# Testing { #testing }

Thanks to <a href="https://www.starlette.dev/testclient/" class="external-link" target="_blank">Starlette</a>, testing **FastAPI** applications is easy and enjoyable.

It is based on <a href="https://www.python-httpx.org" class="external-link" target="_blank">HTTPX</a>, which in turn is designed based on Requests, so it's very familiar and intuitive.

With it, you can use <a href="https://docs.pytest.org/" class="external-link" target="_blank">pytest</a> directly with **FastAPI**.

## Using `TestClient` { #using-testclient }

/// info

To use `TestClient`, first install <a href="https://www.python-httpx.org" class="external-link" target="_blank">`httpx`</a>.

Make sure you create a [virtual environment](../virtual-environments.md){.internal-link target=_blank}, activate it, and then install it, for example:

```console
$ pip install httpx
```

///

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 `httpx`.

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

{* ../../docs_src/app_testing/tutorial001_py39.py hl[2,12,15:18] *}

/// 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](../advanced/async-tests.md){.internal-link target=_blank} in the advanced tutorial.

///

## Separating tests { #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 { #fastapi-app-file }

Let's say you have a file structure as described in [Bigger Applications](bigger-applications.md){.internal-link target=_blank}:

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

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


{* ../../docs_src/app_testing/app_a_py39/main.py *}

### Testing file { #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):

``` hl_lines="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`):

{* ../../docs_src/app_testing/app_a_py39/test_main.py hl[3] *}


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

## Testing: extended example { #testing-extended-example }

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

### Extended **FastAPI** app file { #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.

{* ../../docs_src/app_testing/app_b_an_py310/main.py *}

### Extended testing file { #extended-testing-file }

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

{* ../../docs_src/app_testing/app_b_an_py310/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 `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 <a href="https://www.python-httpx.org" class="external-link" target="_blank">HTTPX documentation</a>.

/// 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 { #run-it }

After that, you just need to install `pytest`.

Make sure you create a [virtual environment](../virtual-environments.md){.internal-link target=_blank}, activate it, and then install it, for example:

<div class="termy">

```console
$ pip install pytest

---> 100%
```

</div>

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

Run the tests with:

<div class="termy">

```console
$ 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>
```

</div>
