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 class _Database:
  32
  33         def __init__(self, file):
  34                 self._dirfile = file + '.dir'
  35                 self._datfile = file + '.dat'
  36                 self._bakfile = file + '.bak'
  37                 # Mod by Jack: create data file if needed
  38                 try:
  39                         f = _open(self._datfile, 'r')
  40                 except IOError:
  41                         f = _open(self._datfile, 'w')
  42                 f.close()
  43                 self._update()
  44
  45         def _update(self):
  46                 self._index = {}
  47                 try:
  48                         f = _open(self._dirfile)
  49                 except IOError:
  50                         pass
  51                 else:
  52                         while 1:
  53                                 line = f.readline()
  54                                 if not line: break
  55                                 key, (pos, siz) = eval(line)
  56                                 self._index[key] = (pos, siz)
  57                         f.close()
  58
  59         def _commit(self):
  60                 try: _os.unlink(self._bakfile)
  61                 except _os.error: pass
  62                 try: _os.rename(self._dirfile, self._bakfile)
  63                 except _os.error: pass
  64                 f = _open(self._dirfile, 'w')
  65                 for key, (pos, siz) in self._index.items():
  66                         f.write("%s, (%s, %s)\n" % (`key`, `pos`, `siz`))
  67                 f.close()
  68
  69         def __getitem__(self, key):
  70                 pos, siz = self._index[key]     # may raise KeyError
  71                 f = _open(self._datfile, 'rb')
  72                 f.seek(pos)
  73                 dat = f.read(siz)
  74                 f.close()
  75                 return dat
  76
  77         def _addval(self, val):
  78                 f = _open(self._datfile, 'rb+')
  79                 f.seek(0, 2)
  80                 pos = f.tell()
  81 ## Does not work under MW compiler
  82 ##              pos = ((pos + _BLOCKSIZE - 1) / _BLOCKSIZE) * _BLOCKSIZE
  83 ##              f.seek(pos)
  84                 npos = ((pos + _BLOCKSIZE - 1) / _BLOCKSIZE) * _BLOCKSIZE
  85                 f.write('\0'*(npos-pos))
  86                 pos = npos
  87
  88                 f.write(val)
  89                 f.close()
  90                 return (pos, len(val))
  91
  92         def _setval(self, pos, val):
  93                 f = _open(self._datfile, 'rb+')
  94                 f.seek(pos)
  95                 f.write(val)
  96                 f.close()
  97                 return (pos, len(val))
  98
  99         def _addkey(self, key, (pos, siz)):
 100                 self._index[key] = (pos, siz)
 101                 f = _open(self._dirfile, 'a')
 102                 f.write("%s, (%s, %s)\n" % (`key`, `pos`, `siz`))
 103                 f.close()
 104
 105         def __setitem__(self, key, val):
 106                 if not type(key) == type('') == type(val):
 107                         raise TypeError, "keys and values must be strings"
 108                 if not self._index.has_key(key):
 109                         (pos, siz) = self._addval(val)
 110                         self._addkey(key, (pos, siz))
 111                 else:
 112                         pos, siz = self._index[key]
 113                         oldblocks = (siz + _BLOCKSIZE - 1) / _BLOCKSIZE
 114                         newblocks = (len(val) + _BLOCKSIZE - 1) / _BLOCKSIZE
 115                         if newblocks <= oldblocks:
 116                                 pos, siz = self._setval(pos, val)
 117                                 self._index[key] = pos, siz
 118                         else:
 119                                 pos, siz = self._addval(val)
 120                                 self._index[key] = pos, siz
 121                         self._addkey(key, (pos, siz))
 122
 123         def __delitem__(self, key):
 124                 del self._index[key]
 125                 self._commit()
 126
 127         def keys(self):
 128                 return self._index.keys()
 129
 130         def has_key(self, key):
 131                 return self._index.has_key(key)
 132
 133         def __len__(self):
 134                 return len(self._index)
 135
 136         def close(self):
 137                 self._index = None
 138                 self._datfile = self._dirfile = self._bakfile = None
 139
 140
 141 def open(file, flag = None, mode = None):
 142         # flag, mode arguments are currently ignored
 143         return _Database(file)