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Path Parameters

You can declare path "parameters" or "variables" with the same syntax used by Python format strings:

from fastapi import FastAPI

app = FastAPI()


@app.get("/items/{item_id}")
async def read_item(item_id):
    return {"item_id": item_id}

The value of the path parameter item_id will be passed to your function as the argument item_id.

So, if you run this example and go to http://127.0.0.1:8000/items/foo, you will see a response of:

{"item_id":"foo"}

Path parameters with types

You can declare the type of a path parameter in the function, using standard Python type annotations:

from fastapi import FastAPI

app = FastAPI()


@app.get("/items/{item_id}")
async def read_item(item_id: int):
    return {"item_id": item_id}

In this case, item_id is declared to be an int.

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This will give you editor support inside of your function, with error checks, completion, etc.

Data conversion

If you run this example and open your browser at http://127.0.0.1:8000/items/3, you will see a response of:

{"item_id":3}

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Notice that the value your function received (and returned) is 3, as a Python int, not a string "3".

So, with that type declaration, FastAPI gives you automatic request "parsing".

Data validation

But if you go to the browser at http://127.0.0.1:8000/items/foo, you will see a nice HTTP error of:

{
    "detail": [
        {
            "loc": [
                "path",
                "item_id"
            ],
            "msg": "value is not a valid integer",
            "type": "type_error.integer"
        }
    ]
}

because the path parameter item_id had a value of "foo", which is not an int.

The same error would appear if you provided a float instead of an int, as in: http://127.0.0.1:8000/items/4.2

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So, with the same Python type declaration, FastAPI gives you data validation.

Notice that the error also clearly states exactly the point where the validation didn't pass.

This is incredibly helpful while developing and debugging code that interacts with your API.

Documentation

And when you open your browser at http://127.0.0.1:8000/docs, you will see an automatic, interactive, API documentation like:

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Again, just with that same Python type declaration, FastAPI gives you automatic, interactive documentation (integrating Swagger UI).

Notice that the path parameter is declared to be an integer.

Standards-based benefits, alternative documentation

And because the generated schema is from the OpenAPI standard, there are many compatible tools.

Because of this, FastAPI itself provides an alternative API documentation (using ReDoc):

The same way, there are many compatible tools. Including code generation tools for many languages.

Pydantic

All the data validation is performed under the hood by Pydantic, so you get all the benefits from it. And you know you are in good hands.

You can use the same type declarations with str, float, bool and many other complex data types.

These are explored in the next chapters of the tutorial.

Order matters

When creating path operations, you can find situations where you have a fixed path.

Like /users/me, let's say that it's to get data about the current user.

And then you can also have a path /users/{user_id} to get data about a specific user by some user ID.

Because path operations are evaluated in order, you need to make sure that the path for /users/me is declared before the one for /users/{user_id}:

from fastapi import FastAPI

app = FastAPI()


@app.get("/users/me")
async def read_user_me():
    return {"user_id": "the current user"}


@app.get("/users/{user_id}")
async def read_user(user_id: str):
    return {"user_id": user_id}

Otherwise, the path for /users/{user_id} would match also for /users/me, "thinking" that it's receiving a parameter user_id with a value of "me".

Recap

With FastAPI, by using short, intuitive and standard Python type declarations, you get:

  • Editor support: error checks, autocompletion, etc.
  • Data "parsing"
  • Data validation
  • API annotation and automatic documentation

And you only have to declare them once.

That's probably the main visible advantage of FastAPI compared to alternative frameworks (apart from the raw performance).