Lib/whichdb.py

   1 """Guess which db package to use to open a db file."""
   2
   3 import struct
   4
   5 def whichdb(filename):
   6     """Guess which db package to use to open a db file.
   7
   8     Return values:
   9
  10     - None if the database file can't be read;
  11     - empty string if the file can be read but can't be recognized
  12     - the module name (e.g. "dbm" or "gdbm") if recognized.
  13
  14     Importing the given module may still fail, and opening the
  15     database using that module may still fail.
  16     """
  17
  18     # Check for dbm first -- this has a .pag and a .dir file
  19     try:
  20         f = open(filename + ".pag", "rb")
  21         f.close()
  22         f = open(filename + ".dir", "rb")
  23         f.close()
  24         return "dbm"
  25     except IOError:
  26         pass
  27
  28     # See if the file exists, return None if not
  29     try:
  30         f = open(filename, "rb")
  31     except IOError:
  32         return None
  33
  34     # Read the start of the file -- the magic number
  35     s16 = f.read(16)
  36     f.close()
  37     s = s16[0:4]
  38
  39     # Return "" if not at least 4 bytes
  40     if len(s) != 4:
  41         return ""
  42
  43     # Convert to 4-byte int in native byte order -- return "" if impossible
  44     try:
  45         (magic,) = struct.unpack("=l", s)
  46     except struct.error:
  47         return ""
  48
  49     # Check for GNU dbm
  50     if magic == 0x13579ace:
  51         return "gdbm"
  52
  53     # Check for BSD hash
  54     if magic in (0x00061561, 0x61150600):
  55         return "dbhash"
  56
  57     # BSD hash v2 has a 12-byte NULL pad in front of the file type
  58     try:
  59         (magic,) = struct.unpack("=l", s16[-4:])
  60     except struct.error:
  61         return ""
  62
  63     # Check for BSD hash
  64     if magic in (0x00061561, 0x61150600):
  65         return "dbhash"
  66
  67     # Unknown
  68     return ""