Pydantic ImportString serialization fails for `sys.stdout`

The Fix

pip install pydantic==1.10.18

Based on closed pydantic/pydantic issue #10122 · 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

@@ -1013,6 +1013,15 @@ def _serialize(v: Any) -> str:
             elif hasattr(v, '__module__') and hasattr(v, '__name__'):
                 return f'{v.__module__}.{v.__name__}'
+            # Handle special cases for sys.XXX streams
+            # if we see more of these, we should consider a more general solution
+            elif hasattr(v, 'name'):

repro.py

from pydantic import BaseModel, ImportString


class ImportThings(BaseModel):
    obj: ImportString


ImportThings(obj='math.cos').model_dump_json()  # ok
ImportThings(obj='sys.stdout').model_dump_json()

Traceback (most recent call last):
  File "...", line 364, in runcode
    coro = func()
           ^^^^^^
  File "<input>", line 1, in <module>
  File ".../python3.12/site-packages/pydantic/main.py", line 415, in model_dump_json
    return self.__pydantic_serializer__.to_json(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.PydanticSerializationError: Error serializing to JSON: UnsupportedOperation: not readable

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 the application relies on other non-standard stream objects.\n\n

Why This Fix Works in Production

Trigger: from pydantic import BaseModel, ImportString
Mechanism: Fixes serialization issues for `ImportString` when using `sys.stdout`.
Why the fix works: Fixes serialization issues for `ImportString` when using `sys.stdout`. (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

Shows up under Python 3.12 in real deployments (not just unit tests).
Surfaces as: from pydantic import BaseModel, ImportString

Proof / Evidence

GitHub issue: #10122
Fix PR: https://github.com/pydantic/pydantic/pull/10137
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: 0.32

Discussion

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

Jump to Sources Open on GitHub

“@dmder, Good find! I'll root around for a fix.”

@sydney-runkle · 2024-08-14 · source

“I should be able to submit a PR tomorrow with a fix for this 👍”

@sydney-runkle · 2024-08-14 · source

“Thank you @sydney-runkle for such a quick turnaround on this 👍”

@dmder · 2024-08-22 · source

Failure Signature (Search String)

from pydantic import BaseModel, ImportString

Error Message

Stack trace

error.txt

Error Message
-------------
from pydantic import BaseModel, ImportString


class ImportThings(BaseModel):
    obj: ImportString


ImportThings(obj='math.cos').model_dump_json()  # ok
ImportThings(obj='sys.stdout').model_dump_json()

Traceback (most recent call last):
  File "...", line 364, in runcode
    coro = func()
           ^^^^^^
  File "<input>", line 1, in <module>
  File ".../python3.12/site-packages/pydantic/main.py", line 415, in model_dump_json
    return self.__pydantic_serializer__.to_json(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.PydanticSerializationError: Error serializing to JSON: UnsupportedOperation: not readable

Minimal Reproduction

repro.py

from pydantic import BaseModel, ImportString


class ImportThings(BaseModel):
    obj: ImportString


ImportThings(obj='math.cos').model_dump_json()  # ok
ImportThings(obj='sys.stdout').model_dump_json()

Traceback (most recent call last):
  File "...", line 364, in runcode
    coro = func()
           ^^^^^^
  File "<input>", line 1, in <module>
  File ".../python3.12/site-packages/pydantic/main.py", line 415, in model_dump_json
    return self.__pydantic_serializer__.to_json(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.PydanticSerializationError: Error serializing to JSON: UnsupportedOperation: not readable

Environment

Python: 3.12
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 the application relies on other non-standard stream objects.

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/10137

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 the application relies on other non-standard stream objects.

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 ImportString serialization fails for `sys.stdout` (Fix)