[capink]

Also if someone can explain what the code below is trying to do — especially the part highlighted in red — I would appreciate it.

                if external_app_path.lower().endswith('calibredb.exe'):
                    print('running a calibredb command.')
                    startupinfo = subprocess.STARTUPINFO()
                    startupinfo.dwFlags |= subprocess.STARTF_USESHOWWINDOW
                    fred = 'calibredb add --library-path="C:/Users/Declan/Documents/Calibre Library" -d "D:/Downloads/Merlin-Thered Been A Time - A_Fallen_Sister.epub"'
                    p = subprocess.Popen(fred, shell=True, startupinfo=startupinfo)
                    rc = p.wait()
                    if rc != 0:
                        if DEBUG: print("FAILURE for: ", fred)
                    else:
                        if DEBUG: print("...success for...", fred)