# Background Tasks - `BackgroundTasks`

You can declare a parameter in a *path operation function* or dependency function with the type `BackgroundTasks`, and then you can use it to schedule the execution of background tasks after the response is sent.

You can import it directly from `fastapi`:

```python
from fastapi import BackgroundTasks
```

You need to import `WSGIMiddleware`.

Then wrap the WSGI (e.g. Flask) app with the middleware.

And then mount that under a path.

{* ../../docs_src/wsgi/tutorial001_py39.py hl[2:3,3] *}

## Check it { #check-it }

Now, every request under the path `/v1/` will be handled by the Flask application.

And the rest will be handled by **FastAPI**.

        return subProjectFolders
    }

    @Test
    fun allSubprojectsAreListed() {
        val knownSubprojectDirs = model.subprojects.subprojects.map { File("../", it.path) }
        subProjectFolderList().forEach {
            assertTrue(
                it in knownSubprojectDirs,
                "Not defined: $it",
            )
        }
    }

    @Test

        response = client.get("/prefix/")
        assert response.status_code == 200, response.text
        assert response.json() == ["OK"]


def test_include_empty():
    # if both include and router.path are empty - it should raise exception
    with pytest.raises(FastAPIError):

        ],
        capture_output=True,
        encoding="utf-8",
        env={**os.environ, "PYTHONIOENCODING": "utf-8"},
    )
    assert result.returncode == 1, result.stdout
    assert "Path does not exist non_existent_file.py" in result.stdout


def test_fastapi_cli_not_installed():
    with patch.object(fastapi.cli, "cli_main", None):
        with pytest.raises(RuntimeError) as exc_info:

import javax.inject.Named;
import javax.inject.Singleton;
import javax.xml.stream.XMLStreamException;

import java.io.IOException;
import java.io.UncheckedIOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.Objects;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

Doing this, our `GzipRequest` will take care of decompressing the data (if necessary) before passing it to our *path operations*.

After that, all of the processing logic is the same.

But because of our changes in `GzipRequest.body`, the request body will be automatically decompressed when it is loaded by **FastAPI** when needed.

from starlette.exceptions import HTTPException as StarletteHTTPException
from starlette.exceptions import WebSocketException as StarletteWebSocketException


class EndpointContext(TypedDict, total=False):
    function: str
    path: str
    file: str
    line: int


class HTTPException(StarletteHTTPException):
    """
    An HTTP exception you can raise in your own code to show errors to the client.

                    value = ReflectionValueExtractor.evaluate(pathExpression, session);
                    if (pathSeparator < expression.length() - 1) {
                        if (value instanceof Path path) {
                            value = path.resolve(expression.substring(pathSeparator + 1));
                        } else {
                            value = value + expression.substring(pathSeparator);
                        }

`BackgroundTasks` の使用は依存性注入システムでも機能し、様々な階層 (*path operations 関数*、依存性 (依存可能性)、サブ依存性など) で `BackgroundTasks` 型のパラメーターを宣言できます。

**FastAPI** は、それぞれの場合の処理​​方法と同じオブジェクトの再利用方法を知っているため、すべてのバックグラウンドタスクがマージされ、バックグラウンドで後で実行されます。

{* ../../docs_src/background_tasks/tutorial002.py hl[13,15,22,25] *}

この例では、レスポンスが送信された *後* にメッセージが `log.txt` ファイルに書き込まれます。

リクエストにクエリがあった場合、バックグラウンドタスクでログに書き込まれます。