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"""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
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.
"""
import pickle
import StringIO
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 self.dict.len()
def has_key(self, key):
return self.dict.has_key(key)
def __getitem__(self, key):
f = StringIO.StringIO(self.dict[key])
return pickle.Unpickler(f).load()
def __setitem__(self, key, value):
f = StringIO.StringIO()
p = pickle.Pickler(f)
p.dump(value)
self.dict[key] = f.getvalue()
def __delitem__(self, key):
del self.dict[key]
def close(self):
if hasattr(self.db, 'close'):
self.db.close()
self.db = None
def __del__(self):
self.close()
class DbShelf(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):
import anydbm
Shelf.__init__(self, anydbm.open(filename))
def open(filename):
"""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 DbShelf(filename)
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