Lib/dumbdbm.py

"""A dumb and slow but simple dbm clone.

For database spam, spam.dir contains the index (a text file),
spam.bak *may* contain a backup of the index (also a text file),
while spam.dat contains the data (a binary file).

XXX TO DO:

- seems to contain a bug when updating...

- reclaim free space (currently, space once occupied by deleted or expanded
items is never reused)

- support concurrent access (currently, if two processes take turns making
updates, they can mess up the index)

- support efficient access to large databases (currently, the whole index
is read when the database is opened, and some updates rewrite the whole index)

- support opening for read-only (flag = 'm')

"""

import os as _os
import __builtin__

_open = __builtin__.open

_BLOCKSIZE = 512

error = IOError                         # For anydbm

class _Database:

    def __init__(self, file, mode):
        self._mode = mode
        self._dirfile = file + _os.extsep + 'dir'
        self._datfile = file + _os.extsep + 'dat'
        self._bakfile = file + _os.extsep + 'bak'
        # Mod by Jack: create data file if needed
        try:
            f = _open(self._datfile, 'r')
        except IOError:
            f = _open(self._datfile, 'w', self._mode)
        f.close()
        self._update()

    def _update(self):
        self._index = {}
        try:
            f = _open(self._dirfile)
        except IOError:
            pass
        else:
            while 1:
                line = f.readline().rstrip()
                if not line: break
                key, (pos, siz) = eval(line)
                self._index[key] = (pos, siz)
            f.close()

    def _commit(self):
        try: _os.unlink(self._bakfile)
        except _os.error: pass
        try: _os.rename(self._dirfile, self._bakfile)
        except _os.error: pass
        f = _open(self._dirfile, 'w', self._mode)
        for key, (pos, siz) in self._index.items():
            f.write("%s, (%s, %s)\n" % (`key`, `pos`, `siz`))
        f.close()

    def __getitem__(self, key):
        pos, siz = self._index[key]     # may raise KeyError
        f = _open(self._datfile, 'rb')
        f.seek(pos)
        dat = f.read(siz)
        f.close()
        return dat

    def _addval(self, val):
        f = _open(self._datfile, 'rb+')
        f.seek(0, 2)
        pos = int(f.tell())
## Does not work under MW compiler
##              pos = ((pos + _BLOCKSIZE - 1) / _BLOCKSIZE) * _BLOCKSIZE
##              f.seek(pos)
        npos = ((pos + _BLOCKSIZE - 1) // _BLOCKSIZE) * _BLOCKSIZE
        f.write('\0'*(npos-pos))
        pos = npos

        f.write(val)
        f.close()
        return (pos, len(val))

    def _setval(self, pos, val):
        f = _open(self._datfile, 'rb+')
        f.seek(pos)
        f.write(val)
        f.close()
        return (pos, len(val))

    def _addkey(self, key, (pos, siz)):
        self._index[key] = (pos, siz)
        f = _open(self._dirfile, 'a', self._mode)
        f.write("%s, (%s, %s)\n" % (`key`, `pos`, `siz`))
        f.close()

    def __setitem__(self, key, val):
        if not type(key) == type('') == type(val):
            raise TypeError, "keys and values must be strings"
        if not key in self._index:
            (pos, siz) = self._addval(val)
            self._addkey(key, (pos, siz))
        else:
            pos, siz = self._index[key]
            oldblocks = (siz + _BLOCKSIZE - 1) / _BLOCKSIZE
            newblocks = (len(val) + _BLOCKSIZE - 1) / _BLOCKSIZE
            if newblocks <= oldblocks:
                pos, siz = self._setval(pos, val)
                self._index[key] = pos, siz
            else:
                pos, siz = self._addval(val)
                self._index[key] = pos, siz

    def __delitem__(self, key):
        del self._index[key]
        self._commit()

    def keys(self):
        return self._index.keys()

    def has_key(self, key):
        return key in self._index

    def __contains__(self, key):
        return key in self._index

    def iterkeys(self):
        return self._index.iterkeys()
    __iter__ = iterkeys

    def __len__(self):
        return len(self._index)

    def close(self):
        self._commit()
        self._index = None
        self._datfile = self._dirfile = self._bakfile = None

    def __del__(self):
        if self._index is not None:
            self._commit()



def open(file, flag=None, mode=0666):
    """Open the database file, filename, and return corresponding object.

    The flag argument, used to control how the database is opened in the
    other DBM implementations, is ignored in the dumbdbm module; the
    database is always opened for update, and will be created if it does
    not exist.

    The optional mode argument is the UNIX mode of the file, used only when
    the database has to be created.  It defaults to octal code 0666 (and
    will be modified by the prevailing umask).

    """
    # flag argument is currently ignored
    return _Database(file, mode)