tiangolo.fastapi/docs/tutorial/testing.md

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.

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).

{!./src/app_testing/tutorial001.py!}

!!! 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.

!!! note "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.

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:

{!./src/app_testing/main.py!}

Testing file

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

{!./src/app_testing/test_main.py!}

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 you have a file main_b.py with your FastAPI app.

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.

{!./src/app_testing/main_b.py!}

Extended testing file

You could then have a test_main_b.py, the same as before, with the extended tests:

{!./src/app_testing/test_main_b.py!}

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` descibed in [JSON Compatible Encoder](encoder.md){.internal-link target=_blank}.

Run it

After that, you just need to install pytest:

pip install pytest

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

Run the tests with:

pytest