tiangolo.fastapi/docs/en/docs/tutorial/path-params-numeric-validations.md

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:

{* ../../docs_src/path_params_numeric_validations/tutorial001_an_py310.py hl[1,3] *}

/// 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{.internal-link target=_blank} 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:

{* ../../docs_src/path_params_numeric_validations/tutorial001_an_py310.py hl[10] *}

/// note

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

//// tab | Python 3.8 non-Annotated

/// tip

Prefer to use the Annotated version if possible.

///

{* ../../docs_src/path_params_numeric_validations/tutorial002.py hl[7] *}

////

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().

{* ../../docs_src/path_params_numeric_validations/tutorial002_an_py39.py *}

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.

{* ../../docs_src/path_params_numeric_validations/tutorial003.py hl[7] *}

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 *.

{* ../../docs_src/path_params_numeric_validations/tutorial003_an_py39.py hl[10] *}

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.

{* ../../docs_src/path_params_numeric_validations/tutorial004_an_py39.py hl[10] *}

Number validations: greater than and less than or equal

The same applies for:

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

{* ../../docs_src/path_params_numeric_validations/tutorial005_an_py39.py hl[10] *}

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.

{* ../../docs_src/path_params_numeric_validations/tutorial006_an_py39.py hl[13] *}

Recap

With Query, Path (and others you haven't seen yet) you can declare metadata and string validations in the same ways as with Query Parameters and String Validations{.internal-link target=_blank}.

And you can also declare numeric validations:

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

/// info

Query, Path, and other classes you will see later are subclasses of a common Param class.

All of them share the same parameters for additional validation and metadata you have seen.

///

/// note | Technical Details

When you import Query, Path and others from fastapi, they are actually functions.

That when called, return instances of classes of the same name.

So, you import Query, which is a function. And when you call it, it returns an instance of a class also named Query.

These functions are there (instead of just using the classes directly) so that your editor doesn't mark errors about their types.

That way you can use your normal editor and coding tools without having to add custom configurations to disregard those errors.

///