Why default value trigger generated typescript class become optional, how can i get rid of it?

[CroMarmot]

Question as title

I just follow this article fastapi generate typescript clients

but my class has some field with default value

class ResponseMessage(BaseModel):
    message: str
+    info: int = 1

after the same step in above article, the generate code belike

export type ResponseMessage = {
    message: string;
    info?: number;
};

Reproduce step

python3 -m venv venv
. venv/bin/active # In linux

pip install fastapi
pip install "uvicorn[standard]"

npm init -y

add "generate-client": "openapi-ts --input http://localhost:8000/openapi.json --output ./src/client --client axios" in package.json

# start backend
uvicorn main:app
# run generate cmd
npm run generate-client

grep -r -A3 "ResponseMessage" src/client/types.gen.ts 

export type ResponseMessage = {
    message: string;
    info?: number;
};
--
export type CreateItemItemsPostResponse = ResponseMessage;

export type $OpenApiTs = {
    '/items/': {
--
                200: ResponseMessage;
                /**
                 * Validation Error
                 */

main.py

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()


class Item(BaseModel):
    name: str
    price: float


class ResponseMessage(BaseModel):
    message: str
    info: int = 1


@app.post("/items/", response_model=ResponseMessage)
async def create_item(item: Item):
    return {"message": "item received"}


@app.get("/items/", response_model=list[Item])
async def get_items():
    return [
        {"name": "Plumbus", "price": 3},
        {"name": "Portal Gun", "price": 9001},
    ]

pip list  | grep pydantic
pydantic                      2.7.1
pydantic_core                 2.18.2

pip list  | grep fastapi
fastapi           0.111.0
fastapi-cli       0.0.4

python3 --version
Python 3.10.12

---

but when frontend received the data, it must be number not optional, what can i do

I searched 'default' and 'optional' both in discussions,no thing related.

i search 'default' only found these two, both not help for me

• https://fastapi.tiangolo.com/zh/how-to/separate-openapi-schemas/
• https://fastapi.tiangolo.com/tutorial/response-model/

i think https://fastapi.tiangolo.com/zh/how-to/separate-openapi-schemas/#model-for-input-and-output-in-docs is might what i want, but it seems not worked, i'm confused what i missing?

---

I do need default value for backend code and i hope the response data field is not optional in generate class in typescript.

how can i achieve it or any easier other way to solve the problem.

My current temporary solution is something like this, using a class without a default value as a description, and then specifically creating a class with a default value based on it. However, when there are many fields or nested fields, this solution will have a very poor experience.

class Item(BaseModel):
    name: str
    description: str
    code: int

class Item200(Item):
    description: str = ""
    code: int = 200


@app.post("/items/",response_model=Item)
def create_item(item: Item)->Item:
    return Item200(name="n")

[mrlubos]

Hey @CroMarmot, that's exactly how you're supposed to define it!

@app.post("/items/")
def create_item(item: Item)->Item200:
    return Item200(name="n")

[CroMarmot]

if the return type is Item200, i don't want the code field be optional in frontend generate code, I want it in "required", and i

---

What strikes me as odd is when using default values.

On the python side, when I assign a value to this class though, this field can be omitted, but when I read and write to it, this field has an explicit type

However in the generated corresponding openai.json and in the front-end code, it's tagged as possibly non-existent, i.e. there's an additional type determination or field existence determination required (although ts has ? operator can be used), but this is a strange asymmetry of experience.

---

Similarly If there are no default values. In python there is the Optional description, for which reading and writing may encounter None, an experience that is generated in ts as , with the expected behavior.

---

I really think here describes what I'm expecting, but In my local environment, Item-Input and Item-Output are not generated, I checked pydantic ,python,fastapi versions and also called print(app.separate_input_output_schemas) and the output is True.

your docs:

My local

how can i enable this feature, what i'm missing?

[mrlubos]

Seems you got the issue right, you need the field to be required in your OpenAPI spec. This sounds like a Pydantic/FastAPI question though?

[CroMarmot]

This should be more of a business-oriented requirement than a bug, i.e., defining structures once, but for the server, the inputs are OPTIONAL and the outputs are NON-OPTIONAL.

Then, fastapi is supposed to implement this requirement based on pydantic v2, but the relevant supported writeups for pydantic changed in v2.4.0, and the corresponding documentation for fastapi has not been updated.

[CroMarmot]

I'm not familiar with openapi, I used protobuf before. Maybe this is a problem with fastapi and not openapi-ts. I don't know.

But I just found another issue fastapi/fastapi#10453 this seems work for me