"""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__ import UserDict _open = __builtin__.open _BLOCKSIZE = 512 error = IOError # For anydbm class _Database(UserDict.DictMixin): def __init__(self, filebasename, mode): self._mode = mode # The directory file is a text file. Each line looks like # "%r, (%d, %d)\n" % (key, pos, siz) # where key is the string key, pos is the offset into the dat # file of the associated value's first byte, and siz is the number # of bytes in the associated value. self._dirfile = filebasename + _os.extsep + 'dir' # The data file is a binary file pointed into by the directory # file, and holds the values associated with keys. Each value # begins at a _BLOCKSIZE-aligned byte offset, and is a raw # binary 8-bit string value. self._datfile = filebasename + _os.extsep + 'dat' self._bakfile = filebasename + _os.extsep + 'bak' # The index is an in-memory dict, mirroring the directory file. self._index = None # maps keys to (pos, siz) pairs # 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() # Read directory file into the in-memory index dict. def _update(self): self._index = {} try: f = _open(self._dirfile) except IOError: pass else: for line in f: key, pos_and_siz_pair = eval(line) self._index[key] = pos_and_siz_pair f.close() # Write the index dict to the directory file. The original directory # file (if any) is renamed with a .bak extension first. If a .bak # file currently exists, it's deleted. 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("%r, (%d, %d)\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 # Append val to the data file, starting at a _BLOCKSIZE-aligned # offset. The data file is first padded with NUL bytes (if needed) # to get to an aligned offset. Return pair # (starting offset of val, len(val)) def _addval(self, val): f = _open(self._datfile, 'rb+') f.seek(0, 2) pos = int(f.tell()) npos = ((pos + _BLOCKSIZE - 1) // _BLOCKSIZE) * _BLOCKSIZE f.write('\0'*(npos-pos)) pos = npos f.write(val) f.close() return (pos, len(val)) # Write val to the data file, starting at offset pos. The caller # is responsible for ensuring that there's enough room starting at # pos to hold val, without overwriting some other value. Return # pair (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)) # key is a new key whose associated value starts in the data file # at offset pos and with length size. Add an index record to # the in-memory index dict, and append one to the index file. def _addkey(self, key, pos_and_siz_pair): self._index[key] = pos_and_siz_pair f = _open(self._dirfile, 'a', self._mode) f.write("%r, %r\n" % (key, pos_and_siz_pair)) f.close() def __setitem__(self, key, val): if not type(key) == type('') == type(val): raise TypeError, "keys and values must be strings" if key not in self._index: self._addkey(key, self._addval(val)) else: # See whether the new value is small enough to fit in the # (padded) space currently occupied by the old value. pos, siz = self._index[key] oldblocks = (siz + _BLOCKSIZE - 1) // _BLOCKSIZE newblocks = (len(val) + _BLOCKSIZE - 1) // _BLOCKSIZE if newblocks <= oldblocks: self._index[key] = self._setval(pos, val) else: # The new value doesn't fit in the (padded) space used # by the old value. The blocks used by the old value are # forever lost. self._index[key] = self._addval(val) # Note that _index may be out of synch with the directory # file now: _setval() and _addval() don't update the directory # file. def __delitem__(self, key): # The blocks used by the associated value are lost. del self._index[key] # XXX It's unclear why we do a _commit() here (the code always # XXX has, so I'm not changing it). _setitem__ doesn't try to # XXX keep the directory file in synch. Why should we? Or # XXX why shouldn't __setitem__? 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)