Lib/dumbdbm.py

   1 """A dumb and slow but simple dbm clone.
   2
   3 For database spam, spam.dir contains the index (a text file),
   4 spam.bak *may* contain a backup of the index (also a text file),
   5 while spam.dat contains the data (a binary file).
   6
   7 XXX TO DO:
   8
   9 - seems to contain a bug when updating...
  10
  11 - reclaim free space (currently, space once occupied by deleted or expanded
  12 items is never reused)
  13
  14 - support concurrent access (currently, if two processes take turns making
  15 updates, they can mess up the index)
  16
  17 - support efficient access to large databases (currently, the whole index
  18 is read when the database is opened, and some updates rewrite the whole index)
  19
  20 - support opening for read-only (flag = 'm')
  21
  22 """
  23
  24 _os = __import__('os')
  25 import __builtin__
  26
  27 _open = __builtin__.open
  28
  29 _BLOCKSIZE = 512
  30
  31 error = IOError                         # For anydbm
  32
  33 class _Database:
  34
  35         def __init__(self, file):
  36                 self._dirfile = file + '.dir'
  37                 self._datfile = file + '.dat'
  38                 self._bakfile = file + '.bak'
  39                 # Mod by Jack: create data file if needed
  40                 try:
  41                         f = _open(self._datfile, 'r')
  42                 except IOError:
  43                         f = _open(self._datfile, 'w')
  44                 f.close()
  45                 self._update()
  46
  47         def _update(self):
  48                 self._index = {}
  49                 try:
  50                         f = _open(self._dirfile)
  51                 except IOError:
  52                         pass
  53                 else:
  54                         while 1:
  55                                 line = f.readline()
  56                                 if not line: break
  57                                 key, (pos, siz) = eval(line)
  58                                 self._index[key] = (pos, siz)
  59                         f.close()
  60
  61         def _commit(self):
  62                 try: _os.unlink(self._bakfile)
  63                 except _os.error: pass
  64                 try: _os.rename(self._dirfile, self._bakfile)
  65                 except _os.error: pass
  66                 f = _open(self._dirfile, 'w')
  67                 for key, (pos, siz) in self._index.items():
  68                         f.write("%s, (%s, %s)\n" % (`key`, `pos`, `siz`))
  69                 f.close()
  70
  71         def __getitem__(self, key):
  72                 pos, siz = self._index[key]     # may raise KeyError
  73                 f = _open(self._datfile, 'rb')
  74                 f.seek(pos)
  75                 dat = f.read(siz)
  76                 f.close()
  77                 return dat
  78
  79         def _addval(self, val):
  80                 f = _open(self._datfile, 'rb+')
  81                 f.seek(0, 2)
  82                 pos = f.tell()
  83 ## Does not work under MW compiler
  84 ##              pos = ((pos + _BLOCKSIZE - 1) / _BLOCKSIZE) * _BLOCKSIZE
  85 ##              f.seek(pos)
  86                 npos = ((pos + _BLOCKSIZE - 1) / _BLOCKSIZE) * _BLOCKSIZE
  87                 f.write('\0'*(npos-pos))
  88                 pos = npos
  89
  90                 f.write(val)
  91                 f.close()
  92                 return (pos, len(val))
  93
  94         def _setval(self, pos, val):
  95                 f = _open(self._datfile, 'rb+')
  96                 f.seek(pos)
  97                 f.write(val)
  98                 f.close()
  99                 return (pos, len(val))
 100
 101         def _addkey(self, key, (pos, siz)):
 102                 self._index[key] = (pos, siz)
 103                 f = _open(self._dirfile, 'a')
 104                 f.write("%s, (%s, %s)\n" % (`key`, `pos`, `siz`))
 105                 f.close()
 106
 107         def __setitem__(self, key, val):
 108                 if not type(key) == type('') == type(val):
 109                         raise TypeError, "keys and values must be strings"
 110                 if not self._index.has_key(key):
 111                         (pos, siz) = self._addval(val)
 112                         self._addkey(key, (pos, siz))
 113                 else:
 114                         pos, siz = self._index[key]
 115                         oldblocks = (siz + _BLOCKSIZE - 1) / _BLOCKSIZE
 116                         newblocks = (len(val) + _BLOCKSIZE - 1) / _BLOCKSIZE
 117                         if newblocks <= oldblocks:
 118                                 pos, siz = self._setval(pos, val)
 119                                 self._index[key] = pos, siz
 120                         else:
 121                                 pos, siz = self._addval(val)
 122                                 self._index[key] = pos, siz
 123                         self._addkey(key, (pos, siz))
 124
 125         def __delitem__(self, key):
 126                 del self._index[key]
 127                 self._commit()
 128
 129         def keys(self):
 130                 return self._index.keys()
 131
 132         def has_key(self, key):
 133                 return self._index.has_key(key)
 134
 135         def __len__(self):
 136                 return len(self._index)
 137
 138         def close(self):
 139                 self._index = None
 140                 self._datfile = self._dirfile = self._bakfile = None
 141
 142
 143 def open(file, flag = None, mode = None):
 144         # flag, mode arguments are currently ignored
 145         return _Database(file)