✨ Add support for disabling the separation of input and output JSON Schemas in OpenAPI with Pydantic v2 (#10145)

* 📝 Add docs for Separate OpenAPI Schemas for Input and Output

* 🔧 Add new docs page to MkDocs config

* ✨ Add separate_input_output_schemas parameter to FastAPI class

* 📝 Add source examples for separating OpenAPI schemas

* ✅ Add tests for separated OpenAPI schemas

* 📝 Add source examples for Python 3.10, 3.9, and 3.7+

* 📝 Update docs for Separate OpenAPI Schemas with new multi-version examples

* ✅ Add and update tests for different Python versions

* ✅ Add tests for corner cases with separate_input_output_schemas

* 📝 Update tutorial to use Union instead of Optional

* 🐛 Fix type annotations

* 🐛 Fix correct import in test

* 💄 Add CSS to simulate browser windows for screenshots

* ➕ Add playwright as a dev dependency to automate generating screenshots

* 🔨 Add Playwright scripts to generate screenshots for new docs

* 📝 Update docs, tweak text to match screenshots

* 🍱 Add screenshots for new docs

docs/en/docs/css/custom.css

@@ -144,3 +144,39 @@ code {
   margin-top: 2em;
   margin-bottom: 2em;
 }
+
+/* Screenshots */
+/*
+Simulate a browser window frame.
+Inspired by Termynal's CSS tricks with modifications
+*/
+
+.screenshot {
+  display: block;
+  background-color: #d3e0de;
+  border-radius: 4px;
+  padding: 45px 5px 5px;
+  position: relative;
+  -webkit-box-sizing: border-box;
+          box-sizing: border-box;
+}
+
+.screenshot img {
+  display: block;
+  border-radius: 2px;
+}
+
+.screenshot:before {
+  content: '';
+  position: absolute;
+  top: 15px;
+  left: 15px;
+  display: inline-block;
+  width: 15px;
+  height: 15px;
+  border-radius: 50%;
+  /* A little hack to display the window buttons in one pseudo element. */
+  background: #d9515d;
+  -webkit-box-shadow: 25px 0 0 #f4c025, 50px 0 0 #3ec930;
+          box-shadow: 25px 0 0 #f4c025, 50px 0 0 #3ec930;
+}

docs/en/docs/how-to/separate-openapi-schemas.md

@@ -0,0 +1,228 @@
+# Separate OpenAPI Schemas for Input and Output or Not
+
+When using **Pydantic v2**, 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 3.10+"
+
+    ```Python hl_lines="7"
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001_py310.py[ln:1-7]!}
+
+    # Code below omitted 👇
+    ```
+
+    <details>
+    <summary>👀 Full file preview</summary>
+
+    ```Python
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001_py310.py!}
+    ```
+
+    </details>
+
+=== "Python 3.9+"
+
+    ```Python hl_lines="9"
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001_py39.py[ln:1-9]!}
+
+    # Code below omitted 👇
+    ```
+
+    <details>
+    <summary>👀 Full file preview</summary>
+
+    ```Python
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001_py39.py!}
+    ```
+
+    </details>
+
+=== "Python 3.7+"
+
+    ```Python hl_lines="9"
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001.py[ln:1-9]!}
+
+    # Code below omitted 👇
+    ```
+
+    <details>
+    <summary>👀 Full file preview</summary>
+
+    ```Python
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001.py!}
+    ```
+
+    </details>
+
+### Model for Input
+
+If you use this model as an input like here:
+
+=== "Python 3.10+"
+
+    ```Python hl_lines="14"
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001_py310.py[ln:1-15]!}
+
+    # Code below omitted 👇
+    ```
+
+    <details>
+    <summary>👀 Full file preview</summary>
+
+    ```Python
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001_py310.py!}
+    ```
+
+    </details>
+
+=== "Python 3.9+"
+
+    ```Python hl_lines="16"
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001_py39.py[ln:1-17]!}
+
+    # Code below omitted 👇
+    ```
+
+    <details>
+    <summary>👀 Full file preview</summary>
+
+    ```Python
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001_py39.py!}
+    ```
+
+    </details>
+
+=== "Python 3.7+"
+
+    ```Python hl_lines="16"
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001.py[ln:1-17]!}
+
+    # Code below omitted 👇
+    ```
+
+    <details>
+    <summary>👀 Full file preview</summary>
+
+    ```Python
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001.py!}
+    ```
+
+    </details>
+
+...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:
+
+<div class="screenshot">
+<img src="/img/tutorial/separate-openapi-schemas/image01.png">
+</div>
+
+### Model for Output
+
+But if you use the same model as an output, like here:
+
+=== "Python 3.10+"
+
+    ```Python hl_lines="19"
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001_py310.py!}
+    ```
+
+=== "Python 3.9+"
+
+    ```Python hl_lines="21"
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001_py39.py!}
+    ```
+
+=== "Python 3.7+"
+
+    ```Python hl_lines="21"
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial001.py!}
+    ```
+
+...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`):
+
+<div class="screenshot">
+<img src="/img/tutorial/separate-openapi-schemas/image02.png">
+</div>
+
+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 **asume 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**:
+
+<div class="screenshot">
+<img src="/img/tutorial/separate-openapi-schemas/image03.png">
+</div>
+
+### 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.
+
+<div class="screenshot">
+<img src="/img/tutorial/separate-openapi-schemas/image04.png">
+</div>
+
+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`.
+
+=== "Python 3.10+"
+
+    ```Python hl_lines="10"
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial002_py310.py!}
+    ```
+
+=== "Python 3.9+"
+
+    ```Python hl_lines="12"
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial002_py39.py!}
+    ```
+
+=== "Python 3.7+"
+
+    ```Python hl_lines="12"
+    {!> ../../../docs_src/separate_openapi_schemas/tutorial002.py!}
+    ```
+
+### 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**:
+
+<div class="screenshot">
+<img src="/img/tutorial/separate-openapi-schemas/image05.png">
+</div>
+
+This is the same behavior as in Pydantic v1. 🤓

docs/en/mkdocs.yml

@@ -176,6 +176,7 @@ nav:
   - how-to/custom-request-and-route.md
   - how-to/conditional-openapi.md
   - how-to/extending-openapi.md
+  - how-to/separate-openapi-schemas.md
   - how-to/custom-docs-ui-assets.md
   - how-to/configure-swagger-ui.md
 - project-generation.md