Guido van Rossum | 8d12a1b | 1996-07-30 16:30:15 +0000 | [diff] [blame] | 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 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: |
| 44 | (magic,) = struct.unpack("l", s) |
| 45 | except struct.error: |
| 46 | try: |
| 47 | (magic,) = struct.unpack("i", s) |
| 48 | except struct.error: |
| 49 | return "" |
| 50 | |
| 51 | # Check for GNU dbm |
| 52 | if magic == 0x13579ace: |
| 53 | return "gdbm" |
| 54 | |
| 55 | # Check for BSD hash |
| 56 | if magic == 0x061561: |
| 57 | return "dbhash" |
| 58 | |
| 59 | # Unknown |
| 60 | return "" |