Pydantic Possible Issue with Internal Representation of

The Fix

pip install pydantic==1.10.19

Based on closed pydantic/pydantic issue #10228 · 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

@@ -88,7 +88,7 @@ def display_as_type(obj: Any) -> str:
     Takes some logic from `typing._type_repr`.
     """
-    if isinstance(obj, types.FunctionType):
+    if isinstance(obj, (types.FunctionType, types.BuiltinFunctionType)):
         return obj.__name__

repro.py

from pydantic import BaseModel
from typing import Any, Callable
from types import FunctionType

import inspect
import time

# Current Issue
print(isinstance(time.time_ns, FunctionType))
# > False


# Suggested Fix
def is_function(obj: Any) -> bool:
    return inspect.isfunction(obj) or inspect.isbuiltin(obj) or inspect.isroutine(obj) or inspect.ismethod(obj)

print(is_function(time.time_ns))
# > True


# Prettier Function Representation
class Dummy(BaseModel):
    def foo(self) -> None: ...

    @staticmethod
    def local_bar() -> Callable[[], None]:
        def bar() -> None: ...
        return bar


def current_function_repr(obj: Any) -> str:
    return obj.__name__

def prettier_function_repr(obj: Any) -> str:
    # If function is locally defined use __name__ instead of __qualname__
    return obj.__name__ if '<locals>' in obj.__qualname__ else obj.__qualname__


print(current_function_repr(Dummy.foo))
# > foo
print(prettier_function_repr(Dummy.foo))
# > Dummy.foo
print(current_function_repr(Dummy.local_bar()))
# > bar
print(prettier_function_repr(Dummy.local_bar()))
# > bar

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==1.10.19\nWhen NOT to use: Do not apply this fix if performance is a critical concern in your application.\n\n

Why This Fix Works in Production

Trigger: In [1]: from types import BuiltinFunctionType, FunctionType
Mechanism: The internal representation of function types fails for some built-in functions due to incorrect type checking
Why the fix works: Fixes representation of builtin function types evaluated in the display_as_type function. (first fixed release: 1.10.19).

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).
The internal representation of function types fails for some built-in functions due to incorrect type checking
Surfaces as: In [1]: from types import BuiltinFunctionType, FunctionType

Proof / Evidence

GitHub issue: #10228
Fix PR: https://github.com/pydantic/pydantic/pull/10479
First fixed release: 1.10.19
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.34

Discussion

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

Jump to Sources Open on GitHub

“@sydney-runkle, I limited the fix to only identifying builtin function types as suggested by @mpkocher”

@kschwab · 2024-09-25 · source

“@kschwab, Thanks for reporting this”

@sydney-runkle · 2024-08-26 · source

“Maybe using types.BuiltinFunctionType and/or other related types defined there could help? https://docs.python.org/3/library/types.html#types.BuiltinFunctionType”

@mpkocher · 2024-08-26 · source

“@mpkocher that's a cleaner fix 👍🏾 I probably won't get to this until sometime next week. Feel free to throw something up sooner if you…”

@kschwab · 2024-08-27 · source

Failure Signature (Search String)

In [1]: from types import BuiltinFunctionType, FunctionType

Error Message

Stack trace

error.txt

Error Message
-------------
In [1]: from types import BuiltinFunctionType, FunctionType
In [2]: import time
In [3]: isinstance(time.time_ns, (BuiltinFunctionType, FunctionType))
Out[3]: True
In [4]: time.time_ns.__name__
Out[4]: 'time_ns'
In [5]: isinstance(len, (BuiltinFunctionType, FunctionType))
Out[5]: True
In [6]: def fx(a, b): return a + b
In [7]: isinstance(fx, (BuiltinFunctionType, FunctionType))
Out[7]: True
In [8]: from functools import partial
In [9]: f1 = partial(fx, 1)
In [10]: isinstance(f1, (BuiltinFunctionType, FunctionType))
Out[10]: False
In [11]: f1.__name__
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[11], line 1
----> 1 f1.__name__

AttributeError: 'functools.partial' object has no attribute '__name__'

Minimal Reproduction

repro.py

from pydantic import BaseModel
from typing import Any, Callable
from types import FunctionType

import inspect
import time

# Current Issue
print(isinstance(time.time_ns, FunctionType))
# > False


# Suggested Fix
def is_function(obj: Any) -> bool:
    return inspect.isfunction(obj) or inspect.isbuiltin(obj) or inspect.isroutine(obj) or inspect.ismethod(obj)

print(is_function(time.time_ns))
# > True


# Prettier Function Representation
class Dummy(BaseModel):
    def foo(self) -> None: ...

    @staticmethod
    def local_bar() -> Callable[[], None]:
        def bar() -> None: ...
        return bar


def current_function_repr(obj: Any) -> str:
    return obj.__name__

def prettier_function_repr(obj: Any) -> str:
    # If function is locally defined use __name__ instead of __qualname__
    return obj.__name__ if '<locals>' in obj.__qualname__ else obj.__qualname__


print(current_function_repr(Dummy.foo))
# > foo
print(prettier_function_repr(Dummy.foo))
# > Dummy.foo
print(current_function_repr(Dummy.local_bar()))
# > bar
print(prettier_function_repr(Dummy.local_bar()))
# > bar

Environment

Python: 3.12
Pydantic: 2

What Broke

Users experience incorrect type representations for built-in functions in settings.

Why It Broke

The internal representation of function types fails for some built-in functions due to incorrect type checking

Fix Options (Details)

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

pip install pydantic==1.10.19

When NOT to use: Do not apply this fix if performance is a critical concern in your application.

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/10479

First fixed release: 1.10.19

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

When NOT to Use This Fix

Do not apply this fix if performance is a critical concern in your application.

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
1.10.19	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 Possible Issue with Internal Representation of Function Types (Fix)