fastapi__fastapi/docs/tutorial/query-params-str-validations.md

**FastAPI** allows you to declare additonal information and validation for your parameters.

Let's take this application as example:

```Python hl_lines="7"
{!./src/query_params_str_validations/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"
{!./src/query_params_str_validations/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"
{!./src/query_params_str_validations/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 path operation.


## Add more validations

You can also add a parameter `min_length`:

```Python hl_lines="7"
{!./src/query_params_str_validations/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"
{!./src/query_params_str_validations/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"
{!./src/query_params_str_validations/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"
{!./src/query_params_str_validations/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.

!!! note
    Have in mind that different tools might have different levels of OpenAPI support.

    Some of them might not show all the extra information declared yet, although in most of the cases, the missing feature is already planned for development.

You can add a `title`:

```Python hl_lines="7"
{!./src/query_params_str_validations/tutorial007.py!}
```

And a `description`:

```Python hl_lines="11"
{!./src/query_params_str_validations/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"
{!./src/query_params_str_validations/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"
{!./src/query_params_str_validations/tutorial010.py!}
```

The docs will show it like this:

<img src="/img/tutorial/query-params-str-validations/image01.png">

## Recap

You can declare additional validations and metadata for your parameters.

Generic validations and metadata:

* `alias`
* `title`
* `description`
* `deprecated`

Validations specific for strings:

* `min_length`
* `max_length`
* `regex`

In these examples you saw how to declare validations for `str` values.

See the next chapters to see how to declare validations for other types, like numbers.