Lib/whichdb.py

   1 """Guess which db package to use to open a db file."""
   2
   3 import os
   4
   5 if os.sep==".":
   6     endsep = "/"
   7 else:
   8     endsep = "."
   9
  10 def whichdb(filename):
  11     """Guess which db package to use to open a db file.
  12
  13     Return values:
  14
  15     - None if the database file can't be read;
  16     - empty string if the file can be read but can't be recognized
  17     - the module name (e.g. "dbm" or "gdbm") if recognized.
  18
  19     Importing the given module may still fail, and opening the
  20     database using that module may still fail.
  21     """
  22
  23     import struct
  24
  25     # Check for dbm first -- this has a .pag and a .dir file
  26     try:
  27         f = open(filename + endsep + "pag", "rb")
  28         f.close()
  29         f = open(filename + endsep + "dir", "rb")
  30         f.close()
  31         return "dbm"
  32     except IOError:
  33         pass
  34
  35     # Check for dumbdbm next -- this has a .dir and and a .dat file
  36     try:
  37         f = open(filename + endsep + "dat", "rb")
  38         f.close()
  39         f = open(filename + endsep + "dir", "rb")
  40         try:
  41             if f.read(1) in ["'", '"']:
  42                 return "dumbdbm"
  43         finally:
  44             f.close()
  45     except IOError:
  46         pass
  47
  48     # See if the file exists, return None if not
  49     try:
  50         f = open(filename, "rb")
  51     except IOError:
  52         return None
  53
  54     # Read the start of the file -- the magic number
  55     s16 = f.read(16)
  56     f.close()
  57     s = s16[0:4]
  58
  59     # Return "" if not at least 4 bytes
  60     if len(s) != 4:
  61         return ""
  62
  63     # Convert to 4-byte int in native byte order -- return "" if impossible
  64     try:
  65         (magic,) = struct.unpack("=l", s)
  66     except struct.error:
  67         return ""
  68
  69     # Check for GNU dbm
  70     if magic == 0x13579ace:
  71         return "gdbm"
  72
  73     # Check for BSD hash
  74     if magic in (0x00061561, 0x61150600):
  75         return "dbhash"
  76
  77     # BSD hash v2 has a 12-byte NULL pad in front of the file type
  78     try:
  79         (magic,) = struct.unpack("=l", s16[-4:])
  80     except struct.error:
  81         return ""
  82
  83     # Check for BSD hash
  84     if magic in (0x00061561, 0x61150600):
  85         return "dbhash"
  86
  87     # Unknown
  88     return ""