How to Download a File after POSTing Data with FastAPI?

How to Download a File after POSTing data using FastAPI?

When working with FastAPI, downloading a file after posting data revolves around utilizing the FileResponse class. To achieve this:

1. Define a Form-data endpoint: Use the Form keyword to define the expected form-data in your endpoint. Ensure the required parameters are set as required in the Form(...).
2. Process and generate a response: After processing the received data and generating the file, use FileResponse to return the file to the user.
3. Set Content-Disposition header: To initiate a download, set the Content-Disposition header in FileResponse using the 'attachment' parameter.
4. Return the FileResponse: Send the FileResponse back to the client, ensuring the headers are correct to trigger the file download.

Here's an example:

<code class="python">@app.post("/download")
async def download_file(request: Request):
    if request.method == "POST":
        form = await request.form()
        if form["message"] and form["language"]:
            # Process the data and generate the file here
            file_path = "path/to/file.mp3"
            headers = {"Content-Disposition": f"attachment; filename=downloaded_file.mp3"}
            return FileResponse(file_path, headers=headers, media_type="audio/mp3")</code>

Remember, if you want the endpoint to handle both GET and POST requests, use either @app.api_route() with methods=["GET", "POST"] or define separate endpoints with @app.post() and @app.get().

Additionally, if you plan to download multiple files or need more flexibility, consider using other concepts such as:

• StreamingResponse: To process large files in chunks, use StreamingResponse instead of FileResponse.
• JavaScript and Fetch API: In the frontend, utilize JavaScript and Fetch API to initiate the file download process.
• Key-Value Stores/Database: To handle multiple users and files simultaneously, consider using a Key-Value store or database to map file paths to unique identifiers.

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