Query Parameters and String Validations

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

Let's take this application as example:Python 3.10+Python 3.8+from fastapi import FastAPI app = FastAPI() @app.get("/items/") async def read_items(q: str | None = None): results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]} if q: results.update({"q": q}) return results

The query parameter q is of type Union[str, None] (or str | None in Python 3.10), that means that it’s of type str but could also be None, and indeed, the default value is None, so FastAPI will know it’s not required.

Note

FastAPI will know that the value of q is not required because of the default value = None.

The Union in Union[str, None] will allow your editor to give you better support and detect errors.

Additional validation

We are going to enforce that even though q is optional, whenever it is provided, its length doesn’t exceed 50 characters.

Import Query and Annotated¶

To achieve that, first import:

• Query from fastapi
• Annotated from typing (or from typing_extensions in Python below 3.9)

Python 3.10+Python 3.8+

In Python 3.9 or above, Annotated is part of the standard library, so you can import it from typing.from typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items(q: Annotated[str | None, Query(max_length=50)] = None): results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]} if q: results.update({"q": q}) return results

Info

FastAPI added support for Annotated (and started recommending it) in version 0.95.0.

If you have an older version, you would get errors when trying to use Annotated.

Make sure you Upgrade the FastAPI version to at least 0.95.1 before using Annotated.

Use Annotated in the type for the q parameter¶

Remember I told you before that Annotated can be used to add metadata to your parameters in the Python Types Intro?

Now it’s the time to use it with FastAPI. 🚀

We had this type annotation:Python 3.10+Python 3.8+q: str | None = None

What we will do is wrap that with Annotated, so it becomes:Python 3.10+Python 3.8+q: Annotated[str | None] = None

Both of those versions mean the same thing, q is a parameter that can be a str or None, and by default, it is None.

Now let’s jump to the fun stuff. 🎉

Add Query to Annotated in the q parameter

Now that we have this Annotated where we can put more metadata, add Query to it, and set the parameter max_length to 50:Python 3.10+Python 3.8+from typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items(q: Annotated[str | None, Query(max_length=50)] = None): results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]} if q: results.update({"q": q}) return results

Notice that the default value is still None, so the parameter is still optional.

But now, having Query(max_length=50) inside of Annotated, we are telling FastAPI that we want it to extract this value from the query parameters (this would have been the default anyway 🤷) and that we want to have additional validation for this value (that’s why we do this, to get the additional validation). 😎

FastAPI will now:

• Validate the data making sure that the max length is 50 characters
• Show a clear error for the client when the data is not valid
• Document the parameter in the OpenAPI schema path operation (so it will show up in the automatic docs UI)

Alternative (old) Query as the default value¶

Previous versions of FastAPI (before 0.95.0) required you to use Query as the default value of your parameter, instead of putting it in Annotated, there’s a high chance that you will see code using it around, so I’ll explain it to you.

Tip

For new code and whenever possible, use Annotated as explained above. There are multiple advantages (explained below) and no disadvantages. 🍰

This is how you would use Query() as the default value of your function parameter, setting the parameter max_length to 50:Python 3.10+Python 3.8+from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items(q: str | None = Query(default=None, max_length=50)): results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]} if q: results.update({"q": q}) return results

As in this case (without using Annotated) we have to replace the default value None in the function with Query(), we now need to set the default value with the parameter Query(default=None), it serves the same purpose of defining that default value (at least for FastAPI).

So:q: Union[str, None] = Query(default=None)

…makes the parameter optional, with a default value of None, the same as:q: Union[str, None] = None

And in Python 3.10 and above:q: str | None = Query(default=None)

…makes the parameter optional, with a default value of None, the same as:q: str | None = None

But it declares it explicitly as being a query parameter.

Info

Keep in mind that the most important part to make a parameter optional is the part:= None

or the:= Query(default=None)

as it will use that None as the default value, and that way make the parameter not required.

The Union[str, None] part allows your editor to provide better support, but it is not what tells FastAPI that this parameter is not required.

Then, we can pass more parameters to Query. In this case, the max_length parameter that applies to strings:q: Union[str, None] = Query(default=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.

Query as the default value or in Annotated¶

Keep in mind that when using Query inside of Annotated you cannot use the default parameter for Query.

Instead use the actual default value of the function parameter. Otherwise, it would be inconsistent.

For example, this is not allowed:q: Annotated[str, Query(default="rick")] = "morty"

…because it’s not clear if the default value should be "rick" or "morty".

So, you would use (preferably):q: Annotated[str, Query()] = "rick"

…or in older code bases you will find:q: str = Query(default="rick")

Advantages of Annotated¶

Using Annotated is recommended instead of the default value in function parameters, it is better for multiple reasons. 🤓

The default value of the function parameter is the actual default value, that’s more intuitive with Python in general. 😌

You could call that same function in other places without FastAPI, and it would work as expected. If there’s a required parameter (without a default value), your editor will let you know with an error, Python will also complain if you run it without passing the required parameter.

When you don’t use Annotated and instead use the (old) default value style, if you call that function without FastAPI in other place, you have to remember to pass the arguments to the function for it to work correctly, otherwise the values will be different from what you expect (e.g. QueryInfo or something similar instead of str). And your editor won’t complain, and Python won’t complain running that function, only when the operations inside error out.

Because Annotated can have more than one metadata annotation, you could now even use the same function with other tools, like Typer.

Add more validations

You can also add a parameter min_length:Python 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items( q: Annotated[str | None, Query(min_length=3, max_length=50)] = None ): 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 pattern that the parameter should match:Python 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items( q: Annotated[ str | None, Query(min_length=3, max_length=50, pattern="^fixedquery$") ] = None, ): results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]} if q: results.update({"q": q}) return results

This specific regular expression pattern 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.

Pydantic v1 regex instead of pattern¶

Before Pydantic version 2 and before FastAPI 0.100.0, the parameter was called regex instead of pattern, but it’s now deprecated.

You could still see some code using it:Python 3.10+ Pydantic v1from typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items( q: Annotated[ str | None, Query(min_length=3, max_length=50, regex="^fixedquery$") ] = None, ): results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]} if q: results.update({"q": q}) return results

But know that this is deprecated and it should be updated to use the new parameter pattern. 🤓

Default values¶

You can, of course, use default values other than None.

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 3.9+Python 3.8+Python 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items(q: Annotated[str, Query(min_length=3)] = "fixedquery"): results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]} if q: results.update({"q": q}) return results

Note

Having a default value of any type, including None, makes the parameter optional (not required).

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: Union[str, None] = None

But we are now declaring it with Query, for example like:Annotatednon-Annotatedq: Annotated[Union[str, None], Query(min_length=3)] = None

So, when you need to declare a value as required while using Query, you can simply not declare a default value:Python 3.9+Python 3.8+Python 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items(q: Annotated[str, Query(min_length=3)]): results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]} if q: results.update({"q": q}) return results

Required with Ellipsis (...)¶

There's an alternative way to explicitly declare that a value is required. You can set the default to the literal value ...:Python 3.9+Python 3.8+Python 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items(q: Annotated[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 special single value, it is part of Python and is called “Ellipsis”.

It is used by Pydantic and FastAPI to explicitly declare that a value is required.

This will let FastAPI know that this parameter is required.

Required with None

You can declare that a parameter can accept None, but that it’s still required. This would force clients to send a value, even if the value is None.

To do that, you can declare that None is a valid type but still use ... as the default:Python 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items(q: Annotated[str | None, Query(min_length=3)] = ...): results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]} if q: results.update({"q": q}) return results

Tip

Pydantic, which is what powers all the data validation and serialization in FastAPI, 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 Required Optional fields.

Tip

Remember that in most of the cases, when something is required, you can simply omit the default, so you normally don’t have to use ....

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:Python 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.9+ non-AnnotatedPython 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items(q: Annotated[list[str] | None, 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:Python 3.9+Python 3.8+Python 3.9+ non-AnnotatedPython 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items(q: Annotated[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] (or list[str] in Python 3.9+):Python 3.9+Python 3.8+Python 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items(q: Annotated[list, Query()] = []): query_items = {"q": q} return query_items

Note

Keep 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

Keep 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 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items( q: Annotated[str | None, Query(title="Query string", min_length=3)] = None ): results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]} if q: results.update({"q": q}) return results

And a description:Python 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items( q: Annotated[ str | None, Query( title="Query string", description="Query string for the items to search in the database that have a good match", min_length=3, ), ] = None, ): 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:Python 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items(q: Annotated[str | None, Query(alias="item-query")] = None): 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:Python 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items( q: Annotated[ str | None, Query( 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, pattern="^fixedquery$", deprecated=True, ), ] = None, ): results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]} if q: results.update({"q": q}) return results

The docs will show it like this:

Exclude from OpenAPI¶

To exclude a query parameter from the generated OpenAPI schema (and thus, from the automatic documentation systems), set the parameter include_in_schema of Query to False:Python 3.10+Python 3.9+Python 3.8+Python 3.10+ non-AnnotatedPython 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Query app = FastAPI() @app.get("/items/") async def read_items( hidden_query: Annotated[str | None, Query(include_in_schema=False)] = None ): if hidden_query: return {"hidden_query": hidden_query} else: return {"hidden_query": "Not found"}