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 import os as _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, mode):
  36         self._mode = mode
  37         self._dirfile = file + _os.extsep + 'dir'
  38         self._datfile = file + _os.extsep + 'dat'
  39         self._bakfile = file + _os.extsep + 'bak'
  40         # Mod by Jack: create data file if needed
  41         try:
  42             f = _open(self._datfile, 'r')
  43         except IOError:
  44             f = _open(self._datfile, 'w', self._mode)
  45         f.close()
  46         self._update()
  47
  48     def _update(self):
  49         self._index = {}
  50         try:
  51             f = _open(self._dirfile)
  52         except IOError:
  53             pass
  54         else:
  55             while 1:
  56                 line = f.readline().rstrip()
  57                 if not line: break
  58                 key, (pos, siz) = eval(line)
  59                 self._index[key] = (pos, siz)
  60             f.close()
  61
  62     def _commit(self):
  63         try: _os.unlink(self._bakfile)
  64         except _os.error: pass
  65         try: _os.rename(self._dirfile, self._bakfile)
  66         except _os.error: pass
  67         f = _open(self._dirfile, 'w', self._mode)
  68         for key, (pos, siz) in self._index.items():
  69             f.write("%s, (%s, %s)\n" % (`key`, `pos`, `siz`))
  70         f.close()
  71
  72     def __getitem__(self, key):
  73         pos, siz = self._index[key]     # may raise KeyError
  74         f = _open(self._datfile, 'rb')
  75         f.seek(pos)
  76         dat = f.read(siz)
  77         f.close()
  78         return dat
  79
  80     def _addval(self, val):
  81         f = _open(self._datfile, 'rb+')
  82         f.seek(0, 2)
  83         pos = int(f.tell())
  84 ## Does not work under MW compiler
  85 ##              pos = ((pos + _BLOCKSIZE - 1) / _BLOCKSIZE) * _BLOCKSIZE
  86 ##              f.seek(pos)
  87         npos = ((pos + _BLOCKSIZE - 1) // _BLOCKSIZE) * _BLOCKSIZE
  88         f.write('\0'*(npos-pos))
  89         pos = npos
  90
  91         f.write(val)
  92         f.close()
  93         return (pos, len(val))
  94
  95     def _setval(self, pos, val):
  96         f = _open(self._datfile, 'rb+')
  97         f.seek(pos)
  98         f.write(val)
  99         f.close()
 100         return (pos, len(val))
 101
 102     def _addkey(self, key, (pos, siz)):
 103         self._index[key] = (pos, siz)
 104         f = _open(self._dirfile, 'a', self._mode)
 105         f.write("%s, (%s, %s)\n" % (`key`, `pos`, `siz`))
 106         f.close()
 107
 108     def __setitem__(self, key, val):
 109         if not type(key) == type('') == type(val):
 110             raise TypeError, "keys and values must be strings"
 111         if not self._index.has_key(key):
 112             (pos, siz) = self._addval(val)
 113             self._addkey(key, (pos, siz))
 114         else:
 115             pos, siz = self._index[key]
 116             oldblocks = (siz + _BLOCKSIZE - 1) / _BLOCKSIZE
 117             newblocks = (len(val) + _BLOCKSIZE - 1) / _BLOCKSIZE
 118             if newblocks <= oldblocks:
 119                 pos, siz = self._setval(pos, val)
 120                 self._index[key] = pos, siz
 121             else:
 122                 pos, siz = self._addval(val)
 123                 self._index[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 __contains__(self, key):
 136         return self._index.has_key(key)
 137
 138     def iterkeys(self):
 139         return self._index.iterkeys()
 140     __iter__ = iterkeys
 141
 142     def __len__(self):
 143         return len(self._index)
 144
 145     def close(self):
 146         self._index = None
 147         self._datfile = self._dirfile = self._bakfile = None
 148
 149
 150 def open(file, flag=None, mode=0666):
 151     # flag, mode arguments are currently ignored
 152     return _Database(file, mode)