V1 models for response and body by chbndrhnns · Pull Request #10223 · fastapi/fastapi

chbndrhnns · 2023-09-08T15:40:47Z

No description provided.

chbndrhnns · 2023-09-08T16:27:48Z

+                        exclude_none=exclude_none,
+                    )
+                except AttributeError:
+                    return [


I am not sure if there is a better way to handle the case where there is a container of model instances.

isinstance()?

if isinstance(value, dict): ...

At least for Python 3.11+, I expect its zero-overhead exception handling to be a bit more efficient. In case that's neglectable, an if/else branch with isinstance might be more readable.

mjspeck · 2023-10-04T16:14:42Z

Any progress on this? Looks like it's been unchanged for almost a month.

conglei · 2023-12-23T20:29:53Z

Any update on this PR? I'm also blocked by the same issue.

aviau · 2024-03-21T01:53:06Z

This would really help large projects migrate to pydantic v2!

YanSte · 2024-04-05T20:15:34Z

Any update ?

zouyi100 · 2024-04-29T03:16:45Z

Any update?

jabbapen · 2024-05-05T22:14:33Z

Any update?

fraser-langton · 2024-06-19T04:46:12Z

Any update on this? FastAPI still supports Pydantic v1 is a fundamentally incorrect statement while this remains open, and we will not upgrade to pydantic v2 or fastapi 0.100 while this remains open. Otherwise there is no path to a smooth transition to v2, the entire repo must be upgraded in one go, defeating the purpose behind pydantic.v1 usage.

https://fastapi.tiangolo.com/release-notes/#pydantic-v1

This version [0.100.0] of FastAPI still supports Pydantic v1. And although Pydantic v1 will be deprecated at some point, it will still be supported for a while.

This means that you can install the new Pydantic v2, and if something fails, you can install Pydantic v1 while you fix any problems you might have, but having the latest FastAPI.

shanumas · 2024-07-06T13:45:08Z

Any update ?

vfazio · 2024-07-18T13:43:17Z

Any update on this? FastAPI still supports Pydantic v1 is a fundamentally incorrect statement while this remains open, and we will not upgrade to pydantic v2 or fastapi 0.100 while this remains open. Otherwise there is no path to a smooth transition to v2, the entire repo must be upgraded in one go, defeating the purpose behind pydantic.v1 usage.

https://fastapi.tiangolo.com/release-notes/#pydantic-v1

This version [0.100.0] of FastAPI still supports Pydantic v1. And although Pydantic v1 will be deprecated at some point, it will still be supported for a while.
This means that you can install the new Pydantic v2, and if something fails, you can install Pydantic v1 while you fix any problems you might have, but having the latest FastAPI.

@tiangolo thoughts on this?

phillipuniverse · 2024-09-17T19:35:29Z

Looks like this will work for serialization/deserialization but needs additional work for schema generation:

../../../Library/Caches/pypoetry/virtualenvs/shipwell-common-python-zjZyLK7l-py3.11/lib/python3.11/site-packages/fastapi/openapi/utils.py:475: in get_openapi
    field_mapping, definitions = get_definitions(
../../../Library/Caches/pypoetry/virtualenvs/shipwell-common-python-zjZyLK7l-py3.11/lib/python3.11/site-packages/fastapi/_compat.py:230: in get_definitions
    field_mapping, definitions = schema_generator.generate_definitions(
../../../Library/Caches/pypoetry/virtualenvs/shipwell-common-python-zjZyLK7l-py3.11/lib/python3.11/site-packages/pydantic/json_schema.py:379: in generate_definitions
    self.generate_inner(schema)
../../../Library/Caches/pypoetry/virtualenvs/shipwell-common-python-zjZyLK7l-py3.11/lib/python3.11/site-packages/pydantic/json_schema.py:552: in generate_inner
    json_schema = current_handler(schema)
../../../Library/Caches/pypoetry/virtualenvs/shipwell-common-python-zjZyLK7l-py3.11/lib/python3.11/site-packages/pydantic/_internal/_schema_generation_shared.py:37: in __call__
    return self.handler(core_schema)
../../../Library/Caches/pypoetry/virtualenvs/shipwell-common-python-zjZyLK7l-py3.11/lib/python3.11/site-packages/pydantic/json_schema.py:511: in handler_func
    json_schema = generate_for_schema_type(schema_or_field)
../../../Library/Caches/pypoetry/virtualenvs/shipwell-common-python-zjZyLK7l-py3.11/lib/python3.11/site-packages/pydantic/json_schema.py:1229: in chain_schema
    return self.generate_inner(schema['steps'][step_index])
../../../Library/Caches/pypoetry/virtualenvs/shipwell-common-python-zjZyLK7l-py3.11/lib/python3.11/site-packages/pydantic/json_schema.py:552: in generate_inner
    json_schema = current_handler(schema)
../../../Library/Caches/pypoetry/virtualenvs/shipwell-common-python-zjZyLK7l-py3.11/lib/python3.11/site-packages/pydantic/_internal/_schema_generation_shared.py:37: in __call__
    return self.handler(core_schema)
../../../Library/Caches/pypoetry/virtualenvs/shipwell-common-python-zjZyLK7l-py3.11/lib/python3.11/site-packages/pydantic/json_schema.py:511: in handler_func
    json_schema = generate_for_schema_type(schema_or_field)
../../../Library/Caches/pypoetry/virtualenvs/shipwell-common-python-zjZyLK7l-py3.11/lib/python3.11/site-packages/pydantic/json_schema.py:1014: in function_plain_schema
    return self.handle_invalid_for_json_schema(
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
self = <pydantic.json_schema.GenerateJsonSchema object at 0x113e3f0d0>
schema = {'function': {'function': <bound method BaseModel.validate of <class 'tests.contrib.fastapi.test_shipwell_fastapi_extension.shipwell_fast_api.<locals>.Price'>>, 'type': 'with-info'}, 'metadata': {}, 'type': 'function-plain'}
error_info = "core_schema.PlainValidatorFunctionSchema ({'type': 'with-info', 'function': <bound method BaseModel.validate of <class 'tests.contrib.fastapi.test_shipwell_fastapi_extension.shipwell_fast_api.<locals>.Price'>>})"
    def handle_invalid_for_json_schema(self, schema: CoreSchemaOrField, error_info: str) -> JsonSchemaValue:
>       raise PydanticInvalidForJsonSchema(f'Cannot generate a JsonSchema for {error_info}')
E       pydantic.errors.PydanticInvalidForJsonSchema: Cannot generate a JsonSchema for core_schema.PlainValidatorFunctionSchema ({'type': 'with-info', 'function': <bound method BaseModel.validate of <class 'tests.contrib.fastapi.test_shipwell_fastapi_extension.shipwell_fast_api.<locals>.Price'>>})
E       
E       For further information visit https://errors.pydantic.dev/2.9/u/invalid-for-json-schema

In this case the v2 schema generation is looking at the validate method on pydantic.v1.BaseModel and trying to generate a schema for it.

YuriiMotov · 2025-07-17T20:36:38Z

@chbndrhnns, thanks for you contribution!
Are you still ready to work on it?

Or, is there anybody who is willing to take this over and work on it?

chbndrhnns · 2025-07-18T16:19:08Z

For now, I updated the code to have passing tests.
Looking into schema generation now.

chbndrhnns · 2025-07-18T16:33:13Z

Schema generation seems a bit more tricky:

FastAPI now needs to handle both Pydantic v1 and v2 models simultaneously when running with Pydantic v2 installed. This is fundamentally different from the previous situation:

Before: FastAPI only needed to handle one version of Pydantic models (v1)
Now: FastAPI needs to handle both v1 and v2 models in the same application

chbndrhnns · 2025-07-21T19:43:38Z

@YuriiMotov How do you suggest to proceed here?

github-actions · 2025-10-08T09:00:39Z

This pull request has a merge conflict that needs to be resolved.

chbndrhnns · 2025-10-08T10:46:01Z

@YuriiMotov I am willing to rebase if we have a clear path to merging

YuriiMotov · 2025-10-08T13:17:49Z

@YuriiMotov I am willing to rebase if we have a clear path to merging

I think there is no need for this now.
Let's wait for Sebastian to review what we have here and decide if it's the right way to go

tiangolo · 2025-10-11T18:00:55Z

Thanks for the interest and effort here! This was covered by #14168, which was just released in FastAPI 0.119.0. 🎉

That adds support for pydantic.v1 models at the same time as v2 models. The new docs are here: https://fastapi.tiangolo.com/how-to/migrate-from-pydantic-v1-to-pydantic-v2/

Given that, I'll close this one now. Thanks for the effort and interest and thanks all for the discussions! 🍰

chbndrhnns commented Sep 8, 2023

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chbndrhnns mentioned this pull request Oct 1, 2023

Working with Pydantic v1 while having v2 installed #10360

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eyurtsev mentioned this pull request Oct 6, 2023

WIP Pydantic v2 compatibility langchain-ai/langserve#10

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tiangolo added feature New feature or request p2 labels Jan 14, 2024

mattt mentioned this pull request Apr 19, 2024

Fix Pydantic v1 incompatibility replicate/cog#1622

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codekiln mentioned this pull request Aug 4, 2024

RuntimeError calling add_routes with chains that have input types in Pydantic v2 langchain-ai/langserve#725

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YuriiMotov added the waiting label Jul 17, 2025

chbndrhnns and others added 10 commits July 17, 2025 22:50

Failing test for v1.BaseModel

b3fbb14

Allow v1.BaseModels to be used when pydantic v2 is installed

6bd9a8e

Inline imports which only exist in v2

0d40db2

Black

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Require pydantic v2 for tests

ad6e4b9

Import Annotated from typing_extensions

0c4fafd

🎨 [pre-commit.ci] Auto format from pre-commit.com hooks

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Black

96ff16d

Use Optional

7ad61cb

Use List instead of list

00dd5c6

chbndrhnns force-pushed the v1-response-model branch from 52de629 to 00dd5c6 Compare July 17, 2025 20:50

chbndrhnns and others added 2 commits July 18, 2025 17:52

Catch UserWarning for v1 models

3d24369

🎨 [pre-commit.ci] Auto format from pre-commit.com hooks

47c375f

github-actions Bot removed the waiting label Jul 18, 2025

Generate schema for pydantic v1 models

30e7a18

chbndrhnns force-pushed the v1-response-model branch from d77bd92 to 30e7a18 Compare July 18, 2025 16:39

github-actions Bot added the conflicts Automatically generated when a PR has a merge conflict label Oct 8, 2025

tiangolo closed this Oct 11, 2025

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