typing.Type field breaks Swagger #8866
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Examplefrom typing import Type
import uvicorn
from fastapi import FastAPI
from pydantic import BaseModel
class Container(BaseModel):
T: Type
app = FastAPI()
@app.get("/", response_model=Container)
def index(container: Container):
return container
if __name__ == "__main__":
uvicorn.run("app:app")Description
Environment
Additional contextStack trace from the server: The functionality that I want in my use case works correctly: I tested the API and custom (de)serialization logic is OK, I get what I want from it, but both I wanted to use generic models for this (and they worked wonderfully), but I also need to send type information to the frontend, and there's no (officially supported, reliable) way to get this in runtime. |
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Answered by
YuriiMotov
Jun 26, 2025
Replies: 3 comments
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What about using |
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FastAPI doesn't know how to deal with such type of field. So, you need to manually define schema, validator and serializer for it. Something like this: from typing import Type, Annotated
from fastapi import FastAPI
from pydantic import BaseModel, field_serializer, field_validator
from pydantic.json_schema import WithJsonSchema
class MyType(BaseModel):
a: str
class Container(BaseModel):
T: Annotated[Type, WithJsonSchema({"type": "string"})]
@field_validator("T", mode="plain")
@classmethod
def validate_type(cls, value: Type) -> Type:
if isinstance(value, type):
return value
elif isinstance(value, str):
try:
return globals()[value]
except KeyError:
raise ValueError(f"Type '{value}' not found in globals.")
return value
@field_serializer("T")
def serialize_type(self, value: Type) -> str:
return value.__name__
app = FastAPI()
@app.post("/", response_model=Container)
def index(container: Container):
return container
from fastapi.testclient import TestClient
def test_success():
client = TestClient(app)
response = client.post("/", json={"T": "MyType"})
assert response.status_code == 200
assert response.json() == {"T": "MyType"}
def test_failure():
client = TestClient(app)
response = client.post("/", json={"T": "NonExistentType"})
assert response.status_code == 422
assert "Type 'NonExistentType' not found in globals." in response.text |
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Answer selected by
YuriiMotov
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FastAPI doesn't know how to deal with such type of field. So, you need to manually define schema, validator and serializer for it.
Something like this: