• listdir
  • path

Confusingly, os.listdir(dir) returns a list of file names relative to dir; if this is not your current working directory (i.e. .) you cannot directly open, copy, or otherwise manipulate the files that it returns. You have to resolve the relative file name with something like

for filename in os.listdir(dir):
    with open(os.path.join(dir, filename)) as filehandle: