Pydantic Suggestion: Add DefaultFactory. (Fix)

The Fix

pip install pydantic==1.10.1

Based on closed pydantic/pydantic issue #9769 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -0,0 +1,2 @@
@@ -0,0 +1,2 @@
+Allow `Field` with a `default_factory` to be used as an argument to a function
+decorated with `validate_arguments`
\ No newline at end of file

repro.py

import datetime
from typing import Annotated, Optional

from pydantic import BaseModel, Field

# Model Definition
class Item(BaseModel):
    id: Annotated[int, Field(title="Item ID")]
    label: Annotated[str | None, Field(title="Item Label")] = None
    created_at: Annotated[datetime.datetime | None, Field(title="Item Created At", default_factory=datetime.datetime.now)] = None

# Ok: Static Analysis Success.
item = Item(id=1)

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==1.10.1\nWhen NOT to use: This fix should not be used if the existing behavior of Field is required.\n\nOption C — Workaround\nmight be to define a small helper function:\nWhen NOT to use: This fix should not be used if the existing behavior of Field is required.\n\n

Why This Fix Works in Production

Trigger: Annotated cannot be used, and Field must be assigned.
Mechanism: The current implementation does not allow using Field with default_factory in Annotated
Why the fix works: Adds preliminary support for Field to validate_arguments, allowing the use of default_factory in function arguments. (first fixed release: 1.10.1).

Why This Breaks in Prod

Shows up under Python 3.12 in real deployments (not just unit tests).
The current implementation does not allow using Field with default_factory in Annotated
Production symptom (often without a traceback): Annotated cannot be used, and Field must be assigned.

Proof / Evidence

GitHub issue: #9769
Fix PR: https://github.com/pydantic/pydantic/pull/2176
First fixed release: 1.10.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.59

Discussion

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

Jump to Sources Open on GitHub

“@sydney-runkle Thanks for the response. Yes. I could not find an existing way to dictate **required** and **default**, and made a suggestion. If you have…”

@ya7010 · 2024-06-26 · source

“Just to clarify - when you specify the default_factory, it's inferred that the field is not required”

@sydney-runkle · 2024-06-26 · source

“Imo seems best for now to have it defined in user code, considering it is a two-line function, but I don't have strong opinions on…”

@Viicos · 2024-06-27 · source

“Hi @yassun7010, Perhaps I'm misunderstanding your question - why does the following not suit your needs?”

@sydney-runkle · 2024-06-26 · source

Failure Signature (Search String)

Annotated cannot be used, and Field must be assigned.
The reason why `Annotated` cannot be used when using `default_factory` is to avoid specifying a field with `default_factory` when initializing in the constructor.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Annotated cannot be used, and Field must be assigned.
The reason why `Annotated` cannot be used when using `default_factory` is to avoid specifying a field with `default_factory` when initializing in the constructor.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Annotated cannot be used, and Field must be assigned.
The reason why `Annotated` cannot be used when using `default_factory` is to avoid specifying a field with `default_factory` when initializing in the constructor.

Minimal Reproduction

repro.py

import datetime
from typing import Annotated, Optional

from pydantic import BaseModel, Field

# Model Definition
class Item(BaseModel):
    id: Annotated[int, Field(title="Item ID")]
    label: Annotated[str | None, Field(title="Item Label")] = None
    created_at: Annotated[datetime.datetime | None, Field(title="Item Created At", default_factory=datetime.datetime.now)] = None

# Ok: Static Analysis Success.
item = Item(id=1)

Environment

Python: 3.12

What Broke

Users cannot consistently declare fields with required and default values.

Why It Broke

The current implementation does not allow using Field with default_factory in Annotated

Fix Options (Details)

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

pip install pydantic==1.10.1

When NOT to use: This fix should not be used if the existing behavior of Field is required.

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

Option C — Workaround Temporary workaround

might be to define a small helper function:

When NOT to use: This fix should not be used if the existing behavior of Field is required.

Use only if you cannot change versions today. Treat this as a stopgap and remove once upgraded.

Fix reference: https://github.com/pydantic/pydantic/pull/2176

First fixed release: 1.10.1

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

When NOT to Use This Fix

This fix should not be used if the existing behavior of Field is 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
1.10.1	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:serialization Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Configuration error Data consistency OpenAPI schema

Sources

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