#230: Fix the PydanticJsonSchemaWarning in FastAPI

While I was preparing the next post, I run into an annoying warning that I wanted to fix before I add one additional feature to the minimalistic to-do application. Let us explore what I had to do to find the reason for the warning and how I could fix the problem.

The warning

When I run the tests for the to-do application, I got this warning message:

====== warnings summary =======
minimal_todo/tests/test_todo.py::test_docs_endpoint_works
C:\*****\Python312\Lib\site-packages\pydantic\json_schema.py:2158: 
PydanticJsonSchemaWarning: Default value is not JSON serializable; 
excluding default from JSON schema [non-serializable-default]
warnings.warn(message, PydanticJsonSchemaWarning)

— Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html

It took me a while to track down the source, but it was the test running against the endpoint behind the API documentation that caused this warning. Knowing this, I could get much faster feedback by running only that single test:

pytest .\minimal_todo\tests\test_todo.py::test_docs_endpoint_works

The source of the problem

The warning showed me that the problem is somewhere within my two Pydantic models. It could either be TaskInput or TaskOutput:

from datetime import date, timedelta
from typing import Annotated
from pydantic import BaseModel, Field, field_validator

class TaskInput(BaseModel):
    name: str = Field(str, min_length=5, max_length=100)
    priority: int = Field(gt=0, lt=10)
    due_date: date
    done: bool

    @field_validator('due_date')
    def due_date_must_be_between_today_and_one_year_in_the_future(cls, v):
        if not date.today() <= v <= date.today() + timedelta(days=365):
            raise ValueError("due_date must be between today and one year in the future")
        return v

class TaskOutput(BaseModel):
    id: int
    name: str
    priority: int
    due_date: date
    done: bool
    created_at: date

I wasted a lot of time with the TaskOutput when in fact the TaskInput model had the problem. Somehow the length validation no longer wanted to serialize for the OpenAPI documentation.

The fix for the problem

Instead of validating the length with a Field, I switched the check to a validator method as we already did for the due_date:

class TaskInput(BaseModel):
    name: str
    priority: int = Field(gt=0, lt=10)
    due_date: date
    done: bool

    @field_validator('due_date')
    def due_date_must_be_between_today_and_one_year_in_the_future(cls, v):
        if not date.today() <= v <= date.today() + timedelta(days=365):
            raise ValueError("due_date must be between today and one year in the future")
        return v

    @field_validator('name')
    def name_must_be_between_5_and_100(cls, v):
        if len(v) < 5 :
            raise ValueError("String should have at least 5 characters")
        if len(v) > 100:
            raise ValueError("String should have at most 100 characters")
        return v

With this change, I could run the test again and this time the warning was gone. Since I kept the warning generic, it matched what the other tests expected and so I did not need to fix any tests at all.

#230: Fix the PydanticJsonSchemaWarning in FastAPI

The warning

The source of the problem

The fix for the problem

Next