Mac/Lib/dbmac.py

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