from builtins import open as _orig_open

def open(file, mode='r', bufsize=-1):
 if 'w' not in mode:
 return _orig_open(file, mode, bufsize)
 import os
 backup = file + '~'
 try:
 os.unlink(backup)
 except OSError:
 pass
 try:
 os.rename(file, backup)
 except OSError:
 return _orig_open(file, mode, bufsize)
 f = _orig_open(file, mode, bufsize)
 _orig_close = f.close
 def close():
 _orig_close()
 import filecmp
 if filecmp.cmp(backup, file, shallow=False):
 import os
 os.unlink(file)
 os.rename(backup, file)
 f.close = close
 return f