Delete an uploaded file after downloading it from Flask

Question

I am currently working on a small web interface which allows different users to upload files, convert the files they have uploaded, and download the converted files. The details of the conversion are not important for my question.

I am currently using flask-uploads to manage the uploaded files, and I am storing them in the file system. Once a user uploads and converts a file, there are all sorts of pretty buttons to delete the file, so that the uploads folder doesn't fill up.

I don't think this is ideal. What I really want is for the files to be deleted right after they are downloaded. I would settle for the files being deleted when the session ends.

I've spent some time trying to figure out how to do this, but I have yet to succeed. It doesn't seem like an uncommon problem, so I figure there must be some solution out there that I am missing. Does anyone have a solution?

Answer 1

There are several ways to do this.

send_file and then immediately delete (Linux only)

Flask has an after_this_request decorator which could work for this use case:

@app.route('/files/<filename>/download')
def download_file(filename):
    file_path = derive_filepath_from_filename(filename)
    file_handle = open(file_path, 'r')
    @after_this_request
    def remove_file(response):
        try:
            os.remove(file_path)
            file_handle.close()
        except Exception as error:
            app.logger.error("Error removing or closing downloaded file handle", error)
        return response
    return send_file(file_handle)

The issue is that this will only work on Linux (which lets the file be read even after deletion if there is still an open file pointer to it). It also won't always work (I've heard reports that sometimes send_file won't wind up making the kernel call before the file is already unlinked by Flask). It doesn't tie up the Python process to send the file though.

Stream file, then delete

Ideally though you'd have the file cleaned up after you know the OS has streamed it to the client. You can do this by streaming the file back through Python by creating a generator that streams the file and then closes it, like is suggested in this answer:

def download_file(filename):
    file_path = derive_filepath_from_filename(filename)
    file_handle = open(file_path, 'r')

    # This *replaces* the `remove_file` + @after_this_request code above
    def stream_and_remove_file():
        yield from file_handle
        file_handle.close()
        os.remove(file_path)

    return current_app.response_class(
        stream_and_remove_file(),
        headers={'Content-Disposition': 'attachment', 'filename': filename}
    )

This approach is nice because it is cross-platform. It isn't a silver bullet however, because it ties up the Python web process until the entire file has been streamed to the client.

Clean up on a timer

Run another process on a timer (using cron, perhaps) or use an in-process scheduler like APScheduler and clean up files that have been on-disk in the temporary location beyond your timeout (e. g. half an hour, one week, thirty days, after they've been marked "downloaded" in RDMBS)

This is the most robust way, but requires additional complexity (cron, in-process scheduler, work queue, etc.)

Answer 2

You can also store the file in memory, delete it, then serve what you have in memory.

For example, if you were serving a PDF:

import io
import os

@app.route('/download')
def download_file():
    file_path = get_path_to_your_file()

    return_data = io.BytesIO()
    with open(file_path, 'rb') as fo:
        return_data.write(fo.read())
    # (after writing, cursor will be at last byte, so move it to start)
    return_data.seek(0)

    os.remove(file_path)

    return send_file(return_data, mimetype='application/pdf',
                     attachment_filename='download_filename.pdf')

(above I'm just assuming it's PDF, but you can get the mimetype programmatically if you need)