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Python Open Source » Email » BoboMail 
BoboMail » bobomail » lib » dumbdbm.py
"""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')

"""

_os = __import__('os')
import __builtin__

_open = __builtin__.open

_BLOCKSIZE = 512

error = IOError        # For anydbm

class _Database:

  def __init__(self, file):
    self._dirfile = file + '.dir'
    self._datfile = file + '.dat'
    self._bakfile = file + '.bak'
    # Mod by Jack: create data file if needed
    try:
      f = _open(self._datfile, 'r')
    except IOError:
      f = _open(self._datfile, 'w')
    f.close()
    self._update()
  
  def _update(self):
    self._index = {}
    try:
      f = _open(self._dirfile)
    except IOError:
      pass
    else:
      while 1:
        line = f.readline()
        if not line: break
        key, (pos, siz) = eval(line)
        self._index[key] = (pos, siz)
      f.close()

  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')
    for key, (pos, siz) in self._index.items():
      f.write("%s, (%s, %s)\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
  
  def _addval(self, val):
    f = _open(self._datfile, 'rb+')
    f.seek(0, 2)
    pos = int(f.tell())
## Does not work under MW compiler
##    pos = ((pos + _BLOCKSIZE - 1) / _BLOCKSIZE) * _BLOCKSIZE
##    f.seek(pos)
    npos = ((pos + _BLOCKSIZE - 1) / _BLOCKSIZE) * _BLOCKSIZE
    f.write('\0'*(npos-pos))
    pos = npos
    
    f.write(val)
    f.close()
    return (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))
  
  def _addkey(self, key, (pos, siz)):
    self._index[key] = (pos, siz)
    f = _open(self._dirfile, 'a')
    f.write("%s, (%s, %s)\n" % (`key`, `pos`, `siz`))
    f.close()
  
  def __setitem__(self, key, val):
    if not type(key) == type('') == type(val):
      raise TypeError, "keys and values must be strings"
    if not self._index.has_key(key):
      (pos, siz) = self._addval(val)
      self._addkey(key, (pos, siz))
    else:
      pos, siz = self._index[key]
      oldblocks = (siz + _BLOCKSIZE - 1) / _BLOCKSIZE
      newblocks = (len(val) + _BLOCKSIZE - 1) / _BLOCKSIZE
      if newblocks <= oldblocks:
        pos, siz = self._setval(pos, val)
        self._index[key] = pos, siz
      else:
        pos, siz = self._addval(val)
        self._index[key] = pos, siz
      self._addkey(key, (pos, siz))
  
  def __delitem__(self, key):
    del self._index[key]
    self._commit()
  
  def keys(self):
    return self._index.keys()
  
  def has_key(self, key):
    return self._index.has_key(key)
  
  def __len__(self):
    return len(self._index)
  
  def close(self):
    self._index = None
    self._datfile = self._dirfile = self._bakfile = None


def open(file, flag = None, mode = None):
  # flag, mode arguments are currently ignored
  return _Database(file)
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