What is the best way to write the contents of a StringIO to a file?

Use shutil.copyfileobj:

with open('file.xml', 'w') as fd:
  buf.seek(0)
  shutil.copyfileobj(buf, fd)

or shutil.copyfileobj(buf, fd, -1) to copy from a file object without using chunks of limited size (used to avoid uncontrolled memory consumption).