# Separate OpenAPI Schemas for Input and Output or Not
Since **Pydantic v2** was released, the generated OpenAPI is a bit more exact and **correct** than before. 😎
In fact, in some cases, it will even have **two JSON Schemas** in OpenAPI for the same Pydantic model, for input and output, depending on if they have **default values**.
Let's see how that works and how to change it if you need to do that.
## Pydantic Models for Input and Output
Let's say you have a Pydantic model with default values, like this one:
```python
from fastapi import FastAPI
from pydantic import BaseModel
class Item(BaseModel):
name: str
description: str | None = None
app = FastAPI()
@app.post("/items/")
def create_item(item: Item):
return item
@app.get("/items/")
def read_items() -> list[Item]:
return [
Item(
name="Portal Gun",
description="Device to travel through the multi-rick-verse",
),
Item(name="Plumbus"),
]
```
### Model for Input
If you use this model as an input like here:
```python
from fastapi import FastAPI
from pydantic import BaseModel
class Item(BaseModel):
name: str
description: str | None = None
app = FastAPI()
@app.post("/items/")
def create_item(item: Item):
return item
@app.get("/items/")
def read_items() -> list[Item]:
return [
Item(
name="Portal Gun",
description="Device to travel through the multi-rick-verse",
),
Item(name="Plumbus"),
]
```
...then the `description` field will **not be required**. Because it has a default value of `None`.
### Input Model in Docs
You can confirm that in the docs, the `description` field doesn't have a **red asterisk**, it's not marked as required:
### Model for Output
But if you use the same model as an output, like here:
```python
from fastapi import FastAPI
from pydantic import BaseModel
class Item(BaseModel):
name: str
description: str | None = None
app = FastAPI()
@app.post("/items/")
def create_item(item: Item):
return item
@app.get("/items/")
def read_items() -> list[Item]:
return [
Item(
name="Portal Gun",
description="Device to travel through the multi-rick-verse",
),
Item(name="Plumbus"),
]
```
...then because `description` has a default value, if you **don't return anything** for that field, it will still have that **default value**.
### Model for Output Response Data
If you interact with the docs and check the response, even though the code didn't add anything in one of the `description` fields, the JSON response contains the default value (`null`):
This means that it will **always have a value**, it's just that sometimes the value could be `None` (or `null` in JSON).
That means that clients using your API don't have to check if the value exists or not, they can **assume the field will always be there**, but just that in some cases it will have the default value of `None`.
The way to describe this in OpenAPI, is to mark that field as **required**, because it will always be there.
Because of that, the JSON Schema for a model can be different depending on if it's used for **input or output**:
- for **input** the `description` will **not be required**
- for **output** it will be **required** (and possibly `None`, or in JSON terms, `null`)
### Model for Output in Docs
You can check the output model in the docs too, **both** `name` and `description` are marked as **required** with a **red asterisk**:
### Model for Input and Output in Docs
And if you check all the available Schemas (JSON Schemas) in OpenAPI, you will see that there are two, one `Item-Input` and one `Item-Output`.
For `Item-Input`, `description` is **not required**, it doesn't have a red asterisk.
But for `Item-Output`, `description` is **required**, it has a red asterisk.
With this feature from **Pydantic v2**, your API documentation is more **precise**, and if you have autogenerated clients and SDKs, they will be more precise too, with a better **developer experience** and consistency. 🎉
## Do not Separate Schemas
Now, there are some cases where you might want to have the **same schema for input and output**.
Probably the main use case for this is if you already have some autogenerated client code/SDKs and you don't want to update all the autogenerated client code/SDKs yet, you probably will want to do it at some point, but maybe not right now.
In that case, you can disable this feature in **FastAPI**, with the parameter `separate_input_output_schemas=False`.
:::callout{intent="note"}
Support for `separate_input_output_schemas` was added in FastAPI `0.102.0`. 🤓
:::
```python
from fastapi import FastAPI
from pydantic import BaseModel
class Item(BaseModel):
name: str
description: str | None = None
app = FastAPI(separate_input_output_schemas=False)
@app.post("/items/")
def create_item(item: Item):
return item
@app.get("/items/")
def read_items() -> list[Item]:
return [
Item(
name="Portal Gun",
description="Device to travel through the multi-rick-verse",
),
Item(name="Plumbus"),
]
```
### Same Schema for Input and Output Models in Docs
And now there will be one single schema for input and output for the model, only `Item`, and it will have `description` as **not required**:
## 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)
- [Learn](./learn-index.md)
- [More](./more-index.md)
- [OpenAPI](./openapi-index.md)
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