FastAPI 框架中提供了下载文件的 Response -> FileResponse，但是默认的 FileResponse 返回类型，没有办法实现大文件的流式下载，需要进一步的加工。

FileResponse 实现

FastAPI（fastapi==0.61.2） 中 FileResponse 对象的定义如下:

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class FileResponse(Response):
    chunk_size = 4096
    
    ....
    
    def set_stat_headers(self, stat_result: os.stat_result) -> None:
        content_length = str(stat_result.st_size)
        last_modified = formatdate(stat_result.st_mtime, usegmt=True)
        etag_base = str(stat_result.st_mtime) + "-" + str(stat_result.st_size)
        etag = hashlib.md5(etag_base.encode()).hexdigest()

        self.headers.setdefault("content-length", content_length)
        self.headers.setdefault("last-modified", last_modified)
        self.headers.setdefault("etag", etag)

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        if self.stat_result is None:
            try:
                stat_result = await aio_stat(self.path)
                self.set_stat_headers(stat_result)
            except FileNotFoundError:
                raise RuntimeError(f"File at path {self.path} does not exist.")
            else:
                mode = stat_result.st_mode
                if not stat.S_ISREG(mode):
                    raise RuntimeError(f"File at path {self.path} is not a file.")
            ....

可以看到，在调用 FileResponse 时，会优先设置 headers，其中包括了 content-length，这个标记将使浏览器默认下载全部的文件内容，再保存到本地

因此，我们可以通过删除 content-length，来实现文件的流式下载

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class QStreamingFileResponse(FileResponse):  # type: ignore

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:

        await send({
            "type": "http.response.start",
            "status": self.status_code,
            "headers": self.raw_headers,
        })
        if self.send_header_only:
            await send({"type": "http.response.body", "body": b"", "more_body": False})
        else:
            async with aiofiles.open(self.path, mode="rb") as file:
                more_body = True
                while more_body:
                    chunk = await file.read(self.chunk_size)
                    more_body = len(chunk) == self.chunk_size
                    await send({
                        "type": "http.response.body",
                        "body": chunk,
                        "more_body": more_body,
                    })
        if self.background is not None:
            await self.background()

@router.get("/file", response_class=QStreamingFileResponse)
async def get_proxy_big_file(
    request: Request,
    file_path: pathlib.Path,
) -> QStreamingFileResponse:
    return QStreamingFileResponse(
            path=str(file_path),
            filename=file_path.name,
            media_type='application/octet-stream',
        )