FastAPI JSON Response

Introduction

When building web APIs, you'll frequently need to return data to clients in a structured format. JSON (JavaScript Object Notation) has become the de facto standard for data exchange in web applications. FastAPI, being a modern API framework, provides excellent support for JSON responses through its JSONResponse class and automatic serialization.

In this tutorial, you'll learn how to:

Return JSON responses using FastAPI
Customize JSON responses with status codes and headers
Handle different data types in JSON responses
Use Pydantic models with JSON responses
Implement advanced JSON response patterns

JSON Response Basics

FastAPI automatically converts Python dictionaries, lists, and Pydantic models to JSON responses. Let's start with the basics:

Automatic JSON Conversion

python
from fastapi import FastAPI

app = FastAPI()

@app.get("/items/")
async def read_items():
    return {"message": "Hello World", "items": [1, 2, 3, 4, 5]}

When you access the /items/ endpoint, FastAPI will:

Take the dictionary you returned
Convert it to JSON
Set the Content-Type header to application/json
Send the JSON response to the client

The response would look like:

json
{
  "message": "Hello World",
  "items": [1, 2, 3, 4, 5]
}

Using the JSONResponse Explicitly

While FastAPI handles JSON conversion automatically, sometimes you need more control. That's where the JSONResponse class comes in:

python
from fastapi import FastAPI
from fastapi.responses import JSONResponse

app = FastAPI()

@app.get("/custom-response/")
async def get_custom_response():
    content = {
        "message": "This is a custom JSON response",
        "status": "success"
    }
    return JSONResponse(content=content, status_code=200)

Setting Custom Status Codes and Headers

You can customize the status code and headers:

python
from fastapi import FastAPI
from fastapi.responses import JSONResponse

app = FastAPI()

@app.get("/not-found/")
async def read_not_found():
    content = {"error": "Resource not found"}
    headers = {"X-Error-Code": "RESOURCE_NOT_FOUND"}
    
    return JSONResponse(
        content=content,
        status_code=404,
        headers=headers
    )

Working with Pydantic Models

Pydantic models work seamlessly with FastAPI's JSON response handling:

python
from fastapi import FastAPI
from pydantic import BaseModel
from typing import List, Optional

app = FastAPI()

class Item(BaseModel):
    id: int
    name: str
    description: Optional[str] = None
    price: float
    tags: List[str] = []

@app.get("/items/{item_id}")
async def read_item(item_id: int):
    # In a real app, you would fetch from a database
    item = Item(
        id=item_id,
        name="Example Item",
        description="This is an example item",
        price=45.5,
        tags=["example", "item"]
    )
    return item  # FastAPI automatically converts the Pydantic model to JSON

The response will be:

json
{
  "id": 1,
  "name": "Example Item",
  "description": "This is an example item",
  "price": 45.5,
  "tags": ["example", "item"]
}

Handling Different Data Types

FastAPI handles various Python data types in JSON responses:

Lists and Nested Objects

python
from fastapi import FastAPI

app = FastAPI()

@app.get("/complex-data/")
async def get_complex_data():
    return {
        "string_value": "hello",
        "integer_value": 42,
        "float_value": 3.14,
        "boolean_value": True,
        "null_value": None,
        "array": [1, 2, 3, 4, 5],
        "nested_object": {
            "name": "John",
            "age": 30,
            "address": {
                "street": "123 Main St",
                "city": "New York"
            }
        },
        "mixed_array": [
            {"id": 1, "name": "Item 1"},
            {"id": 2, "name": "Item 2"}
        ]
    }

Date and Time Objects

python
from fastapi import FastAPI
from datetime import datetime, date

app = FastAPI()

@app.get("/datetime-example/")
async def get_datetime_example():
    return {
        "current_datetime": datetime.now(),
        "current_date": date.today(),
        "specific_date": datetime(2023, 1, 1, 12, 0, 0)
    }

FastAPI's JSON encoder will convert datetime objects to ISO format strings automatically.

Real-World Example: Building an API with JSON Responses

Let's build a simple API for a book repository:

python
from fastapi import FastAPI, HTTPException, Query
from pydantic import BaseModel
from typing import List, Optional
from fastapi.responses import JSONResponse

app = FastAPI()

# Define our data model
class Book(BaseModel):
    id: int
    title: str
    author: str
    year: int
    genre: Optional[str] = None

# Sample data (in a real app, you'd use a database)
books_db = [
    Book(id=1, title="To Kill a Mockingbird", author="Harper Lee", year=1960, genre="Fiction"),
    Book(id=2, title="1984", author="George Orwell", year=1949, genre="Dystopian"),
    Book(id=3, title="The Great Gatsby", author="F. Scott Fitzgerald", year=1925, genre="Fiction"),
]

@app.get("/books/", response_model=List[Book])
async def get_all_books(
    genre: Optional[str] = Query(None, description="Filter books by genre")
):
    if genre:
        filtered_books = [book for book in books_db if book.genre == genre]
        return filtered_books
    return books_db

@app.get("/books/{book_id}", response_model=Book)
async def get_book(book_id: int):
    for book in books_db:
        if book.id == book_id:
            return book
    
    # Custom error response
    error_content = {
        "error": "Book not found",
        "details": f"Book with ID {book_id} does not exist"
    }
    return JSONResponse(status_code=404, content=error_content)

@app.post("/books/", response_model=Book, status_code=201)
async def create_book(book: Book):
    # In a real app, you would save to a database
    # Check if book with same ID exists
    if any(b.id == book.id for b in books_db):
        error_content = {
            "error": "Duplicate ID",
            "details": f"Book with ID {book.id} already exists"
        }
        return JSONResponse(status_code=400, content=error_content)
        
    books_db.append(book)
    return book

Advanced Techniques

Custom JSON Encoding

Sometimes you need to handle custom objects that aren't natively serializable to JSON:

python
from fastapi import FastAPI
from fastapi.responses import JSONResponse
import json
from typing import Any

class CustomJSONEncoder(json.JSONEncoder):
    def default(self, obj: Any) -> Any:
        if isinstance(obj, set):
            return list(obj)  # Convert sets to lists
        # Add other custom type conversions here
        return super().default(obj)

app = FastAPI()

@app.get("/custom-types/")
async def get_custom_types():
    data = {
        "my_set": {1, 2, 3, 4},  # Sets aren't JSON serializable by default
        "regular_list": [5, 6, 7]
    }
    
    # Use custom JSON encoder
    json_data = json.dumps(data, cls=CustomJSONEncoder)
    return JSONResponse(content=json.loads(json_data))

Streaming JSON Responses

For large datasets, you might want to stream the response:

python
from fastapi import FastAPI
from fastapi.responses import StreamingResponse
import json

app = FastAPI()

@app.get("/stream-data/")
async def stream_data():
    async def generate_large_data():
        # Yield the opening bracket
        yield "{"
        
        # Stream items one by one
        for i in range(10000):
            prefix = "" if i == 0 else ","
            yield f'{prefix}"item_{i}": {i}'
        
        # Yield the closing bracket
        yield "}"
    
    return StreamingResponse(
        generate_large_data(), 
        media_type="application/json"
    )

Error Handling with JSON Responses

FastAPI allows you to create custom exception handlers that return JSON:

python
from fastapi import FastAPI, HTTPException, Request
from fastapi.responses import JSONResponse

app = FastAPI()

class CustomException(Exception):
    def __init__(self, name: str, code: str, message: str):
        self.name = name
        self.code = code
        self.message = message

@app.exception_handler(CustomException)
async def custom_exception_handler(request: Request, exc: CustomException):
    return JSONResponse(
        status_code=400,
        content={
            "error": exc.name,
            "code": exc.code,
            "message": exc.message,
            "path": request.url.path
        }
    )

@app.get("/trigger-error/")
async def trigger_error():
    raise CustomException(
        name="ValidationError",
        code="USER_ID_INVALID",
        message="The user ID provided is not valid"
    )

Summary

FastAPI provides robust support for JSON responses:

Automatic conversion: FastAPI automatically converts Python dictionaries, lists, and Pydantic models to JSON
Explicit responses: Use JSONResponse when you need more control over status codes and headers
Pydantic integration: Pydantic models are automatically serialized to JSON
Data type handling: FastAPI handles various Python data types, including datetime objects
Custom encoding: You can implement custom JSON encoders for special types
Error handling: Use custom exception handlers to return formatted JSON error responses

JSON responses are a fundamental part of building RESTful APIs with FastAPI. With the techniques covered in this tutorial, you have the knowledge to create standardized, well-structured APIs that communicate effectively with clients using JSON.

Additional Resources

Exercises

Create a FastAPI endpoint that returns a nested JSON structure with at least three levels of nesting.
Implement an endpoint that returns a list of Pydantic models with at least 5 fields each.
Create a custom exception handler for ValueError that returns appropriate JSON error responses.
Build a simple REST API with CRUD operations (Create, Read, Update, Delete) for a resource of your choice, using appropriate status codes and JSON responses.
Implement a custom JSON encoder that can handle custom Python classes by converting them to dictionaries.

If you spot any mistakes on this website, please let me know at [email protected]. I’d greatly appreciate your feedback! :)

Introduction​

JSON Response Basics​

Automatic JSON Conversion​

Using the JSONResponse Explicitly​

Setting Custom Status Codes and Headers​

Working with Pydantic Models​

Handling Different Data Types​

Lists and Nested Objects​

Date and Time Objects​

Real-World Example: Building an API with JSON Responses​

Advanced Techniques​

Custom JSON Encoding​

Streaming JSON Responses​

Error Handling with JSON Responses​

Summary​

Additional Resources​

Exercises​