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317 lines
11 KiB
317 lines
11 KiB
5 months ago
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"""A dumb and slow but simple dbm clone.
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For database spam, spam.dir contains the index (a text file),
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spam.bak *may* contain a backup of the index (also a text file),
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while spam.dat contains the data (a binary file).
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XXX TO DO:
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- seems to contain a bug when updating...
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- reclaim free space (currently, space once occupied by deleted or expanded
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items is never reused)
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- support concurrent access (currently, if two processes take turns making
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updates, they can mess up the index)
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- support efficient access to large databases (currently, the whole index
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is read when the database is opened, and some updates rewrite the whole index)
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- support opening for read-only (flag = 'm')
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"""
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import ast as _ast
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import io as _io
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import os as _os
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import collections.abc
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__all__ = ["error", "open"]
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_BLOCKSIZE = 512
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error = OSError
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class _Database(collections.abc.MutableMapping):
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# The on-disk directory and data files can remain in mutually
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# inconsistent states for an arbitrarily long time (see comments
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# at the end of __setitem__). This is only repaired when _commit()
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# gets called. One place _commit() gets called is from __del__(),
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# and if that occurs at program shutdown time, module globals may
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# already have gotten rebound to None. Since it's crucial that
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# _commit() finish successfully, we can't ignore shutdown races
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# here, and _commit() must not reference any globals.
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_os = _os # for _commit()
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_io = _io # for _commit()
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def __init__(self, filebasename, mode, flag='c'):
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self._mode = mode
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self._readonly = (flag == 'r')
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# The directory file is a text file. Each line looks like
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# "%r, (%d, %d)\n" % (key, pos, siz)
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# where key is the string key, pos is the offset into the dat
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# file of the associated value's first byte, and siz is the number
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# of bytes in the associated value.
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self._dirfile = filebasename + '.dir'
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# The data file is a binary file pointed into by the directory
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# file, and holds the values associated with keys. Each value
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# begins at a _BLOCKSIZE-aligned byte offset, and is a raw
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# binary 8-bit string value.
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self._datfile = filebasename + '.dat'
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self._bakfile = filebasename + '.bak'
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# The index is an in-memory dict, mirroring the directory file.
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self._index = None # maps keys to (pos, siz) pairs
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# Handle the creation
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self._create(flag)
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self._update(flag)
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def _create(self, flag):
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if flag == 'n':
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for filename in (self._datfile, self._bakfile, self._dirfile):
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try:
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_os.remove(filename)
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except OSError:
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pass
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# Mod by Jack: create data file if needed
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try:
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f = _io.open(self._datfile, 'r', encoding="Latin-1")
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except OSError:
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if flag not in ('c', 'n'):
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raise
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with _io.open(self._datfile, 'w', encoding="Latin-1") as f:
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self._chmod(self._datfile)
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else:
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f.close()
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# Read directory file into the in-memory index dict.
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def _update(self, flag):
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self._modified = False
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self._index = {}
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try:
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f = _io.open(self._dirfile, 'r', encoding="Latin-1")
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except OSError:
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if flag not in ('c', 'n'):
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raise
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self._modified = True
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else:
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with f:
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for line in f:
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line = line.rstrip()
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key, pos_and_siz_pair = _ast.literal_eval(line)
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key = key.encode('Latin-1')
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self._index[key] = pos_and_siz_pair
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# Write the index dict to the directory file. The original directory
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# file (if any) is renamed with a .bak extension first. If a .bak
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# file currently exists, it's deleted.
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def _commit(self):
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# CAUTION: It's vital that _commit() succeed, and _commit() can
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# be called from __del__(). Therefore we must never reference a
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# global in this routine.
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if self._index is None or not self._modified:
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return # nothing to do
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try:
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self._os.unlink(self._bakfile)
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except OSError:
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pass
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try:
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self._os.rename(self._dirfile, self._bakfile)
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except OSError:
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pass
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with self._io.open(self._dirfile, 'w', encoding="Latin-1") as f:
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self._chmod(self._dirfile)
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for key, pos_and_siz_pair in self._index.items():
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# Use Latin-1 since it has no qualms with any value in any
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# position; UTF-8, though, does care sometimes.
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entry = "%r, %r\n" % (key.decode('Latin-1'), pos_and_siz_pair)
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f.write(entry)
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sync = _commit
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def _verify_open(self):
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if self._index is None:
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raise error('DBM object has already been closed')
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def __getitem__(self, key):
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if isinstance(key, str):
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key = key.encode('utf-8')
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self._verify_open()
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pos, siz = self._index[key] # may raise KeyError
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with _io.open(self._datfile, 'rb') as f:
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f.seek(pos)
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dat = f.read(siz)
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return dat
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# Append val to the data file, starting at a _BLOCKSIZE-aligned
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# offset. The data file is first padded with NUL bytes (if needed)
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# to get to an aligned offset. Return pair
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# (starting offset of val, len(val))
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def _addval(self, val):
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with _io.open(self._datfile, 'rb+') as f:
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f.seek(0, 2)
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pos = int(f.tell())
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npos = ((pos + _BLOCKSIZE - 1) // _BLOCKSIZE) * _BLOCKSIZE
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f.write(b'\0'*(npos-pos))
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pos = npos
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f.write(val)
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return (pos, len(val))
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# Write val to the data file, starting at offset pos. The caller
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# is responsible for ensuring that there's enough room starting at
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# pos to hold val, without overwriting some other value. Return
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# pair (pos, len(val)).
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def _setval(self, pos, val):
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with _io.open(self._datfile, 'rb+') as f:
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f.seek(pos)
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f.write(val)
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return (pos, len(val))
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# key is a new key whose associated value starts in the data file
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# at offset pos and with length siz. Add an index record to
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# the in-memory index dict, and append one to the directory file.
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def _addkey(self, key, pos_and_siz_pair):
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self._index[key] = pos_and_siz_pair
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with _io.open(self._dirfile, 'a', encoding="Latin-1") as f:
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self._chmod(self._dirfile)
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f.write("%r, %r\n" % (key.decode("Latin-1"), pos_and_siz_pair))
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def __setitem__(self, key, val):
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if self._readonly:
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raise error('The database is opened for reading only')
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if isinstance(key, str):
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key = key.encode('utf-8')
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elif not isinstance(key, (bytes, bytearray)):
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raise TypeError("keys must be bytes or strings")
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if isinstance(val, str):
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val = val.encode('utf-8')
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elif not isinstance(val, (bytes, bytearray)):
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raise TypeError("values must be bytes or strings")
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self._verify_open()
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self._modified = True
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if key not in self._index:
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self._addkey(key, self._addval(val))
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else:
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# See whether the new value is small enough to fit in the
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# (padded) space currently occupied by the old value.
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pos, siz = self._index[key]
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oldblocks = (siz + _BLOCKSIZE - 1) // _BLOCKSIZE
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newblocks = (len(val) + _BLOCKSIZE - 1) // _BLOCKSIZE
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if newblocks <= oldblocks:
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self._index[key] = self._setval(pos, val)
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else:
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# The new value doesn't fit in the (padded) space used
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# by the old value. The blocks used by the old value are
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# forever lost.
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self._index[key] = self._addval(val)
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# Note that _index may be out of synch with the directory
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# file now: _setval() and _addval() don't update the directory
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# file. This also means that the on-disk directory and data
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# files are in a mutually inconsistent state, and they'll
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# remain that way until _commit() is called. Note that this
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# is a disaster (for the database) if the program crashes
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# (so that _commit() never gets called).
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def __delitem__(self, key):
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if self._readonly:
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raise error('The database is opened for reading only')
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if isinstance(key, str):
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key = key.encode('utf-8')
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self._verify_open()
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self._modified = True
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# The blocks used by the associated value are lost.
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del self._index[key]
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# XXX It's unclear why we do a _commit() here (the code always
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# XXX has, so I'm not changing it). __setitem__ doesn't try to
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# XXX keep the directory file in synch. Why should we? Or
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# XXX why shouldn't __setitem__?
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self._commit()
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def keys(self):
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try:
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return list(self._index)
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except TypeError:
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raise error('DBM object has already been closed') from None
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def items(self):
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self._verify_open()
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return [(key, self[key]) for key in self._index.keys()]
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def __contains__(self, key):
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if isinstance(key, str):
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key = key.encode('utf-8')
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try:
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return key in self._index
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except TypeError:
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if self._index is None:
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raise error('DBM object has already been closed') from None
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else:
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raise
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def iterkeys(self):
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try:
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return iter(self._index)
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except TypeError:
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raise error('DBM object has already been closed') from None
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__iter__ = iterkeys
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def __len__(self):
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try:
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return len(self._index)
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except TypeError:
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raise error('DBM object has already been closed') from None
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def close(self):
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try:
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self._commit()
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finally:
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self._index = self._datfile = self._dirfile = self._bakfile = None
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__del__ = close
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def _chmod(self, file):
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self._os.chmod(file, self._mode)
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def __enter__(self):
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return self
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def __exit__(self, *args):
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self.close()
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def open(file, flag='c', mode=0o666):
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"""Open the database file, filename, and return corresponding object.
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The flag argument, used to control how the database is opened in the
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other DBM implementations, supports only the semantics of 'c' and 'n'
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values. Other values will default to the semantics of 'c' value:
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the database will always opened for update and will be created if it
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does not exist.
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The optional mode argument is the UNIX mode of the file, used only when
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the database has to be created. It defaults to octal code 0o666 (and
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will be modified by the prevailing umask).
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"""
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# Modify mode depending on the umask
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try:
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um = _os.umask(0)
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_os.umask(um)
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except AttributeError:
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pass
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else:
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# Turn off any bits that are set in the umask
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mode = mode & (~um)
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if flag not in ('r', 'w', 'c', 'n'):
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raise ValueError("Flag must be one of 'r', 'w', 'c', or 'n'")
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return _Database(file, mode, flag=flag)
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