Testing

Thanks to Starlette, testing FastAPI applications is easy and enjoyable.

It is based on Requests, so it's very familiar and intuitive.

With it, you can use pytest directly with FastAPI.

Using TestClient

Import TestClient.

Create a TestClient passing to it your FastAPI application.

Create functions with a name that starts with test_ (this is standard pytest conventions).

Use the TestClient object the same way as you do with requests.

Write simple assert statements with the standard Python expressions that you need to check (again, standard pytest).

from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()


@app.get("/")
async def read_main():
    return {"msg": "Hello World"}


client = TestClient(app)


def test_read_main():
    response = client.get("/")
    assert response.status_code == 200
    assert response.json() == {"msg": "Hello World"}

Tip

Notice that the testing functions are normal def, not async def.

And the calls to the client are also normal calls, not using await.

This allows you to use pytest directly without complications.

Technical Details

You could also use from starlette.testclient import TestClient.

FastAPI provides the same starlette.testclient as fastapi.testclient just as a convenience for you, the developer. But it comes directly from Starlette.

Tip

If you want to call async functions in your tests apart from sending requests to your FastAPI application (e.g. asynchronous database functions), have a look at the Async Tests in the advanced tutorial.

Separating tests

In a real application, you probably would have your tests in a different file.

And your FastAPI application might also be composed of several files/modules, etc.

FastAPI app file

Let's say you have a file main.py with your FastAPI app:

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
async def read_main():
    return {"msg": "Hello World"}

Testing file

Then you could have a file test_main.py with your tests, and import your app from the main module (main.py):

from fastapi.testclient import TestClient

from .main import app

client = TestClient(app)


def test_read_main():
    response = client.get("/")
    assert response.status_code == 200
    assert response.json() == {"msg": "Hello World"}

Testing: extended example

Now let's extend this example and add more details to see how to test different parts.

Extended FastAPI app file

Let's say that now the file main.py with your FastAPI app has some other path operations.

It has a GET operation that could return an error.

It has a POST operation that could return several errors.

Both path operations require an X-Token header.

Python 3.6 and abovePython 3.10 and above

from typing import Optional

from fastapi import FastAPI, Header, HTTPException
from pydantic import BaseModel

fake_secret_token = "coneofsilence"

fake_db = {
    "foo": {"id": "foo", "title": "Foo", "description": "There goes my hero"},
    "bar": {"id": "bar", "title": "Bar", "description": "The bartenders"},
}

app = FastAPI()


class Item(BaseModel):
    id: str
    title: str
    description: Optional[str] = None


@app.get("/items/{item_id}", response_model=Item)
async def read_main(item_id: str, x_token: str = Header(...)):
    if x_token != fake_secret_token:
        raise HTTPException(status_code=400, detail="Invalid X-Token header")
    if item_id not in fake_db:
        raise HTTPException(status_code=404, detail="Item not found")
    return fake_db[item_id]


@app.post("/items/", response_model=Item)
async def create_item(item: Item, x_token: str = Header(...)):
    if x_token != fake_secret_token:
        raise HTTPException(status_code=400, detail="Invalid X-Token header")
    if item.id in fake_db:
        raise HTTPException(status_code=400, detail="Item already exists")
    fake_db[item.id] = item
    return item

from fastapi import FastAPI, Header, HTTPException
from pydantic import BaseModel

fake_secret_token = "coneofsilence"

fake_db = {
    "foo": {"id": "foo", "title": "Foo", "description": "There goes my hero"},
    "bar": {"id": "bar", "title": "Bar", "description": "The bartenders"},
}

app = FastAPI()


class Item(BaseModel):
    id: str
    title: str
    description: str | None = None


@app.get("/items/{item_id}", response_model=Item)
async def read_main(item_id: str, x_token: str = Header(...)):
    if x_token != fake_secret_token:
        raise HTTPException(status_code=400, detail="Invalid X-Token header")
    if item_id not in fake_db:
        raise HTTPException(status_code=404, detail="Item not found")
    return fake_db[item_id]


@app.post("/items/", response_model=Item)
async def create_item(item: Item, x_token: str = Header(...)):
    if x_token != fake_secret_token:
        raise HTTPException(status_code=400, detail="Invalid X-Token header")
    if item.id in fake_db:
        raise HTTPException(status_code=400, detail="Item already exists")
    fake_db[item.id] = item
    return item

Extended testing file

You could then update test_main.py with the extended tests:

from fastapi.testclient import TestClient

from .main import app

client = TestClient(app)


def test_read_item():
    response = client.get("/items/foo", headers={"X-Token": "coneofsilence"})
    assert response.status_code == 200
    assert response.json() == {
        "id": "foo",
        "title": "Foo",
        "description": "There goes my hero",
    }


def test_read_item_bad_token():
    response = client.get("/items/foo", headers={"X-Token": "hailhydra"})
    assert response.status_code == 400
    assert response.json() == {"detail": "Invalid X-Token header"}


def test_read_inexistent_item():
    response = client.get("/items/baz", headers={"X-Token": "coneofsilence"})
    assert response.status_code == 404
    assert response.json() == {"detail": "Item not found"}


def test_create_item():
    response = client.post(
        "/items/",
        headers={"X-Token": "coneofsilence"},
        json={"id": "foobar", "title": "Foo Bar", "description": "The Foo Barters"},
    )
    assert response.status_code == 200
    assert response.json() == {
        "id": "foobar",
        "title": "Foo Bar",
        "description": "The Foo Barters",
    }


def test_create_item_bad_token():
    response = client.post(
        "/items/",
        headers={"X-Token": "hailhydra"},
        json={"id": "bazz", "title": "Bazz", "description": "Drop the bazz"},
    )
    assert response.status_code == 400
    assert response.json() == {"detail": "Invalid X-Token header"}


def test_create_existing_item():
    response = client.post(
        "/items/",
        headers={"X-Token": "coneofsilence"},
        json={
            "id": "foo",
            "title": "The Foo ID Stealers",
            "description": "There goes my stealer",
        },
    )
    assert response.status_code == 400
    assert response.json() == {"detail": "Item already exists"}

Whenever you need the client to pass information in the request and you don't know how to, you can search (Google) how to do it in requests.

Then you just do the same in your tests.

E.g.:

• To pass a path or query parameter, add it to the URL itself.
• To pass a JSON body, pass a Python object (e.g. a dict) to the parameter json.
• If you need to send Form Data instead of JSON, use the data parameter instead.
• To pass headers, use a dict in the headers parameter.
• For cookies, a dict in the cookies parameter.

For more information about how to pass data to the backend (using requests or the TestClient) check the Requests documentation.

Info

Note that the TestClient receives data that can be converted to JSON, not Pydantic models.

If you have a Pydantic model in your test and you want to send its data to the application during testing, you can use the jsonable_encoder described in JSON Compatible Encoder.

Run it

After that, you just need to install pytest:

$ pip install pytest

---> 100%

It will detect the files and tests automatically, execute them, and report the results back to you.

Run the tests with:

$ pytest

================ test session starts ================
platform linux -- Python 3.6.9, pytest-5.3.5, py-1.8.1, pluggy-0.13.1
rootdir: /home/user/code/superawesome-cli/app
plugins: forked-1.1.3, xdist-1.31.0, cov-2.8.1
collected 6 items

---> 100%

test_main.py <span style="color: green; white-space: pre;">......                            [100%]</span>

<span style="color: green;">================= 1 passed in 0.03s =================</span>