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.

Check

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}

Check

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

Check

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:

Check

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.

Several of 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".

Predefined values

If you have a path operation that receives a path parameter, but you want the possible valid path parameter values to be predefined, you can use a standard Python Enum.

Create an Enum class

Import Enum and create a sub-class that inherits from str and from Enum.

By inheriting from str the API docs will be able to know that the values must be of type string and will be able to render correctly.

And create class attributes with fixed values, those fixed values will be the available valid values:

from enum import Enum

from fastapi import FastAPI


class ModelName(str, Enum):
    alexnet = "alexnet"
    resnet = "resnet"
    lenet = "lenet"


app = FastAPI()


@app.get("/model/{model_name}")
async def get_model(model_name: ModelName):
    if model_name == ModelName.alexnet:
        return {"model_name": model_name, "message": "Deep Learning FTW!"}
    if model_name.value == "lenet":
        return {"model_name": model_name, "message": "LeCNN all the images"}
    return {"model_name": model_name, "message": "Have some residuals"}

Tip

If you are wondering, "AlexNet", "ResNet", and "LeNet" are just names of Machine Learning models.

Declare a path parameter

Then create a path parameter with a type annotation using the enum class you created (ModelName):

from enum import Enum

from fastapi import FastAPI


class ModelName(str, Enum):
    alexnet = "alexnet"
    resnet = "resnet"
    lenet = "lenet"


app = FastAPI()


@app.get("/model/{model_name}")
async def get_model(model_name: ModelName):
    if model_name == ModelName.alexnet:
        return {"model_name": model_name, "message": "Deep Learning FTW!"}
    if model_name.value == "lenet":
        return {"model_name": model_name, "message": "LeCNN all the images"}
    return {"model_name": model_name, "message": "Have some residuals"}

Check the docs

Because the available values for the path parameter are specified, the interactive docs can show them nicely:

Working with Python enumerations

The value of the path parameter will be an enumeration member.

Compare enumeration members

You can compare it with the enumeration member in your created enum ModelName:

from enum import Enum

from fastapi import FastAPI


class ModelName(str, Enum):
    alexnet = "alexnet"
    resnet = "resnet"
    lenet = "lenet"


app = FastAPI()


@app.get("/model/{model_name}")
async def get_model(model_name: ModelName):
    if model_name == ModelName.alexnet:
        return {"model_name": model_name, "message": "Deep Learning FTW!"}
    if model_name.value == "lenet":
        return {"model_name": model_name, "message": "LeCNN all the images"}
    return {"model_name": model_name, "message": "Have some residuals"}

Get the enumeration value

You can get the actual value (a str in this case) using model_name.value, or in general, your_enum_member.value:

from enum import Enum

from fastapi import FastAPI


class ModelName(str, Enum):
    alexnet = "alexnet"
    resnet = "resnet"
    lenet = "lenet"


app = FastAPI()


@app.get("/model/{model_name}")
async def get_model(model_name: ModelName):
    if model_name == ModelName.alexnet:
        return {"model_name": model_name, "message": "Deep Learning FTW!"}
    if model_name.value == "lenet":
        return {"model_name": model_name, "message": "LeCNN all the images"}
    return {"model_name": model_name, "message": "Have some residuals"}

Tip

You could also access the value "lenet" with ModelName.lenet.value.

Return enumeration members

You can return enum members from your path operation, even nested in a JSON body (e.g. a dict).

They will be converted to their corresponding values before returning them to the client:

from enum import Enum

from fastapi import FastAPI


class ModelName(str, Enum):
    alexnet = "alexnet"
    resnet = "resnet"
    lenet = "lenet"


app = FastAPI()


@app.get("/model/{model_name}")
async def get_model(model_name: ModelName):
    if model_name == ModelName.alexnet:
        return {"model_name": model_name, "message": "Deep Learning FTW!"}
    if model_name.value == "lenet":
        return {"model_name": model_name, "message": "LeCNN all the images"}
    return {"model_name": model_name, "message": "Have some residuals"}

Path parameters containing paths

Let's say you have a path operation with a path /files/{file_path}.

But you need file_path itself to contain a path, like home/johndoe/myfile.txt.

So, the URL for that file would be something like: /files/home/johndoe/myfile.txt.

OpenAPI support

OpenAPI doesn't support a way to declare a path parameter to contain a path inside, as that could lead to scenarios that are difficult to test and define.

Nevertheless, you can still do it in FastAPI, using one of the internal tools from Starlette.

And the docs would still work, although not adding any documentation telling that the parameter should contain a path.

Path convertor

Using an option directly from Starlette you can declare a path parameter containing a path using a URL like:

/files/{file_path:path}

In this case, the name of the parameter is file_path, and the last part, :path, tells it that the parameter should match any path.

So, you can use it with:

from fastapi import FastAPI

app = FastAPI()


@app.get("/files/{file_path:path}")
async def read_user_me(file_path: str):
    return {"file_path": file_path}

Tip

You could need the parameter to contain /home/johndoe/myfile.txt, with a leading slash (/).

In that case, the URL would be: /files//home/johndoe/myfile.txt, with a double slash (//) between files and home.

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).