How to document default None/null in OpenAPI/Swagger using FastAPI?

Using a ORM, I want to do a POST request letting some fields with a null value, which will be translated in the database for the default value specified there.

The problem is that OpenAPI (Swagger) docs, ignores the default None and still prompts a UUID by default.

from fastapi import FastAPI
from pydantic import BaseModel
from typing import Optional
from uuid import UUID
import uvicorn


class Table(BaseModel):
    # ID: Optional[UUID]      # the docs show a example UUID, ok
    ID: Optional[UUID] = None # the docs still shows a uuid, when it should show a null or valid None value.

app = FastAPI()  
    
@app.post("/table/", response_model=Table)
def create_table(table: Table):
    # here we call to sqlalchey orm etc.
    return 'nothing important, the important thing is in the docs'
    
if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8000)

In the OpenAPI schema example (request body) which is at the docs we find:

{
 "ID": "3fa85f64-5717-4562-b3fc-2c963f66afa6"
}

This is not ok, because I specified that the default value is None,so I expected this instead:

{
 "ID": null, # null is the equivalent of None here
}

Which will pass a null to the ID and finally will be parsed in the db to the default value (that is a new generated UUID).

Solution

When you declare Optional parameters, users shouldn't include those parameters in the request specified with null or None (in Python), in order to be None. By default, the value of such parameters will be None, unless the user specifies some other value when sending the request.

Hence, all you have to do is to declare a custom example for the Pydantic model using Config and schema_extra, as described in the documentation and as shown below. The below example will create an empty (i.e., {}) request body in OpenAPI (Swagger UI), which can be successfully submitted (as ID is the only attribute of the model and is optional).

class Table(BaseModel):
    ID: Optional[UUID] = None
    
    class Config:
        schema_extra = {
            "example": {
            }
        }

@app.post("/table/", response_model=Table)
def create_table(table: Table):
    return table

If the Table model included some other required attributes, you could add example values for those, as demonstrated below:

class Table(BaseModel):
    ID: Optional[UUID] = None
    some_attr: str
    
    class Config:
        schema_extra = {
            "example": {
                "some_attr": "Foo"
            }
        }

If you would like to keep the auto-generated examples for the rest of the attributes except the one for the ID attribute, you could use the below to remove ID from the model's properties in the generated schema (inspired by Schema customization):

class Table(BaseModel):
    ID: Optional[UUID] = None
    some_attr: str
    some_attr2: float
    some_attr3: bool
    
    class Config:
        @staticmethod
        def schema_extra(schema: Dict[str, Any], model: Type['Table']) -> None:
            del schema.get('properties')['ID']

Also, if you would like to add custom example to some of the attributes, you could use Field() (as described here); for example, some_attr: str = Field(example="Foo").

Another possible solution would be to modify the generated OpenAPI schema, as described in Solution 3 of this answer. Though, the above solution is likely more suited to this case.

Note

ID: Optional[UUID] = None is the same as ID: UUID = None. As previously documented in FastAPI website (see this answer as well):

The Optional in Optional[str] is not used by FastAPI, but will allow your editor to give you better support and detect errors.

Since then, FastAPI has revised their documentation with the following:

The Union in Union[str, None] will allow your editor to give you better support and detect errors.

Hence, ID: Union[UUID, None] = None is the same as ID: Optional[UUID] = None and ID: UUID = None. In Python 3.10+, one could also use ID: UUID| None = None (see here).

As per FastAPI documentation (see Info section in the link provided):

Have in mind that the most important part to make a parameter optional is the part:
= None
or the:
= Query(default=None)
as it will use that None as the default value, and that way make the parameter not required.

The Union[str, None] part allows your editor to provide better support, but it is not what tells FastAPI that this parameter is not required.