Delete an uploaded file after downloading it from Flask

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.)