File Explorer

1"""Generic interface to all dbm clones.2 3Use4 5        import dbm6        d = dbm.open(file, 'w', 0o666)7 8The returned object is a dbm.gnu, dbm.ndbm or dbm.dumb object, dependent on the9type of database being opened (determined by the whichdb function) in the case10of an existing dbm. If the dbm does not exist and the create or new flag ('c'11or 'n') was specified, the dbm type will be determined by the availability of12the modules (tested in the above order).13 14It has the following interface (key and data are strings):15 16        d[key] = data   # store data at key (may override data at17                        # existing key)18        data = d[key]   # retrieve data at key (raise KeyError if no19                        # such key)20        del d[key]      # delete data stored at key (raises KeyError21                        # if no such key)22        flag = key in d # true if the key exists23        list = d.keys() # return a list of all existing keys (slow!)24 25Future versions may change the order in which implementations are26tested for existence, and add interfaces to other dbm-like27implementations.28"""29 30__all__ = ['open', 'whichdb', 'error']31 32import io33import os34import struct35import sys36 37 38class error(Exception):39    pass40 41_names = ['dbm.gnu', 'dbm.ndbm', 'dbm.dumb']42_defaultmod = None43_modules = {}44 45error = (error, OSError)46 47try:48    from dbm import ndbm49except ImportError:50    ndbm = None51 52 53def open(file, flag='r', mode=0o666):54    """Open or create database at path given by *file*.55 56    Optional argument *flag* can be 'r' (default) for read-only access, 'w'57    for read-write access of an existing database, 'c' for read-write access58    to a new or existing database, and 'n' for read-write access to a new59    database.60 61    Note: 'r' and 'w' fail if the database doesn't exist; 'c' creates it62    only if it doesn't exist; and 'n' always creates a new database.63    """64    global _defaultmod65    if _defaultmod is None:66        for name in _names:67            try:68                mod = __import__(name, fromlist=['open'])69            except ImportError:70                continue71            if not _defaultmod:72                _defaultmod = mod73            _modules[name] = mod74        if not _defaultmod:75            raise ImportError("no dbm clone found; tried %s" % _names)76 77    # guess the type of an existing database, if not creating a new one78    result = whichdb(file) if 'n' not in flag else None79    if result is None:80        # db doesn't exist or 'n' flag was specified to create a new db81        if 'c' in flag or 'n' in flag:82            # file doesn't exist and the new flag was used so use default type83            mod = _defaultmod84        else:85            raise error[0]("db file doesn't exist; "86                           "use 'c' or 'n' flag to create a new db")87    elif result == "":88        # db type cannot be determined89        raise error[0]("db type could not be determined")90    elif result not in _modules:91        raise error[0]("db type is {0}, but the module is not "92                       "available".format(result))93    else:94        mod = _modules[result]95    return mod.open(file, flag, mode)96 97 98def whichdb(filename):99    """Guess which db package to use to open a db file.100 101    Return values:102 103    - None if the database file can't be read;104    - empty string if the file can be read but can't be recognized105    - the name of the dbm submodule (e.g. "ndbm" or "gnu") if recognized.106 107    Importing the given module may still fail, and opening the108    database using that module may still fail.109    """110 111    # Check for ndbm first -- this has a .pag and a .dir file112    try:113        f = io.open(filename + ".pag", "rb")114        f.close()115        f = io.open(filename + ".dir", "rb")116        f.close()117        return "dbm.ndbm"118    except OSError:119        # some dbm emulations based on Berkeley DB generate a .db file120        # some do not, but they should be caught by the bsd checks121        try:122            f = io.open(filename + ".db", "rb")123            f.close()124            # guarantee we can actually open the file using dbm125            # kind of overkill, but since we are dealing with emulations126            # it seems like a prudent step127            if ndbm is not None:128                d = ndbm.open(filename)129                d.close()130                return "dbm.ndbm"131        except OSError:132            pass133 134    # Check for dumbdbm next -- this has a .dir and a .dat file135    try:136        # First check for presence of files137        os.stat(filename + ".dat")138        size = os.stat(filename + ".dir").st_size139        # dumbdbm files with no keys are empty140        if size == 0:141            return "dbm.dumb"142        f = io.open(filename + ".dir", "rb")143        try:144            if f.read(1) in (b"'", b'"'):145                return "dbm.dumb"146        finally:147            f.close()148    except OSError:149        pass150 151    # See if the file exists, return None if not152    try:153        f = io.open(filename, "rb")154    except OSError:155        return None156 157    with f:158        # Read the start of the file -- the magic number159        s16 = f.read(16)160    s = s16[0:4]161 162    # Return "" if not at least 4 bytes163    if len(s) != 4:164        return ""165 166    # Convert to 4-byte int in native byte order -- return "" if impossible167    try:168        (magic,) = struct.unpack("=l", s)169    except struct.error:170        return ""171 172    # Check for GNU dbm173    if magic in (0x13579ace, 0x13579acd, 0x13579acf):174        return "dbm.gnu"175 176    # Later versions of Berkeley db hash file have a 12-byte pad in177    # front of the file type178    try:179        (magic,) = struct.unpack("=l", s16[-4:])180    except struct.error:181        return ""182 183    # Unknown184    return ""185 186 187if __name__ == "__main__":188    for filename in sys.argv[1:]:189        print(whichdb(filename) or "UNKNOWN", filename)190