing.FrozenSet, collections.abc.Set]
-DICT_TYPES: list[type] = [dict, typing.Dict, collections.abc.MutableMapping, collections.abc.Mapping]
+DICT_TYPES: list[type] = [dict, typing.Dict]
 IP_TYPES: list[type] = [IPv4Address, IPv4Interface, IPv4Network, IPv6Address, IPv6Interface, IPv6Network]

repro.py

import collections.abc

import pydantic

config = pydantic.ConfigDict(arbitrary_types_allowed=True, strict=True)
adapter = pydantic.TypeAdapter(collections.abc.MutableMapping, config=config)
adapter.validate_python(collections.UserDict())

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==1.10.18\nWhen NOT to use: This fix should not be used if strict mode validation is not required.\n\n

Why This Fix Works in Production

Trigger: C:\code\pipelines cratch\venv\Scripts\python.exe C:\code\pipelines cratch\devtools\pydantic_userdict.py
Mechanism: Fixes the issue where a type adapter for collections.abc.MutableMapping fails to validate instances of collections.UserDict in strict mode.
Why the fix works: Fixes the issue where a type adapter for collections.abc.MutableMapping fails to validate instances of collections.UserDict in strict mode. (first fixed release: 1.10.18).

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

Surfaces as: C:\code\pipelines\scratch\venv\Scripts\python.exe C:\code\pipelines\scratch\devtools\pydantic_userdict.py

Proof / Evidence

GitHub issue: #9549
Fix PR: https://github.com/pydantic/pydantic/pull/10020
First fixed release: 1.10.18
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.36

Discussion

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

Jump to Sources Open on GitHub

“@sydney-runkle, I'm interested in taking a look at this. BTW, can you unassign me from #9100?”

@mikeleppane · 2024-06-03 · source

“@hb2638, Thanks for reporting this. I can reproduce. Looks like a bug. Some more context:”

@sydney-runkle · 2024-06-03 · source

“Could be a good first issue if someone is looking to get their hands dirty with pydantic's internal validation logic. I think MutableMapping should probably…”

@sydney-runkle · 2024-06-03 · source

“In summary, I think the fix might actually be in pydantic, when we do the schema building. The way we do it is a bit…”

@sydney-runkle · 2024-06-09 · source

Failure Signature (Search String)

C:\code\pipelines cratch\venv\Scripts\python.exe C:\code\pipelines cratch\devtools\pydantic_userdict.py

Error Message

Stack trace

error.txt

Error Message
-------------
C:\code\pipelines\scratch\venv\Scripts\python.exe C:\code\pipelines\scratch\devtools\pydantic_userdict.py 
Traceback (most recent call last):
  File "C:\code\pipelines\scratch\devtools\pydantic_userdict.py", line 6, in <module>
    adapter.validate_python(collections.UserDict())
  File "C:\code\pipelines\scratch\venv\Lib\site-packages\pydantic\type_adapter.py", line 260, in validate_python
    return self.validator.validate_python(object, strict=strict, from_attributes=from_attributes, context=context)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 1 validation error for dict[any,any]
  Input should be a valid dictionary [type=dict_type, input_value={}, input_type=UserDict]
    For further information visit https://errors.pydantic.dev/2.7/v/dict_type

Process finished with exit code 1

Minimal Reproduction

repro.py

import collections.abc

import pydantic

config = pydantic.ConfigDict(arbitrary_types_allowed=True, strict=True)
adapter = pydantic.TypeAdapter(collections.abc.MutableMapping, config=config)
adapter.validate_python(collections.UserDict())

Environment

Pydantic: 2

Fix Options (Details)

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

pip install pydantic==1.10.18

When NOT to use: This fix should not be used if strict mode validation is not required.

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/10020

First fixed release: 1.10.18

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

When NOT to Use This Fix

This fix should not be used if strict mode validation is not required.

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
1.10.18	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 TypeAdapter for collections.abc.MutableMapping fails when validate_python called on a (Fix)