python-2.5.2/win32/Lib/shelve.py
author Deepak Modgil <Deepak.Modgil@Nokia.com>
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"""Manage shelves of pickled objects.
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A "shelf" is a persistent, dictionary-like object.  The difference
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with dbm databases is that the values (not the keys!) in a shelf can
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be essentially arbitrary Python objects -- anything that the "pickle"
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module can handle.  This includes most class instances, recursive data
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types, and objects containing lots of shared sub-objects.  The keys
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are ordinary strings.
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To summarize the interface (key is a string, data is an arbitrary
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object):
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        import shelve
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        d = shelve.open(filename) # open, with (g)dbm filename -- no suffix
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        d[key] = data   # store data at key (overwrites old data if
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                        # using an existing key)
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        data = d[key]   # retrieve a COPY of the data at key (raise
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                        # KeyError if no such key) -- NOTE that this
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                        # access returns a *copy* of the entry!
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        del d[key]      # delete data stored at key (raises KeyError
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                        # if no such key)
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        flag = d.has_key(key)   # true if the key exists; same as "key in d"
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        list = d.keys() # a list of all existing keys (slow!)
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        d.close()       # close it
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Dependent on the implementation, closing a persistent dictionary may
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or may not be necessary to flush changes to disk.
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Normally, d[key] returns a COPY of the entry.  This needs care when
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mutable entries are mutated: for example, if d[key] is a list,
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        d[key].append(anitem)
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does NOT modify the entry d[key] itself, as stored in the persistent
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mapping -- it only modifies the copy, which is then immediately
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discarded, so that the append has NO effect whatsoever.  To append an
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item to d[key] in a way that will affect the persistent mapping, use:
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        data = d[key]
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        data.append(anitem)
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        d[key] = data
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To avoid the problem with mutable entries, you may pass the keyword
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argument writeback=True in the call to shelve.open.  When you use:
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        d = shelve.open(filename, writeback=True)
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then d keeps a cache of all entries you access, and writes them all back
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to the persistent mapping when you call d.close().  This ensures that
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such usage as d[key].append(anitem) works as intended.
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However, using keyword argument writeback=True may consume vast amount
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of memory for the cache, and it may make d.close() very slow, if you
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access many of d's entries after opening it in this way: d has no way to
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check which of the entries you access are mutable and/or which ones you
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actually mutate, so it must cache, and write back at close, all of the
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entries that you access.  You can call d.sync() to write back all the
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entries in the cache, and empty the cache (d.sync() also synchronizes
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the persistent dictionary on disk, if feasible).
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"""
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# Try using cPickle and cStringIO if available.
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try:
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    from cPickle import Pickler, Unpickler
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except ImportError:
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    from pickle import Pickler, Unpickler
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try:
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    from cStringIO import StringIO
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except ImportError:
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    from StringIO import StringIO
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import UserDict
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import warnings
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__all__ = ["Shelf","BsdDbShelf","DbfilenameShelf","open"]
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class Shelf(UserDict.DictMixin):
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    """Base class for shelf implementations.
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    This is initialized with a dictionary-like object.
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    See the module's __doc__ string for an overview of the interface.
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    """
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    def __init__(self, dict, protocol=None, writeback=False):
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        self.dict = dict
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        if protocol is None:
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            protocol = 0
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        self._protocol = protocol
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        self.writeback = writeback
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        self.cache = {}
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    def keys(self):
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        return self.dict.keys()
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    def __len__(self):
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        return len(self.dict)
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    def has_key(self, key):
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        return self.dict.has_key(key)
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    def __contains__(self, key):
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        return self.dict.has_key(key)
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    def get(self, key, default=None):
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        if self.dict.has_key(key):
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            return self[key]
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        return default
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    def __getitem__(self, key):
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        try:
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            value = self.cache[key]
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        except KeyError:
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            f = StringIO(self.dict[key])
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            value = Unpickler(f).load()
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            if self.writeback:
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                self.cache[key] = value
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        return value
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    def __setitem__(self, key, value):
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        if self.writeback:
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            self.cache[key] = value
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        f = StringIO()
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        p = Pickler(f, self._protocol)
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        p.dump(value)
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        self.dict[key] = f.getvalue()
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    def __delitem__(self, key):
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        del self.dict[key]
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        try:
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            del self.cache[key]
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        except KeyError:
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            pass
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    def close(self):
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        self.sync()
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        try:
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            self.dict.close()
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        except AttributeError:
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            pass
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        self.dict = 0
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    def __del__(self):
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        if not hasattr(self, 'writeback'):
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            # __init__ didn't succeed, so don't bother closing
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            return
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        self.close()
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    def sync(self):
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        if self.writeback and self.cache:
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            self.writeback = False
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            for key, entry in self.cache.iteritems():
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                self[key] = entry
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            self.writeback = True
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            self.cache = {}
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        if hasattr(self.dict, 'sync'):
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            self.dict.sync()
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class BsdDbShelf(Shelf):
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    """Shelf implementation using the "BSD" db interface.
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    This adds methods first(), next(), previous(), last() and
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    set_location() that have no counterpart in [g]dbm databases.
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    The actual database must be opened using one of the "bsddb"
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    modules "open" routines (i.e. bsddb.hashopen, bsddb.btopen or
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    bsddb.rnopen) and passed to the constructor.
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    See the module's __doc__ string for an overview of the interface.
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    """
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    def __init__(self, dict, protocol=None, writeback=False):
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        Shelf.__init__(self, dict, protocol, writeback)
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    def set_location(self, key):
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        (key, value) = self.dict.set_location(key)
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        f = StringIO(value)
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        return (key, Unpickler(f).load())
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    def next(self):
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        (key, value) = self.dict.next()
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        f = StringIO(value)
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        return (key, Unpickler(f).load())
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    def previous(self):
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        (key, value) = self.dict.previous()
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        f = StringIO(value)
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        return (key, Unpickler(f).load())
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    def first(self):
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        (key, value) = self.dict.first()
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        f = StringIO(value)
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        return (key, Unpickler(f).load())
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    def last(self):
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        (key, value) = self.dict.last()
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        f = StringIO(value)
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        return (key, Unpickler(f).load())
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class DbfilenameShelf(Shelf):
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    """Shelf implementation using the "anydbm" generic dbm interface.
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    This is initialized with the filename for the dbm database.
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    See the module's __doc__ string for an overview of the interface.
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    """
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    def __init__(self, filename, flag='c', protocol=None, writeback=False):
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        import anydbm
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        Shelf.__init__(self, anydbm.open(filename, flag), protocol, writeback)
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def open(filename, flag='c', protocol=None, writeback=False):
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    """Open a persistent dictionary for reading and writing.
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    The filename parameter is the base filename for the underlying
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    database.  As a side-effect, an extension may be added to the
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    filename and more than one file may be created.  The optional flag
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    parameter has the same interpretation as the flag parameter of
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    anydbm.open(). The optional protocol parameter specifies the
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    version of the pickle protocol (0, 1, or 2).
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    See the module's __doc__ string for an overview of the interface.
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    """
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    return DbfilenameShelf(filename, flag, protocol, writeback)