📝 Use Optional in docs (#1644)

* Updated .py files with Optional tag (up to body_nested_models)

* Update optionals

* docs_src/ all updates, few I was unsure of

* Updated markdown files with Optional param

* es: Add Optional typing to index.md

* Last of markdown files updated with Optional param

* Update highlight lines

* it: Add Optional typings

* README.md: Update with Optional typings

* Update more highlight increments

* Update highlights

* schema-extra-example.md: Update highlights

* updating highlighting on website to reflect .py changes

* Update highlighting for query-params & response-directly

* Address PR comments

* Get rid of unnecessary comment

* ⏪ Revert Optional in Chinese docs as it probably also requires changes in text

* 🎨 Apply format

* ⏪ Revert modified example

* ♻️ Simplify example in docs

* 📝 Update OpenAPI callback example to use Optional

* ✨ Add Optional types to tests

* 📝 Update docs about query params, default to using Optional

* 🎨 Update code examples line highlighting

* 📝 Update nested models docs to use "type parameters" instead of "subtypes"

* 📝 Add notes about FastAPI usage of None

including:

= None

and

= Query(None)

and clarify relationship with Optional[str]

* 📝 Add note about response_model_by_alias

* ♻️ Simplify query param list example

* 🔥 Remove test for removed example

* ✅ Update test for updated example

Co-authored-by: Christopher Nguyen <chrisngyn99@gmail.com>
Co-authored-by: yk396 <yk396@cornell.edu>
Co-authored-by: Kai Chen <kaichen120@gmail.com>

docs_src/openapi_callbacks/tutorial001.py

@@ -1,3 +1,5 @@
+from typing import Optional
+
 from fastapi import APIRouter, FastAPI
 from fastapi.responses import JSONResponse
 from pydantic import BaseModel, HttpUrl
@@ -7,7 +9,7 @@ app = FastAPI()
 
 class Invoice(BaseModel):
     id: str
-    title: str = None
+    title: Optional[str] = None
     customer: str
     total: float
 
@@ -32,7 +34,7 @@ def invoice_notification(body: InvoiceEvent):
 
 
 @app.post("/invoices/", callbacks=invoices_callback_router.routes)
-def create_invoice(invoice: Invoice, callback_url: HttpUrl = None):
+def create_invoice(invoice: Invoice, callback_url: Optional[HttpUrl] = None):
     """
     Create an invoice.