How to Return JSON Response in FastAPI: Simple Guide

In FastAPI, you return a JSON response by simply returning a Python dictionary or Pydantic model from your path operation function. FastAPI automatically converts it to JSON using JSONResponse behind the scenes.

Syntax

In FastAPI, you define a function for a route and return a Python dictionary or a Pydantic model instance. FastAPI converts this return value into a JSON response automatically.

• @app.get("/path"): Decorator to define a GET route.
• Function returns a dict or BaseModel instance.
• FastAPI uses JSONResponse internally to send JSON.

python

from fastapi import FastAPI

app = FastAPI()

@app.get("/example")
def example():
    return {"key": "value"}

Example

This example shows a FastAPI app with a GET endpoint that returns a JSON response containing a message and a number. When you visit /json, you get a JSON object automatically.

python

from fastapi import FastAPI

app = FastAPI()

@app.get("/json")
def get_json():
    return {"message": "Hello, FastAPI!", "number": 123}

Output

{"message":"Hello, FastAPI!","number":123}

Common Pitfalls

Some common mistakes when returning JSON in FastAPI include:

• Returning non-serializable objects like sets or custom classes without Pydantic models.
• Forgetting to return a dictionary or Pydantic model, which causes errors.
• Trying to manually create JSON strings instead of returning Python objects.

FastAPI handles conversion, so just return Python data structures.

python

from fastapi import FastAPI

app = FastAPI()

# Wrong: returning a set (not JSON serializable)
@app.get("/wrong")
def wrong():
    return {1, 2, 3}  # This will cause an error

# Right: return a list or dict
@app.get("/right")
def right():
    return [1, 2, 3]

Quick Reference

Tips for returning JSON in FastAPI:

• Return dict, list, or Pydantic models.
• Use Pydantic models for validation and documentation.
• FastAPI auto-converts return values to JSON responses.
• Use from fastapi.responses import JSONResponse only for custom JSON responses.

Key Takeaways

Return Python dicts or Pydantic models from your FastAPI route functions to get JSON responses.

FastAPI automatically converts returned data to JSON using JSONResponse.

Avoid returning non-serializable types like sets or custom objects without Pydantic.

Use Pydantic models for better validation and clear API docs.

Manual JSON string returns are unnecessary and error-prone in FastAPI.