1_orig_open = open
2
3class _BkFile:
4    def __init__(self, file, mode, bufsize):
5        import os
6        self.__filename = file
7        self.__backup = file + '~'
8        try:
9            os.unlink(self.__backup)
10        except os.error:
11            pass
12        try:
13            os.rename(file, self.__backup)
14        except os.error:
15            self.__backup = None
16        self.__file = _orig_open(file, mode, bufsize)
17        self.closed = self.__file.closed
18        self.fileno = self.__file.fileno
19        self.flush = self.__file.flush
20        self.isatty = self.__file.isatty
21        self.mode = self.__file.mode
22        self.name = self.__file.name
23        self.read = self.__file.read
24        self.readinto = self.__file.readinto
25        self.readline = self.__file.readline
26        self.readlines = self.__file.readlines
27        self.seek = self.__file.seek
28        self.softspace = self.__file.softspace
29        self.tell = self.__file.tell
30        self.truncate = self.__file.truncate
31        self.write = self.__file.write
32        self.writelines = self.__file.writelines
33
34    def close(self):
35        self.__file.close()
36        if self.__backup is None:
37            return
38        import filecmp
39        if filecmp.cmp(self.__backup, self.__filename, shallow = 0):
40            import os
41            os.unlink(self.__filename)
42            os.rename(self.__backup, self.__filename)
43
44def open(file, mode = 'r', bufsize = -1):
45    if 'w' not in mode:
46        return _orig_open(file, mode, bufsize)
47    return _BkFile(file, mode, bufsize)
48