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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

!!! info To use TestClient, first install requests.

E.g. `pip install requests`.

Import TestClient.

Create a TestClient by passing your FastAPI application to it.

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).

{!../../../docs_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.

!!! 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{.internal-link target=_blank} 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 structure as described in Bigger Applications{.internal-link target=_blank}:

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

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

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

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):

.
├── 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):

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

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

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 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 3.6 and above"

```Python
{!> ../../../docs_src/app_testing/app_b/main.py!}
```

=== "Python 3.10 and above"

```Python
{!> ../../../docs_src/app_testing/app_b_py310/main.py!}
```

Extended testing file

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

{!> ../../../docs_src/app_testing/app_b/test_main.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` described in [JSON Compatible Encoder](encoder.md){.internal-link target=_blank}.

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>