FastAPI doesn't format datetime.datetime output to ISO 8601 as documented

First Check

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Example Code

from pydantic import BaseModel
from fastapi import FastAPI
import datetime


class TestOutput(BaseModel):
    date: datetime.datetime

def app():
    fast_api_app = FastAPI()

    @fast_api_app.get("/", response_model=TestOutput)
    def test():
        return {'date': datetime.datetime.utcnow()}

    return fast_api_app

Description

Opening the web browser, I got the response:

{
  "date": "2022-06-14T19:25:55.821177"
}

The timezone information is missing, I expected “+00:00” in the end

The docs indicates that I should receive an ISO 8601 formatted date, that includes timezone https://fastapi.tiangolo.com/tutorial/extra-data-types/

Even the example provided in OpenAPI (/docs) have the timezone information as the follow:

{
  "date": "2022-06-14T19:25:55.846Z"
}

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.75.0

Python Version

3.8.10

Additional Context

No response

[slackner]

A small side note, there is no need to use pytz to generate timezone aware timestamps. You can also use:

datetime.datetime.now(datetime.timezone.utc)

[slackner]

You can add a custom pydantic type to achieve that. See the following code snippet as an example:

from pydantic import BaseModel
from pydantic.datetime_parse import parse_datetime
from fastapi import FastAPI
import datetime

class UTCDatetime(datetime.datetime):
    @classmethod
    def __get_validators__(cls):
        yield parse_datetime  # default Pydantic behavior
        yield cls.validate

    @classmethod
    def validate(cls, value) -> str:
        if value.tzinfo is None:
            return value.replace(tzinfo=datetime.timezone.utc)

        # Convert to UTC
        return value.astimezone(datetime.timezone.utc)
        # or alternatively, return the original value:
        # return value

class TestOutput(BaseModel):
    date: UTCDatetime

def app():
    fast_api_app = FastAPI()

    @fast_api_app.get("/", response_model=TestOutput)
    def test():
        return {'date': datetime.datetime.utcnow()}

    return fast_api_app

As long as you are careful where you are using timezone-aware and non-timezone-aware timestamps, using such an approach is fine.

(Note: I’m using this myself, but for input parameters instead of output parameters. The type hint datetime.datetime accepts both timezone-aware and non-timezone-aware timestamps, and comparing both in the API endpoint can lead to an exception otherwise.)