Lib/shelve.py

"""Manage shelves of pickled objects.

A "shelf" is a persistent, dictionary-like object.  The difference
with dbm databases is that the values (not the keys!) in a shelf can
be essentially arbitrary Python objects -- anything that the "pickle"
module can handle.  This includes most class instances, recursive data
types, and objects containing lots of shared sub-objects.  The keys
are ordinary strings.

To summarize the interface (key is a string, data is an arbitrary
object):

        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 data at key (raise KeyError if no
                        # such key)
        del d[key]      # delete data stored at key (raises KeyError
                        # if no such key)
        flag = d.has_key(key)   # true if the key exists
        list = d.keys() # a list of all existing keys (slow!)

        d.close()       # close it

Dependent on the implementation, closing a persistent dictionary may
or may not be necessary to flush changes to disk.
"""

# Try using cPickle and cStringIO if available.

try:
    from cPickle import Pickler, Unpickler
except ImportError:
    from pickle import Pickler, Unpickler

try:
    from cStringIO import StringIO
except ImportError:
    from StringIO import StringIO

__all__ = ["Shelf","BsdDbShelf","DbfilenameShelf","open"]

class Shelf:
    """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):
        self.dict = dict

    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 get(self, key, default=None):
        if self.dict.has_key(key):
            return self[key]
        return default

    def __getitem__(self, key):
        f = StringIO(self.dict[key])
        return Unpickler(f).load()

    def __setitem__(self, key, value):
        f = StringIO()
        p = Pickler(f)
        p.dump(value)
        self.dict[key] = f.getvalue()

    def __delitem__(self, key):
        del self.dict[key]

    def close(self):
        try:
            self.dict.close()
        except:
            pass
        self.dict = 0

    def __del__(self):
        self.close()

    def sync(self):
        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):
        Shelf.__init__(self, dict)

    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'):
        import anydbm
        Shelf.__init__(self, anydbm.open(filename, flag))


def open(filename, flag='c'):
    """Open a persistent dictionary for reading and writing.

    Argument is the filename for the dbm database.
    See the module's __doc__ string for an overview of the interface.
    """

    return DbfilenameShelf(filename, flag)