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"""Guess which db package to use to open a db file."""
import struct
def whichdb(filename):
"""Guess which db package to use to open a db file.
Return values:
- None if the database file can't be read;
- empty string if the file can be read but can't be recognized
- the module name (e.g. "dbm" or "gdbm") if recognized.
Importing the given module may still fail, and opening the
database using that module may still fail.
"""
# Check for dbm first -- this has a .pag and a .dir file
try:
f = open(filename + ".pag", "rb")
f.close()
f = open(filename + ".dir", "rb")
f.close()
return "dbm"
except IOError:
pass
# See if the file exists, return None if not
try:
f = open(filename, "rb")
except IOError:
return None
# Read the start of the file -- the magic number
s16 = f.read(16)
f.close()
s = s16[0:4]
# Return "" if not at least 4 bytes
if len(s) != 4:
return ""
# Convert to 4-byte int in native byte order -- return "" if impossible
try:
(magic,) = struct.unpack("=l", s)
except struct.error:
return ""
# Check for GNU dbm
if magic == 0x13579ace:
return "gdbm"
# Check for BSD hash
if magic in (0x00061561, 0x61150600):
return "dbhash"
# BSD hash v2 has a 12-byte NULL pad in front of the file type
try:
(magic,) = struct.unpack("=l", s16[-4:])
except struct.error:
return ""
# Check for BSD hash
if magic in (0x00061561, 0x61150600):
return "dbhash"
# Unknown
return ""
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