JSON Compatible Encoder

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There are some cases where you might need to convert a data type (like a Pydantic model) to something compatible with JSON (like a dict, list, etc).

For example, if you need to store it in a database.

For that, FastAPI provides a jsonable_encoder() function.

Using the jsonable_encoder

Let's imagine that you have a database fake_db that only receives JSON compatible data.

For example, it doesn't receive datetime objects, as those are not compatible with JSON.

So, a datetime object would have to be converted to a str containing the data in ISO format.

The same way, this database wouldn't receive a Pydantic model (an object with attributes), only a dict.

You can use jsonable_encoder for that.

It receives an object, like a Pydantic model, and returns a JSON compatible version:

Python 3.10+

from datetime import datetime

from fastapi import FastAPI
from fastapi.encoders import jsonable_encoder
from pydantic import BaseModel

fake_db = {}


class Item(BaseModel):
    title: str
    timestamp: datetime
    description: str | None = None


app = FastAPI()


@app.put("/items/{id}")
def update_item(id: str, item: Item):
    json_compatible_item_data = jsonable_encoder(item)
    fake_db[id] = json_compatible_item_data

🤓 Other versions and variants

Python 3.8+

from datetime import datetime
from typing import Union

from fastapi import FastAPI
from fastapi.encoders import jsonable_encoder
from pydantic import BaseModel

fake_db = {}


class Item(BaseModel):
    title: str
    timestamp: datetime
    description: Union[str, None] = None


app = FastAPI()


@app.put("/items/{id}")
def update_item(id: str, item: Item):
    json_compatible_item_data = jsonable_encoder(item)
    fake_db[id] = json_compatible_item_data

In this example, it would convert the Pydantic model to a dict, and the datetime to a str.

The result of calling it is something that can be encoded with the Python standard json.dumps().

It doesn't return a large str containing the data in JSON format (as a string). It returns a Python standard data structure (e.g. a dict) with values and sub-values that are all compatible with JSON.

Note

jsonable_encoder is actually used by FastAPI internally to convert data. But it is useful in many other scenarios.