Query Parameters and String Validations

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

Let's take this application as example:

from fastapi import FastAPI

app = FastAPI()


@app.get("/items/")
async def read_items(q: str = None):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

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:

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: str = Query(None, max_length=50)):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

Use Query as the default value

And now use it as the default value of your parameter, setting the parameter max_length to 50:

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: str = Query(None, max_length=50)):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

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:

q: str = Query(None)

...makes the parameter optional, the same as:

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:

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:

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: str = Query(None, min_length=3, max_length=50)):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

Add regular expressions

You can define a regular expression that the parameter should match:

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(
    q: str = Query(None, min_length=3, max_length=50, regex="^fixedquery$")
):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

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":

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: str = Query("fixedquery", min_length=3)):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

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:

q: str

instead of:

q: str = None

But we are now declaring it with Query, for example like:

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:

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: str = Query(..., min_length=3)):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

Info

If you hadn't seen that ... before: it is a a special single value, it is part of Python and is called "Ellipsis".

This will let FastAPI know that this parameter is required.

Query parameter list / multiple values

When you define a query parameter explicitly with Query you can also declare it to receive a list of values, or said in other way, to receive multiple values.

For example, to declare a query parameter q that can appear multiple times in the URL, you can write:

from typing import List

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: List[str] = Query(None)):
    query_items = {"q": q}
    return query_items

Then, with a URL like:

http://localhost:8000/items/?q=foo&q=bar

you would receive the multiple q query parameters' values (foo and bar) in a Python list inside your path operation function, in the function parameter q.

So, the response to that URL would be:

{
  "q": [
    "foo",
    "bar"
  ]
}

Tip

To declare a query parameter with a type of list, like in the example above, you need to explicitly use Query, otherwise it would be interpreted as a request body.

The interactive API docs will update accordingly, to allow multiple values:

Query parameter list / multiple values with defaults

And you can also define a default list of values if none are provided:

from typing import List

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: List[str] = Query(["foo", "bar"])):
    query_items = {"q": q}
    return query_items

If you go to:

http://localhost:8000/items/

the default of q will be: ["foo", "bar"] and your response will be:

{
  "q": [
    "foo",
    "bar"
  ]
}

Using list

You can also use list directly instead of List[str]:

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: list = Query(None)):
    query_items = {"q": q}
    return query_items

Note

Have in mind that in this case, FastAPI won't check the contents of the list.

For example, List[int] would check (and document) that the contents of the list are integers. But list alone wouldn't.

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:

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: str = Query(None, title="Query string", min_length=3)):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

And a description:

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(
    q: str = Query(
        None,
        title="Query string",
        description="Query string for the items to search in the database that have a good match",
        min_length=3,
    )
):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

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:

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: str = Query(None, alias="item-query")):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

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 deprecated.

Then pass the parameter deprecated=True to Query:

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(
    q: str = Query(
        None,
        alias="item-query",
        title="Query string",
        description="Query string for the items to search in the database that have a good match",
        min_length=3,
        max_length=50,
        regex="^fixedquery$",
        deprecated=True,
    )
):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})
    return results

The docs will show it like this:

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.