Body - updates

Update replacing with PUT

To update an item you can use the HTTP PUT operation.

You can use the jsonable_encoder to convert the input data to data that can be stored as JSON (e.g. with a NoSQL database). For example, converting datetime to str.

from typing import List

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

app = FastAPI()


class Item(BaseModel):
    name: str = None
    description: str = None
    price: float = None
    tax: float = 10.5
    tags: List[str] = []


items = {
    "foo": {"name": "Foo", "price": 50.2},
    "bar": {"name": "Bar", "description": "The bartenders", "price": 62, "tax": 20.2},
    "baz": {"name": "Baz", "description": None, "price": 50.2, "tax": 10.5, "tags": []},
}


@app.get("/items/{item_id}", response_model=Item)
async def read_item(item_id: str):
    return items[item_id]


@app.put("/items/{item_id}", response_model=Item)
async def update_item(item_id: str, item: Item):
    update_item_encoded = jsonable_encoder(item)
    items[item_id] = update_item_encoded
    return update_item_encoded

PUT is used to receive data that should replace the existing data.

Warning about replacing

That means that if you want to update the item bar using PUT with a body containing:

{
    "name": "Barz",
    "price": 3,
    "description": None,
}

because it doesn't include the already stored attribute "tax": 20.2, the input model would take the default value of "tax": 10.5.

And the data would be saved with that "new" tax of 10.5.

Partial updates with PATCH

You can also use the HTTP PATCH operation to partially update data.

This means that you can send only the data that you want to update, leaving the rest intact.

Note

PATCH is less commonly used and known than PUT.

And many teams use only PUT, even for partial updates.

You are free to use them however you want, FastAPI doesn't impose any restrictions.

But this guide shows you, more or less, how they are intended to be used.

Using Pydantic's exclude_unset parameter

If you want to receive partial updates, it's very useful to use the parameter exclude_unset in Pydantic's model's .dict().

Like item.dict(exclude_unset=True).

That would generate a dict with only the data that was set when creating the item model, excluding default values.

Then you can use this to generate a dict with only the data that was set (sent in the request), omitting default values:

from typing import List

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

app = FastAPI()


class Item(BaseModel):
    name: str = None
    description: str = None
    price: float = None
    tax: float = 10.5
    tags: List[str] = []


items = {
    "foo": {"name": "Foo", "price": 50.2},
    "bar": {"name": "Bar", "description": "The bartenders", "price": 62, "tax": 20.2},
    "baz": {"name": "Baz", "description": None, "price": 50.2, "tax": 10.5, "tags": []},
}


@app.get("/items/{item_id}", response_model=Item)
async def read_item(item_id: str):
    return items[item_id]


@app.patch("/items/{item_id}", response_model=Item)
async def update_item(item_id: str, item: Item):
    stored_item_data = items[item_id]
    stored_item_model = Item(**stored_item_data)
    update_data = item.dict(exclude_unset=True)
    updated_item = stored_item_model.copy(update=update_data)
    items[item_id] = jsonable_encoder(updated_item)
    return updated_item

Using Pydantic's update parameter

Now, you can create a copy of the existing model using .copy(), and pass the update parameter with a dict containing the data to update.

Like stored_item_model.copy(update=update_data):

from typing import List

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

app = FastAPI()


class Item(BaseModel):
    name: str = None
    description: str = None
    price: float = None
    tax: float = 10.5
    tags: List[str] = []


items = {
    "foo": {"name": "Foo", "price": 50.2},
    "bar": {"name": "Bar", "description": "The bartenders", "price": 62, "tax": 20.2},
    "baz": {"name": "Baz", "description": None, "price": 50.2, "tax": 10.5, "tags": []},
}


@app.get("/items/{item_id}", response_model=Item)
async def read_item(item_id: str):
    return items[item_id]


@app.patch("/items/{item_id}", response_model=Item)
async def update_item(item_id: str, item: Item):
    stored_item_data = items[item_id]
    stored_item_model = Item(**stored_item_data)
    update_data = item.dict(exclude_unset=True)
    updated_item = stored_item_model.copy(update=update_data)
    items[item_id] = jsonable_encoder(updated_item)
    return updated_item

Partial updates recap

In summary, to apply partial updates you would:

  • (Optionally) use PATCH instead of PUT.
  • Retrieve the stored data.
  • Put that data in a Pydantic model.
  • Generate a dict without default values from the input model (using exclude_unset).
    • This way you can update only the values actually set by the user, instead of overriding values already stored with default values in your model.
  • Create a copy of the stored model, updating it's attributes with the received partial updates (using the update parameter).
  • Convert the copied model to something that can be stored in your DB (for example, using the jsonable_encoder).
    • This is comparable to using the model's .dict() method again, but it makes sure (and converts) the values to data types that can be converted to JSON, for example, datetime to str.
  • Save the data to your DB.
  • Return the updated model.
from typing import List

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

app = FastAPI()


class Item(BaseModel):
    name: str = None
    description: str = None
    price: float = None
    tax: float = 10.5
    tags: List[str] = []


items = {
    "foo": {"name": "Foo", "price": 50.2},
    "bar": {"name": "Bar", "description": "The bartenders", "price": 62, "tax": 20.2},
    "baz": {"name": "Baz", "description": None, "price": 50.2, "tax": 10.5, "tags": []},
}


@app.get("/items/{item_id}", response_model=Item)
async def read_item(item_id: str):
    return items[item_id]


@app.patch("/items/{item_id}", response_model=Item)
async def update_item(item_id: str, item: Item):
    stored_item_data = items[item_id]
    stored_item_model = Item(**stored_item_data)
    update_data = item.dict(exclude_unset=True)
    updated_item = stored_item_model.copy(update=update_data)
    items[item_id] = jsonable_encoder(updated_item)
    return updated_item

Tip

You can actually use this same technique with an HTTP PUT operation.

But the example here uses PATCH because it was created for these use cases.

Note

Notice that the input model is still validated.

So, if you want to receive partial updates that can omit all the attributes, you need to have a model with all the attributes marked as optional (with default values or None).

To distinguish from the models with all optional values for updates and models with required values for creation, you can use the ideas described in Extra Models.