FastApi Tutorial 2 (Path parameters)

Path parameters

You can give parameters to apis in the same format as in standard python:

from fastapi import FastAPI

app = FastAPI()


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

If you run the example: http://127.0.0.1:8000/items/foo , you will get the following response:

{"item_id":"foo"}

Path parameters with types

You can declare the type of a path parameter using standard python type annotation:

from fastapi import FastAPI

app = FastAPI()


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

In this case, item_id is declared as an int.

If you run http://127.0.0.1:8000/items/1 , you will get

{"item_id":3}

If you run http://127.0.0.1:8000/items/foo , you will get a nice http error:

{
  "detail": [
    {
      "type": "int_parsing",
      "loc": [
        "path",
        "item_id"
      ],
      "msg": "Input should be a valid integer, unable to parse string as an integer",
      "input": "foo"
    }
  ]
}

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

With the same python declaration FastApi gives you a data validation showing where exactly error happened which is nice when debugging.

Order matters

Let’s say we’ve “/items/me” and “/items/{item_id}”:

from fastapi import FastAPI

app = FastAPI()


@app.get("/items/me")
async def get_item_me():
    return {"item_id": 1}


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

In FastApi path operations are evaluated in order, that’s why “/items/me” must come before “/items/{item_id}”. Otherwise when we try to reach “/items/me”, “/items/{item_id}” will take the request and return error saying “Input should be a valid integer…”.

Predefined values

from enum import Enum

from fastapi import FastAPI

app = FastAPI()


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


@app.get("/models/{model_name}")
async def get_model(model_name: ModelName):
    return {"model_name": model_name}

This way our get_model endpoint can only accept either [“alexnet”, “resnet”, “lenet”]. If other string is provided as a path parameter, we get an error:

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http://127.0.0.1:8000/models/my_model

{
  "detail": [
    {
      "type": "enum",
      "loc": [
        "path",
        "model_name"
      ],
      "msg": "Input should be 'alexnet', 'resnet' or 'lenet'",
      "input": "my_model",
      "ctx": {
        "expected": "'alexnet', 'resnet' or 'lenet'"
      }
    }
  ]
}

If everything is good, http://127.0.0.1:8000/models/alexnet :

{
  "model_name": "alexnet"
}

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

http://127.0.0.1:8000/doc Swagger UI

If you open the above url, you can interact with the API in a more easier to understand way.

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