📝 Add docs recommending Union over Optional and migrate source ex…

…amples (#4908)

* 📝 Add docs recommending Union over Optional

* 📝 Update docs recommending Union over Optional

* 📝 Update source examples for docs, recommend Union over Optional

* 📝 Update highlighted lines with updated source examples

* 📝 Update highlighted lines in Markdown with recent code changes

* 📝 Update docs, use Union instead of Optional

* ♻️ Update source examples to recommend Union over Optional

* 🎨 Update highlighted code in Markdown after moving from Optional to Union

README.md

@@ -151,7 +151,7 @@ $ pip install "uvicorn[standard]"
 * Create a file `main.py` with:
 
 ```Python
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 
@@ -164,7 +164,7 @@ def read_root():
 
 
 @app.get("/items/{item_id}")
-def read_item(item_id: int, q: Optional[str] = None):
+def read_item(item_id: int, q: Union[str, None] = None):
     return {"item_id": item_id, "q": q}
 ```
 
@@ -174,7 +174,7 @@ def read_item(item_id: int, q: Optional[str] = None):
 If your code uses `async` / `await`, use `async def`:
 
 ```Python hl_lines="9  14"
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 
@@ -187,7 +187,7 @@ async def read_root():
 
 
 @app.get("/items/{item_id}")
-async def read_item(item_id: int, q: Optional[str] = None):
+async def read_item(item_id: int, q: Union[str, None] = None):
     return {"item_id": item_id, "q": q}
 ```
 
@@ -266,7 +266,7 @@ Now modify the file `main.py` to receive a body from a `PUT` request.
 Declare the body using standard Python types, thanks to Pydantic.
 
 ```Python hl_lines="4  9-12  25-27"
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 from pydantic import BaseModel
@@ -277,7 +277,7 @@ app = FastAPI()
 class Item(BaseModel):
     name: str
     price: float
-    is_offer: Optional[bool] = None
+    is_offer: Union[bool, None] = None
 
 
 @app.get("/")
@@ -286,7 +286,7 @@ def read_root():
 
 
 @app.get("/items/{item_id}")
-def read_item(item_id: int, q: Optional[str] = None):
+def read_item(item_id: int, q: Union[str, None] = None):
     return {"item_id": item_id, "q": q}
 
 

docs/de/docs/index.md

@@ -149,7 +149,7 @@ $ pip install uvicorn[standard]
 * Create a file `main.py` with:
 
 ```Python
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 
@@ -162,7 +162,7 @@ def read_root():
 
 
 @app.get("/items/{item_id}")
-def read_item(item_id: int, q: Optional[str] = None):
+def read_item(item_id: int, q: Union[str, None] = None):
     return {"item_id": item_id, "q": q}
 ```
 
@@ -172,7 +172,7 @@ def read_item(item_id: int, q: Optional[str] = None):
 If your code uses `async` / `await`, use `async def`:
 
 ```Python hl_lines="9  14"
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 
@@ -185,7 +185,7 @@ async def read_root():
 
 
 @app.get("/items/{item_id}")
-async def read_item(item_id: int, q: Optional[str] = None):
+async def read_item(item_id: int, q: Union[str, None] = None):
     return {"item_id": item_id, "q": q}
 ```
 
@@ -264,7 +264,7 @@ Now modify the file `main.py` to receive a body from a `PUT` request.
 Declare the body using standard Python types, thanks to Pydantic.
 
 ```Python hl_lines="4  9-12  25-27"
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 from pydantic import BaseModel
@@ -275,7 +275,7 @@ app = FastAPI()
 class Item(BaseModel):
     name: str
     price: float
-    is_offer: Optional[bool] = None
+    is_offer: Union[bool, None] = None
 
 
 @app.get("/")
@@ -284,7 +284,7 @@ def read_root():
 
 
 @app.get("/items/{item_id}")
-def read_item(item_id: int, q: Optional[str] = None):
+def read_item(item_id: int, q: Union[str, None] = None):
     return {"item_id": item_id, "q": q}
 
 

docs/en/docs/advanced/additional-status-codes.md

@@ -14,7 +14,7 @@ But you also want it to accept new items. And when the items didn't exist before
 
 To achieve that, import `JSONResponse`, and return your content there directly, setting the `status_code` that you want:
 
-```Python hl_lines="4  23"
+```Python hl_lines="4  25"
 {!../../../docs_src/additional_status_codes/tutorial001.py!}
 ```
 

docs/en/docs/advanced/testing-dependencies.md

@@ -28,7 +28,7 @@ To override a dependency for testing, you put as a key the original dependency (
 
 And then **FastAPI** will call that override instead of the original dependency.
 
-```Python hl_lines="26-27  30"
+```Python hl_lines="28-29  32"
 {!../../../docs_src/dependency_testing/tutorial001.py!}
 ```
 

docs/en/docs/deployment/docker.md

@@ -142,7 +142,7 @@ Successfully installed fastapi pydantic uvicorn
 * Create a `main.py` file with:
 
 ```Python
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 
@@ -155,7 +155,7 @@ def read_root():
 
 
 @app.get("/items/{item_id}")
-def read_item(item_id: int, q: Optional[str] = None):
+def read_item(item_id: int, q: Union[str, None] = None):
     return {"item_id": item_id, "q": q}
 ```
 

docs/en/docs/index.md

@@ -148,7 +148,7 @@ $ pip install "uvicorn[standard]"
 * Create a file `main.py` with:
 
 ```Python
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 
@@ -161,7 +161,7 @@ def read_root():
 
 
 @app.get("/items/{item_id}")
-def read_item(item_id: int, q: Optional[str] = None):
+def read_item(item_id: int, q: Union[str, None] = None):
     return {"item_id": item_id, "q": q}
 ```
 
@@ -171,7 +171,7 @@ def read_item(item_id: int, q: Optional[str] = None):
 If your code uses `async` / `await`, use `async def`:
 
 ```Python hl_lines="9  14"
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 
@@ -184,7 +184,7 @@ async def read_root():
 
 
 @app.get("/items/{item_id}")
-async def read_item(item_id: int, q: Optional[str] = None):
+async def read_item(item_id: int, q: Union[str, None] = None):
     return {"item_id": item_id, "q": q}
 ```
 
@@ -263,7 +263,7 @@ Now modify the file `main.py` to receive a body from a `PUT` request.
 Declare the body using standard Python types, thanks to Pydantic.
 
 ```Python hl_lines="4  9-12  25-27"
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 from pydantic import BaseModel
@@ -274,7 +274,7 @@ app = FastAPI()
 class Item(BaseModel):
     name: str
     price: float
-    is_offer: Optional[bool] = None
+    is_offer: Union[bool, None] = None
 
 
 @app.get("/")
@@ -283,7 +283,7 @@ def read_root():
 
 
 @app.get("/items/{item_id}")
-def read_item(item_id: int, q: Optional[str] = None):
+def read_item(item_id: int, q: Union[str, None] = None):
     return {"item_id": item_id, "q": q}
 
 

docs/en/docs/python-types.md

@@ -317,6 +317,45 @@ This also means that in Python 3.10, you can use `Something | None`:
     {!> ../../../docs_src/python_types/tutorial009_py310.py!}
     ```
 
+#### Using `Union` or `Optional`
+
+If you are using a Python version below 3.10, here's a tip from my very **subjective** point of view:
+
+* 🚨 Avoid using `Optional[SomeType]`
+* Instead ✨ **use `Union[SomeType, None]`** ✨.
+
+Both are equivalent and underneath they are the same, but I would recommend `Union` instead of `Optional` because the word "**optional**" would seem to imply that the value is optional, and it actually means "it can be `None`", even if it's not optional and is still required.
+
+I think `Union[str, SomeType]` is more explicit about what it means.
+
+It's just about the words and names. But those words can affect how you and your teammates think about the code.
+
+As an example, let's take this function:
+
+```Python hl_lines="1  4"
+{!../../../docs_src/python_types/tutorial009c.py!}
+```
+
+The parameter `name` is defined as `Optional[str]`, but it is **not optional**, you cannot call the function without the parameter:
+
+```Python
+say_hi()  # Oh, no, this throws an error! 😱
+```
+
+The `name` parameter is **still required** (not *optional*) because it doesn't have a default value. Still, `name` accepts `None` as the value:
+
+```Python
+say_hi(name=None)  # This works, None is valid 🎉
+```
+
+The good news is, once you are on Python 3.10 you won't have to worry about that, as you will be able to simply use `|` to define unions of types:
+
+```Python hl_lines="1  4"
+{!../../../docs_src/python_types/tutorial009c_py310.py!}
+```
+
+And then you won't have to worry about names like `Optional` and `Union`. 😎
+
 #### Generic types
 
 These types that take type parameters in square brackets are called **Generic types** or **Generics**, for example:
@@ -422,6 +461,9 @@ An example from the official Pydantic docs:
 
 You will see a lot more of all this in practice in the [Tutorial - User Guide](tutorial/index.md){.internal-link target=_blank}.
 
+!!! tip
+    Pydantic has a special behavior when you use `Optional` or `Union[Something, None]` without a default value, you can read more about it in the Pydantic docs about <a href="https://pydantic-docs.helpmanual.io/usage/models/#required-optional-fields" class="external-link" target="_blank">Required Optional fields</a>.
+
 ## Type hints in **FastAPI**
 
 **FastAPI** takes advantage of these type hints to do several things.

docs/en/docs/tutorial/body-multiple-params.md

@@ -89,13 +89,13 @@ But you can instruct **FastAPI** to treat it as another body key using `Body`:
 
 === "Python 3.6 and above"
 
-    ```Python hl_lines="23"
+    ```Python hl_lines="22"
     {!> ../../../docs_src/body_multiple_params/tutorial003.py!}
     ```
 
 === "Python 3.10 and above"
 
-    ```Python hl_lines="21"
+    ```Python hl_lines="20"
     {!> ../../../docs_src/body_multiple_params/tutorial003_py310.py!}
     ```
 
@@ -126,7 +126,7 @@ Of course, you can also declare additional query parameters whenever you need, a
 As, by default, singular values are interpreted as query parameters, you don't have to explicitly add a `Query`, you can just do:
 
 ```Python
-q: Optional[str] = None
+q: Union[str, None] = None
 ```
 
 Or in Python 3.10 and above:
@@ -139,7 +139,7 @@ For example:
 
 === "Python 3.6 and above"
 
-    ```Python hl_lines="28"
+    ```Python hl_lines="27"
     {!> ../../../docs_src/body_multiple_params/tutorial004.py!}
     ```
 
@@ -152,7 +152,6 @@ For example:
 !!! info
     `Body` also has all the same extra validation and metadata parameters as `Query`,`Path` and others you will see later.
 
-
 ## Embed a single body parameter
 
 Let's say you only have a single `item` body parameter from a Pydantic model `Item`.
@@ -162,7 +161,7 @@ By default, **FastAPI** will then expect its body directly.
 But if you want it to expect a JSON with a key `item` and inside of it the model contents, as it does when you declare extra body parameters, you can use the special `Body` parameter `embed`:
 
 ```Python
-item: Item = Body(..., embed=True)
+item: Item = Body(embed=True)
 ```
 
 as in:

docs/en/docs/tutorial/body.md

@@ -206,7 +206,7 @@ The function parameters will be recognized as follows:
 !!! note
     FastAPI will know that the value of `q` is not required because of the default value `= None`.
 
-    The `Optional` in `Optional[str]` is not used by FastAPI, but will allow your editor to give you better support and detect errors.
+    The `Union` in `Union[str, None]` is not used by FastAPI, but will allow your editor to give you better support and detect errors.
 
 ## Without Pydantic
 

docs/en/docs/tutorial/dependencies/classes-as-dependencies.md

@@ -109,7 +109,7 @@ Pay attention to the `__init__` method used to create the instance of the class:
 
 === "Python 3.6 and above"
 
-    ```Python hl_lines="8"
+    ```Python hl_lines="9"
     {!> ../../../docs_src/dependencies/tutorial001.py!}
     ```
 

docs/en/docs/tutorial/dependencies/index.md

@@ -33,7 +33,7 @@ It is just a function that can take all the same parameters that a *path operati
 
 === "Python 3.6 and above"
 
-    ```Python hl_lines="8-9"
+    ```Python hl_lines="8-11"
     {!> ../../../docs_src/dependencies/tutorial001.py!}
     ```
 
@@ -81,7 +81,7 @@ The same way you use `Body`, `Query`, etc. with your *path operation function* p
 
 === "Python 3.6 and above"
 
-    ```Python hl_lines="13  18"
+    ```Python hl_lines="15  20"
     {!> ../../../docs_src/dependencies/tutorial001.py!}
     ```
 

docs/en/docs/tutorial/dependencies/sub-dependencies.md

@@ -55,7 +55,7 @@ Then we can use the dependency with:
 
 === "Python 3.6 and above"
 
-    ```Python hl_lines="21"
+    ```Python hl_lines="22"
     {!> ../../../docs_src/dependencies/tutorial005.py!}
     ```
 

docs/en/docs/tutorial/path-params-numeric-validations.md

@@ -59,7 +59,7 @@ It doesn't matter for **FastAPI**. It will detect the parameters by their names,
 
 So, you can declare your function as:
 
-```Python hl_lines="8"
+```Python hl_lines="7"
 {!../../../docs_src/path_params_numeric_validations/tutorial002.py!}
 ```
 
@@ -71,7 +71,7 @@ Pass `*`, as the first parameter of the function.
 
 Python won't do anything with that `*`, but it will know that all the following parameters should be called as keyword arguments (key-value pairs), also known as <abbr title="From: K-ey W-ord Arg-uments"><code>kwargs</code></abbr>. Even if they don't have a default value.
 
-```Python hl_lines="8"
+```Python hl_lines="7"
 {!../../../docs_src/path_params_numeric_validations/tutorial003.py!}
 ```