[Python] Custom Pydantic classes as FastAPI body parameters

Pydantic says that you can create custom classes by simply defining the __get_validators__ method. This is useful if you want to parse into a class with its own metaclass or for some other reason do not want to inherit from BaseModel.

However, this fails in strange places in FastAPI. For example, FastAPI does not detect such a class as a body parameter, but always thinks it is a query parameter.

from fastapi import FastAPI, Body
from fastapi.testclient import TestClient

app = FastAPI()

class NastyMetaClass(type):
pass

class Foo(metaclass=NastyMetaClass):
@classmethod
def __get_validators__(cls):
yield lambda value: True

@app.post("/implicit")
def foo(foo: Foo): # This is supposed to work, but does not
return "It worked"

@app.post("/explicit")
def foo_body(foo: Foo = Body(...)): # The `= Body(...)` fixes it
return "It worked"

client = TestClient(app)

response = client.post("/implicit", json={})
print(response.json())
# {'detail': [{'loc': ['query', 'foo'], 'msg': 'field required', 'type': 'value_error.missing'}]}

response = client.post("/explicit", json={})
print(response.json())
# It worked


How can I make FastAPI recognize custom Pydantic classes?

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