--- /dev/null
+**FastAPI** allows you to declare additonal information and validation for your parameters.
+
+Let's take this application as example:
+
+```Python hl_lines="7"
+{!./tutorial/src/query-params-schema/tutorial001.py!}
+```
+
+The query parameter `q` is of type `str`, and by default is `None`, so it is optional.
+
+## Additional validation
+
+We are going to enforce that even though `q` is optional, whenever it is provided, it **doesn't exceed a length of 50 characters**.
+
+
+### Import `Query`
+
+To achieve that, first import `Query` from `fastapi`:
+
+```Python hl_lines="1"
+{!./tutorial/src/query-params-schema/tutorial002.py!}
+```
+
+## Use `Query` as the default value
+
+And now use it as the default value of your parameter, setting the parameter `max_length` to 50:
+
+```Python hl_lines="7"
+{!./tutorial/src/query-params-schema/tutorial002.py!}
+```
+
+As we have to replace the default value `None` with `Query(None)`, the first parameter to `Query` serves the same purpose of defining that default value.
+
+So:
+
+```Python
+q: str = Query(None)
+```
+
+...makes the parameter optional, the same as:
+
+```Python
+q: str = None
+```
+
+But it declares it explicitly as being a query parameter.
+
+And then, we can pass more parameters to `Query`. In this case, the `max_length` parameter that applies to strings:
+
+```Python
+q: str = Query(None, max_length=50)
+```
+
+This will validate the data, show a clear error when the data is not valid, and document the parameter in the OpenAPI schema endpoint.
+
+
+## Add more validations
+
+You can also add a parameter `min_length`:
+
+```Python hl_lines="7"
+{!./tutorial/src/query-params-schema/tutorial003.py!}
+```
+
+## Add regular expressions
+
+You can define a <abbr title="A regular expression, regex or regexp is a sequence of characters that define a search pattern for strings.">regular expression</abbr> that the parameter should match:
+
+```Python hl_lines="8"
+{!./tutorial/src/query-params-schema/tutorial004.py!}
+```
+
+This specific regular expression checks that the received parameter value:
+
+* `^`: starts with the following characters, doesn't have characters before.
+* `fixedquery`: has the exact value `fixedquery`.
+* `$`: ends there, doesn't have any more characters after `fixedquery`.
+
+If you feel lost with all these **"regular expression"** ideas, don't worry. They are a hard topic for many people. You can still do a lot of stuff without needing regular expressions yet.
+
+But whenever you need them and go and learn them, know that you can already use them directly in **FastAPI**.
+
+## Default values
+
+The same way that you can pass `None` as the first argument to be used as the default value, you can pass other values.
+
+Let's say that you want to declare the `q` query parameter to have a `min_length` of `3`, and to have a default value of `"fixedquery"`:
+
+```Python hl_lines="7"
+{!./tutorial/src/query-params-schema/tutorial005.py!}
+```
+
+!!! note
+ Having a default value also makes the parameter optional.
+
+## Make it required
+
+When we don't need to declare more validations or metadata, we can make the `q` query parameter required just by not declaring a default value, like:
+
+```Python
+q: str
+```
+
+instead of:
+
+```Python
+q: str = None
+```
+
+But we are now declaring it with `Query`, for example like:
+
+```Python
+q: str = Query(None, min_length=3)
+```
+
+So, when you need to declare a value as required while using `Query`, you can use `...` as the first argument:
+
+```Python hl_lines="7"
+{!./tutorial/src/query-params-schema/tutorial006.py!}
+```
+
+!!! info
+ If you hadn't seen that `...` before: it is a a special single value, it is <a href="https://docs.python.org/3/library/constants.html#Ellipsis" target="_blank">part of Python and is called "Ellipsis"</a>.
+
+This will let **FastAPI** know that this parameter is required.
+
+## Declare more metadata
+
+You can add more information about the parameter.
+
+That information will be included in the generated OpenAPI and used by the documentation user interfaces and external tools.
+
+You can add a `title`:
+
+```Python hl_lines="7"
+{!./tutorial/src/query-params-schema/tutorial007.py!}
+```
+
+And a `description`:
+
+```Python hl_lines="11"
+{!./tutorial/src/query-params-schema/tutorial008.py!}
+```
+
+## Alias parameters
+
+Imagine that you want the parameter to be `item-query`.
+
+Like in:
+
+```
+http://127.0.0.1:8000/items/?item-query=foobaritems
+```
+
+But `item-query` is not a valid Python variable name.
+
+The closest would be `item_query`.
+
+But you still need it to be exactly `item-query`...
+
+Then you can declare an `alias`, and that alias is what will be used to find the parameter value:
+
+```Python hl_lines="7"
+{!./tutorial/src/query-params-schema/tutorial009.py!}
+```
+
+## Deprecating parameters
+
+Now let's say you don't like this parameter anymore.
+
+You have to leave it there a while because there are clients using it, but you want the docs to clearly show it as <abbr title="obsolete, recommended not to use it">deprecated</abbr>.
+
+Then pass the parameter `deprecated=True` to `Query`:
+
+```Python hl_lines="16"
+{!./tutorial/src/query-params-schema/tutorial010.py!}
+```
+
+The docs will show it like this:
+
+<img src="/img/tutorial/query-params-schema/image01.png">
projects / thirdparty / fastapi / fastapi.git / commitdiff
