# Body - Updates

## Update replacing with `PUT`

To update an item you can use the [HTTP `PUT`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/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`.

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

app = FastAPI()


class Item(BaseModel):
    name: str | None = None
    description: str | None = None
    price: float | None = 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:

```Python
{
    "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`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/PATCH) operation to _partially_ update data.

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

:::callout{intent="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 `.model_dump()`.

Like `item.model_dump(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:

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

app = FastAPI()


class Item(BaseModel):
    name: str | None = None
    description: str | None = None
    price: float | None = 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}")
async def update_item(item_id: str, item: Item) -> Item:
    stored_item_data = items[item_id]
    stored_item_model = Item(**stored_item_data)
    update_data = item.model_dump(exclude_unset=True)
    updated_item = stored_item_model.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 `.model_copy()`, and pass the `update` parameter with a `dict` containing the data to update.

Like `stored_item_model.model_copy(update=update_data)`:

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

app = FastAPI()


class Item(BaseModel):
    name: str | None = None
    description: str | None = None
    price: float | None = 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}")
async def update_item(item_id: str, item: Item) -> Item:
    stored_item_data = items[item_id]
    stored_item_model = Item(**stored_item_data)
    update_data = item.model_dump(exclude_unset=True)
    updated_item = stored_item_model.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 its 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 `.model_dump()` 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.

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

app = FastAPI()


class Item(BaseModel):
    name: str | None = None
    description: str | None = None
    price: float | None = 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}")
async def update_item(item_id: str, item: Item) -> Item:
    stored_item_data = items[item_id]
    stored_item_model = Item(**stored_item_data)
    update_data = item.model_dump(exclude_unset=True)
    updated_item = stored_item_model.model_copy(update=update_data)
    items[item_id] = jsonable_encoder(updated_item)
    return updated_item
```

:::callout{intent="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.
:::

:::callout{intent="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](/guides/tutorial-user-guide-extra-models).
:::

## Related pages

- [About](./about-index.md)
- [Advanced User Guide](./advanced-user-guide-index.md)
- [Deployment](./deployment-index.md)
- [FastAPI](./fastapi-index.md)
- [FastAPI Docs](../index.md)
- [Features](./features-index.md)
- [How To - Recipes](./how-to-recipes-index.md)
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