blob: 8985062d5d97875dd517bdfd15bd025d9431f0bf [file] [log] [blame]
Guido van Rossum8d12a1b1996-07-30 16:30:15 +00001"""Guess which db package to use to open a db file."""
2
3import struct
4
5def 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 first 4 bytes of the file -- the magic number
35 s = f.read(4)
36 f.close()
37
38 # Return "" if not at least 4 bytes
39 if len(s) != 4:
40 return ""
41
42 # Convert to 4-byte int in native byte order -- return "" if impossible
43 try:
Guido van Rossum265b5b31997-01-11 19:22:11 +000044 (magic,) = struct.unpack("=l", s)
Guido van Rossum8d12a1b1996-07-30 16:30:15 +000045 except struct.error:
Guido van Rossum265b5b31997-01-11 19:22:11 +000046 return ""
Guido van Rossum8d12a1b1996-07-30 16:30:15 +000047
48 # Check for GNU dbm
49 if magic == 0x13579ace:
50 return "gdbm"
51
52 # Check for BSD hash
53 if magic == 0x061561:
54 return "dbhash"
55
56 # Unknown
57 return ""