Path Parameters and Numeric Validations

In the same way that you can declare more validations and metadata for query parameters with Query, you can declare the same type of validations and metadata for path parameters with Path.

Import Path

First, import Path from fastapi, and import Annotated: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, Path, Query app = FastAPI() @app.get("/items/{item_id}") async def read_items( item_id: Annotated[int, Path(title="The ID of the item to get")], q: Annotated[str | None, Query(alias="item-query")] = None, ): results = {"item_id": item_id} 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.

Declare metadata¶

You can declare all the same parameters as for Query.

For example, to declare a title metadata value for the path parameter item_id you can type: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, Path, Query app = FastAPI() @app.get("/items/{item_id}") async def read_items( item_id: Annotated[int, Path(title="The ID of the item to get")], q: Annotated[str | None, Query(alias="item-query")] = None, ): results = {"item_id": item_id} if q: results.update({"q": q}) return results

Note

A path parameter is always required as it has to be part of the path.

So, you should declare it with ... to mark it as required.

Nevertheless, even if you declared it with None or set a default value, it would not affect anything, it would still be always required.

Order the parameters as you need¶

Tip

This is probably not as important or necessary if you use Annotated.

Let’s say that you want to declare the query parameter q as a required str.

And you don’t need to declare anything else for that parameter, so you don’t really need to use Query.

But you still need to use Path for the item_id path parameter. And you don’t want to use Annotated for some reason.

Python will complain if you put a value with a “default” before a value that doesn’t have a “default”.

But you can re-order them, and have the value without a default (the query parameter q) first.

It doesn’t matter for FastAPI. It will detect the parameters by their names, types and default declarations (Query, Path, etc), it doesn’t care about the order.

So, you can declare your function as:Python 3.8 non-Annotated

Tip

Prefer to use the Annotated version if possible.from fastapi import FastAPI, Path app = FastAPI() @app.get("/items/{item_id}") async def read_items(q: str, item_id: int = Path(title="The ID of the item to get")): results = {"item_id": item_id} if q: results.update({"q": q}) return results

But keep in mind that if you use Annotated, you won't have this problem, it won't matter as you're not using the function parameter default values for Query() or Path().Python 3.9+Python 3.8+from typing import Annotated from fastapi import FastAPI, Path app = FastAPI() @app.get("/items/{item_id}") async def read_items( q: str, item_id: Annotated[int, Path(title="The ID of the item to get")] ): results = {"item_id": item_id} if q: results.update({"q": q}) return results

Order the parameters as you need, tricks¶

Tip

This is probably not as important or necessary if you use Annotated.

Here’s a small trick that can be handy, but you won’t need it often.

If you want to:

• declare the q query parameter without a Query nor any default value
• declare the path parameter item_id using Path
• have them in a different order
• not use Annotated

…Python has a little special syntax for that.

Pass *, as the first parameter of the function.

Python won't do anything with that *, but it will know that all the following parameters should be called as keyword arguments (key-value pairs), also known as kwargs. Even if they don't have a default value.from fastapi import FastAPI, Path app = FastAPI() @app.get("/items/{item_id}") async def read_items(*, item_id: int = Path(title="The ID of the item to get"), q: str): results = {"item_id": item_id} if q: results.update({"q": q}) return results

Better with Annotated¶

Keep in mind that if you use Annotated, as you are not using function parameter default values, you won't have this problem, and you probably won't need to use *.Python 3.9+Python 3.8+from typing import Annotated from fastapi import FastAPI, Path app = FastAPI() @app.get("/items/{item_id}") async def read_items( item_id: Annotated[int, Path(title="The ID of the item to get")], q: str ): results = {"item_id": item_id} if q: results.update({"q": q}) return results

Number validations: greater than or equal¶

With Query and Path (and others you’ll see later) you can declare number constraints.

Here, with ge=1, item_id will need to be an integer number "greater than or equal" to 1.Python 3.9+Python 3.8+Python 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Path app = FastAPI() @app.get("/items/{item_id}") async def read_items( item_id: Annotated[int, Path(title="The ID of the item to get", ge=1)], q: str ): results = {"item_id": item_id} if q: results.update({"q": q}) return results

Number validations: greater than and less than or equal¶

The same applies for:

• gt: greater than
• le: less than or equal

Python 3.9+Python 3.8+Python 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Path app = FastAPI() @app.get("/items/{item_id}") async def read_items( item_id: Annotated[int, Path(title="The ID of the item to get", gt=0, le=1000)], q: str, ): results = {"item_id": item_id} if q: results.update({"q": q}) return results

Number validations: floats, greater than and less than

Number validations also work for float values.

Here’s where it becomes important to be able to declare gt and not just ge. As with it you can require, for example, that a value must be greater than 0, even if it is less than 1.

So, 0.5 would be a valid value. But 0.0 or 0 would not.

And the same for lt.Python 3.9+Python 3.8+Python 3.8+ non-Annotatedfrom typing import Annotated from fastapi import FastAPI, Path, Query app = FastAPI() @app.get("/items/{item_id}") async def read_items( *, item_id: Annotated[int, Path(title="The ID of the item to get", ge=0, le=1000)], q: str, size: Annotated[float, Query(gt=0, lt=10.5)], ): results = {"item_id": item_id} if q: results.update({"q": q}) return results