How Can I Efficiently Return JSON-Formatted Data in a FastAPI Application?

Returning JSON Formatted Data with FastAPI

When developing a FastAPI application, it's essential to understand the nuances of returning JSON formatted data. This requires delving into the inner workings of FastAPI and comprehending the role played by JSON serialization.

The Dilemma with JSON Serialization

At the heart of the issue lies the use of json.dumps() to serialize objects before returning them. While this approach may seem logical, it introduces redundant serialization as FastAPI automatically JSON-encodes the return value during response generation. This leads to a seemingly incorrect string representation of JSON data rather than a neatly formatted dict.

The Solution

To rectify this, you must allow FastAPI to handle the JSON serialization process. This can be achieved by returning data objects (dicts, lists, etc.) directly. FastAPI will seamlessly convert them to JSON-compatible data using the jsonable_encoder and wrap it in a JSONResponse. The resulting response will contain application/json encoded data, ensuring the desired JSON format.

Implementation Options

Option 1: Using Return Values

Return data objects as you would expect:

@app.get('/')
async def main():
    return d

Behind the scenes, FastAPI will serialize the dict (d) using JSONResponse and encode it with json.dumps().

Option 2: Custom Response Objects

If you require precise control over the response, utilize the Response object directly:

@app.get('/')
async def main():
    return Response(content=json.dumps(d, indent=4, default=str), media_type='application/json')

This approach grants you freedom over the media_type (e.g., 'application/json'), providing customizability.

Note: The default argument to json.dumps() (str) allows for the serialization of datetime objects. By passing an indent, you can control the formatting of the JSON output.

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