- The choice of which database package will be used
(such as dbm, gdbm or bsddb) depends on
which interface is available. Therefore it is not safe to open the database
directly using dbm. The database is also (unfortunately) subject
to the limitations of dbm, if it is used -- this means
that (the pickled representation of) the objects stored in the
database should be fairly small, and in rare cases key collisions may
cause the database to refuse updates.
- Depending on the implementation, closing a persistent dictionary may
or may not be necessary to flush changes to disk. The __del__
method of the Shelf class calls the close method, so the
programmer generally need not do this explicitly.
- The shelve module does not support concurrent read/write
access to shelved objects. (Multiple simultaneous read accesses are
safe.) When a program has a shelf open for writing, no other program
should have it open for reading or writing. Unix file locking can
be used to solve this, but this differs across Unix versions and
requires knowledge about the database implementation used.
class Shelf( |
dict[, protocol=None[, writeback=False]]) |
-
A subclass of UserDict.DictMixin which stores pickled values in the
dict object.
By default, version 0 pickles are used to serialize values. The
version of the pickle protocol can be specified with the
protocol parameter. See the pickle documentation for a
discussion of the pickle protocols.
Changed in version 2.3:
The protocol
parameter was added.
If the writeback parameter is True , the object will hold a
cache of all entries accessed and write them back to the dict at
sync and close times. This allows natural operations on mutable entries,
but can consume much more memory and make sync and close take a long time.
class BsdDbShelf( |
dict[, protocol=None[, writeback=False]]) |
-
A subclass of Shelf which exposes first,
next, previous, last and
set_location which are available in the bsddb module
but not in other database modules. The dict object passed to
the constructor must support those methods. This is generally
accomplished by calling one of bsddb.hashopen,
bsddb.btopen or bsddb.rnopen. The optional
protocol and writeback parameters have the
same interpretation as for the Shelf class.
class DbfilenameShelf( |
filename[, flag='c'[, protocol=None[, writeback=False]]]) |
-
A subclass of Shelf which accepts a filename instead of
a dict-like object. The underlying file will be opened using
anydbm.open. By default, the file will be created and
opened for both read and write. The optional flag parameter has
the same interpretation as for the open function. The
optional protocol and writeback parameters
have the same interpretation as for the Shelf class.
Release 2.5.2, documentation updated on 21st February, 2008.
See About this document... for information on suggesting changes.
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