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Testing

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

It is based on HTTPX, which in turn is designed based on Requests, so it's very familiar and intuitive.

With it, you can use pytest directly with FastAPI.

Using TestClient

Section titled “Using TestClient”

Import TestClient.

Create a TestClient by passing your FastAPI application to it.

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

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

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

Python
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"}

Separating tests

Section titled “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

Section titled “FastAPI app file”

Let's say you have a file structure as described in Bigger Applications:

.
├── app
│   ├── __init__.py
│   └── main.py

In the file main.py you have your FastAPI app:

Python
from fastapi import FastAPI

app = FastAPI()


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

Testing file

Section titled “Testing file”

Then you could have a file test_main.py with your tests. It could live on the same Python package (the same directory with a __init__.py file):

{5}
.
├── app
│   ├── __init__.py
│   ├── main.py
│   └── test_main.py

Because this file is in the same package, you can use relative imports to import the object app from the main module (main.py):

Python
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"}

...and have the code for the tests just like before.

Testing: extended example

Section titled “Testing: extended example”

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

Extended FastAPI app file

Section titled “Extended FastAPI app file”

Let's continue with the same file structure as before:

.
├── app
│   ├── __init__.py
│   ├── main.py
│   └── test_main.py

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
from typing import Annotated

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: Annotated[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/")
async def create_item(item: Item, x_token: Annotated[str, Header()]) -> Item:
    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=409, detail="Item already exists")
    fake_db[item.id] = item.model_dump()
    return item

Extended testing file

Section titled “Extended testing file”

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

Python
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_nonexistent_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 == 409
    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 httpx, or even how to do it with requests, as HTTPX's design is based on Requests' design.

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 httpx or the TestClient) check the HTTPX documentation.

Run it

Section titled “Run it”

After that, you just need to install pytest.

Add it to your project:

console
$ uv add pytest

---> 100%

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

Run the tests with:

console
$ uv run 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>
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