# Query Parameters

When you declare other function parameters that are not part of the path parameters, they are automatically interpreted as "query" parameters.

```Python hl_lines="9"
{!../../../docs_src/query_params/tutorial001.py!}
```

The query is the set of key-value pairs that go after the `?` in a URL, separated by `&` characters.

For example, in the URL:

```
http://127.0.0.1:8000/items/?skip=0&limit=10
```

...the query parameters are:

* `skip`: with a value of `0`
* `limit`: with a value of `10`

As they are part of the URL, they are "naturally" strings.

But when you declare them with Python types (in the example above, as `int`), they are converted to that type and validated against it.

All the same process that applied for path parameters also applies for query parameters:

* Editor support (obviously)
* Data <abbr title="converting the string that comes from an HTTP request into Python data">"parsing"</abbr>
* Data validation
* Automatic documentation

## Defaults

As query parameters are not a fixed part of a path, they can be optional and can have default values.

In the example above they have default values of `skip=0` and `limit=10`.

So, going to the URL:

```
http://127.0.0.1:8000/items/
```

would be the same as going to:

```
http://127.0.0.1:8000/items/?skip=0&limit=10
```

But if you go to, for example:

```
http://127.0.0.1:8000/items/?skip=20
```

The parameter values in your function will be:

* `skip=20`: because you set it in the URL
* `limit=10`: because that was the default value

## Optional parameters

The same way, you can declare optional query parameters, by setting their default to `None`:

```Python hl_lines="9"
{!../../../docs_src/query_params/tutorial002.py!}
```

In this case, the function parameter `q` will be optional, and will be `None` by default.

!!! check
    Also notice that **FastAPI** is smart enough to notice that the path parameter `item_id` is a path parameter and `q` is not, so, it's a query parameter.

!!! note
    FastAPI will know that `q` is optional because of the `= None`.

    The `Optional` in `Optional[str]` is not used by FastAPI (FastAPI will only use the `str` part), but the `Optional[str]` will let your editor help you finding errors in your code.

## Query parameter type conversion

You can also declare `bool` types, and they will be converted:

```Python hl_lines="9"
{!../../../docs_src/query_params/tutorial003.py!}
```

In this case, if you go to:

```
http://127.0.0.1:8000/items/foo?short=1
```

or

```
http://127.0.0.1:8000/items/foo?short=True
```

or

```
http://127.0.0.1:8000/items/foo?short=true
```

or

```
http://127.0.0.1:8000/items/foo?short=on
```

or

```
http://127.0.0.1:8000/items/foo?short=yes
```

or any other case variation (uppercase, first letter in uppercase, etc), your function will see the parameter `short` with a `bool` value of `True`. Otherwise as `False`.


## Multiple path and query parameters

You can declare multiple path parameters and query parameters at the same time, **FastAPI** knows which is which.

And you don't have to declare them in any specific order.

They will be detected by name:

```Python hl_lines="8  10"
{!../../../docs_src/query_params/tutorial004.py!}
```

## Required query parameters

When you declare a default value for non-path parameters (for now, we have only seen query parameters), then it is not required.

If you don't want to add a specific value but just make it optional, set the default as `None`.

But when you want to make a query parameter required, you can just not declare any default value:

```Python hl_lines="6-7"
{!../../../docs_src/query_params/tutorial005.py!}
```

Here the query parameter `needy` is a required query parameter of type `str`.

If you open in your browser a URL like:

```
http://127.0.0.1:8000/items/foo-item
```

...without adding the required parameter `needy`, you will see an error like:

```JSON
{
    "detail": [
        {
            "loc": [
                "query",
                "needy"
            ],
            "msg": "field required",
            "type": "value_error.missing"
        }
    ]
}
```

As `needy` is a required parameter, you would need to set it in the URL:

```
http://127.0.0.1:8000/items/foo-item?needy=sooooneedy
```

...this would work:

```JSON
{
    "item_id": "foo-item",
    "needy": "sooooneedy"
}
```

And of course, you can define some parameters as required, some as having a default value, and some entirely optional:

```Python hl_lines="10"
{!../../../docs_src/query_params/tutorial006.py!}
```

In this case, there are 3 query parameters:

* `needy`, a required `str`.
* `skip`, an `int` with a default value of `0`.
* `limit`, an optional `int`.

!!! tip
    You could also use `Enum`s the same way as with [Path Parameters](path-params.md#predefined-values){.internal-link target=_blank}.
