# Testing

Thanks to [Starlette](https://starlette.dev/testclient/), testing **FastAPI** applications is easy and enjoyable.

It is based on [HTTPX](https://www.python-httpx.org), which in turn is designed based on Requests, so it's very familiar and intuitive.

With it, you can use [pytest](https://docs.pytest.org/) directly with **FastAPI**.

## Using `TestClient`

:::callout{intent="note"}
To use `TestClient`, first install [`httpx`](https://www.python-httpx.org).

Add it to your project:

```console
$ uv add httpx
```
:::

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"}
```

:::callout{intent="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.
:::

:::callout{intent="note" title="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.
:::

:::callout{intent="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](/guides/advanced-user-guide-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 structure as described in [Bigger Applications](/guides/tutorial-user-guide-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

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

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
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

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](https://www.python-httpx.org).

:::callout{intent="note"}
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](/guides/tutorial-user-guide-encoder).
:::

## 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>
```

## Related pages

- [About](./about-index.md)
- [Advanced User Guide](./advanced-user-guide-index.md)
- [Deployment](./deployment-index.md)
- [FastAPI](./fastapi-index.md)
- [FastAPI Docs](../index.md)
- [Features](./features-index.md)
- [How To - Recipes](./how-to-recipes-index.md)
- [Learn](./learn-index.md)
- [More](./more-index.md)
- [OpenAPI](./openapi-index.md)

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