Lib/whichdb.py

   1 """Guess which db package to use to open a db file."""
   2
   3 import os
   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     import struct
  19
  20     # Check for dbm first -- this has a .pag and a .dir file
  21     try:
  22         f = open(filename + os.extsep + "pag", "rb")
  23         f.close()
  24         f = open(filename + os.extsep + "dir", "rb")
  25         f.close()
  26         return "dbm"
  27     except IOError:
  28         pass
  29
  30     # Check for dumbdbm next -- this has a .dir and and a .dat file
  31     try:
  32         f = open(filename + os.extsep + "dat", "rb")
  33         f.close()
  34         f = open(filename + os.extsep + "dir", "rb")
  35         try:
  36             if f.read(1) in ["'", '"']:
  37                 return "dumbdbm"
  38         finally:
  39             f.close()
  40     except IOError:
  41         pass
  42
  43     # See if the file exists, return None if not
  44     try:
  45         f = open(filename, "rb")
  46     except IOError:
  47         return None
  48
  49     # Read the start of the file -- the magic number
  50     s16 = f.read(16)
  51     f.close()
  52     s = s16[0:4]
  53
  54     # Return "" if not at least 4 bytes
  55     if len(s) != 4:
  56         return ""
  57
  58     # Convert to 4-byte int in native byte order -- return "" if impossible
  59     try:
  60         (magic,) = struct.unpack("=l", s)
  61     except struct.error:
  62         return ""
  63
  64     # Check for GNU dbm
  65     if magic == 0x13579ace:
  66         return "gdbm"
  67
  68     # Check for BSD hash
  69     if magic in (0x00061561, 0x61150600):
  70         return "dbhash"
  71
  72     # BSD hash v2 has a 12-byte NULL pad in front of the file type
  73     try:
  74         (magic,) = struct.unpack("=l", s16[-4:])
  75     except struct.error:
  76         return ""
  77
  78     # Check for BSD hash
  79     if magic in (0x00061561, 0x61150600):
  80         return "dbhash"
  81
  82     # Unknown
  83     return ""