Using default for datetime query parameter without timezone leads to OpenAPI SpecValidationException

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Example

Here’s a self-contained, minimal, reproducible, example with my use case:

from datetime import datetime

from fastapi import FastAPI
import uvicorn

app = FastAPI()

start = datetime.today()


@app.get("/")
async def root(start_date: datetime = start):
    print(start_date)
    return {"start_date": start_date}

if __name__ == "__main__":
    uvicorn.run(app, host="127.0.0.1", port=8000)

Description

Run openapi-generator and see the following error:

$ openapi-generator --version
openapi-generator-cli 4.3.1
  commit : 003165c
  built  : 2020-05-06T09:38:39Z
  source : https://github.com/openapitools/openapi-generator
  docs   : https://openapi-generator.tech/

$ openapi-generator generate --input-spec http://localhost:8000/openapi.json --output api --generator-name typescript-axios
Exception in thread "main" org.openapitools.codegen.SpecValidationException: There were issues with the specification. The option can be disabled via validateSpec (Maven/Gradle) or --skip-validate-spec (CLI).
 | Error count: 1, Warning count: 0
Errors: 
	-attribute paths.'/'(get).parameters.[start_date].schemas.default=`2020-12-15T07:57:25.277303` is not of type `date-time`

	at org.openapitools.codegen.config.CodegenConfigurator.toContext(CodegenConfigurator.java:480)
	at org.openapitools.codegen.config.CodegenConfigurator.toClientOptInput(CodegenConfigurator.java:507)
	at org.openapitools.codegen.cmd.Generate.execute(Generate.java:423)
	at org.openapitools.codegen.cmd.OpenApiGeneratorCommand.run(OpenApiGeneratorCommand.java:32)
	at org.openapitools.codegen.OpenAPIGenerator.main(OpenAPIGenerator.java:61)

I would expect FastAPI to handle default datetime objects in a way that is compatible with standard OpenAPI generation tools.

• datetime is listed as a supported extra data type: https://fastapi.tiangolo.com/tutorial/extra-data-types/#other-data-types

Environment

• OS: macOS 10.15.7
• FastAPI Version: 0.62
• Python version: 3.8.6

[johnthagen]

The isoformat() returns a str so it is not needed. FastAPI converts to ISO format correctly by default. I think the issue is that if timezone information is not available, FastAPI still just serializes the datetime, but it is not a properly formed OpenAPI date-time (if missing the timezone).

A simplification of my earlier working example is:

async def root(start_date: datetime = datetime.now(timezone.utc)):
    ...

But I before closing this issue, I think a documentation note would be helpful, and perhaps maintainers of FastAPI want to decide if passing non-timezone datetimes should raise a warning or error given they don’t conform to the Spec they generate.

[johnthagen]

According to the Python docs, a better way to get a timezone aware datetime is:

from datetime import datetime, timezone

from fastapi import FastAPI
import uvicorn

app = FastAPI()

start = datetime.now(timezone.utc)


@app.get("/")
async def root(start_date: datetime = start):
    print(start_date)
    return {"start_date": start_date}

if __name__ == "__main__":
    uvicorn.run(app, host="127.0.0.1", port=8000)