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