How to Return JSON Data in FastAPI: Automatic vs. Manual Conversion?

How to return data in JSON format using FastAPI?

To return data in JSON format using FastAPI, you can use the jsonable_encoder encoder to convert Python data structures into JSON-compatible data. This can be achieved using either of the following options:

Option 1: Using the jsonable_encoder Automatically

Return data as usual, and FastAPI will automatically handle the JSON conversion. FastAPI internally uses the jsonable_encoder to convert the data to JSON-compatible format. The jsonable_encoder ensures that unsupported objects, like datetime objects, are converted to strings. FastAPI then wraps the data in a JSONResponse object with an application/json media type, which the client receives as a JSON response.

from fastapi.encoders import jsonable_encoder
from fastapi.responses import JSONResponse

def return_dict():
    data_dict = {"name": "John Doe", "age": 30}
    return JSONResponse(content=jsonable_encoder(data_dict))

Option 2: Manual JSON Conversion

If you need to perform custom JSON conversion, you can directly return a Response object with the media_type set to 'application/json' and the content set to the JSON-encoded data. Remember to use the json.dumps() function with the default=str argument to ensure that unsupported objects are converted to strings before being encoded as JSON.

import json
from fastapi import Response

def return_response():
    data_dict = {"name": "John Doe", "age": 30}
    json_data = json.dumps(data_dict, default=str)
    return Response(content=json_data, media_type="application/json")

Additional Notes:

• By default, FastAPI adds a Content-Length and Content-Type header to the response.
• You can specify a custom status code for the response by setting the status_code attribute of the Response or JSONResponse object.

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