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Path Parameters and Numeric Validations

The same way 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:

from fastapi import FastAPI, Path, Query

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 = Query(None, alias="item-query"),
):
    results = {"item_id": item_id}
    if q:
        results.update({"q": q})
    return results

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:

from fastapi import FastAPI, Path, Query

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 = Query(None, alias="item-query"),
):
    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

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.

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:

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

Order the parameters as you need, tricks

If you want to declare the q query parameter without a Query nor any default value, and the path parameter item_id using Path, and have them in a different order, 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

Number validations: greater than or equal

With Query and Path (and other's you'll see later) you can declare string constraints, but also number constraints.

Here, with ge=1, item_id will need to be an integer number "greater than or equal" to 1.

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", 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
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", 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.

from fastapi import FastAPI, Path, Query

app = FastAPI()


@app.get("/items/{item_id}")
async def read_items(
    *,
    item_id: int = Path(..., title="The ID of the item to get", ge=0, le=1000),
    q: str,
    size: float = Query(..., gt=0, lt=10.5)
):
    results = {"item_id": item_id}
    if q:
        results.update({"q": q})
    return results

Recap

With Query, Path (and others you haven't seen yet) you can declare metadata and string validations (the previous chapter).

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 others you will see later subclasses of a common Param class (that you don't need to use).

And all of them share the same all these same parameters of additional validation and metadata you have seen.

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.