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; 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 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

import UserDict

__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, binary=False):
        self.dict = dict
        self._binary = binary

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

    def __setitem__(self, key, value):
        f = StringIO()
        p = Pickler(f, self._binary)
        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, binary=False):
        Shelf.__init__(self, dict, binary)

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


def open(filename, flag='c', binary=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 binary parameter may be set to True to force the use of binary
    pickles for serializing data values.

    See the module's __doc__ string for an overview of the interface.
    """

    return DbfilenameShelf(filename, flag, binary)