"""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 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 = key in d # 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.
Normally, d[key] returns a COPY of the entry. This needs care when
mutable 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 persistent
mapping -- it only modifies the copy, which is then immediately
discarded, so that the append has NO effect whatsoever. To append an
item 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 keyword
argument 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 back
to the persistent mapping when you call d.close(). This ensures that
such usage as d[key].append(anitem) works as intended.
However, using keyword argument writeback=True may consume vast amount
of memory for the cache, and it may make d.close() very slow, if you
access many of d's entries after opening it in this way: d has no way to
check which of the entries you access are mutable and/or which ones you
actually mutate, so it must cache, and write back at close, all of the
entries that you access. You can call d.sync() to write back all the
entries in the cache, and empty the cache (d.sync() also synchronizes
the persistent dictionary on disk, if feasible).
"""
from pickle import Pickler, Unpickler
from io import BytesIO
import collections
__all__ = ["Shelf", "BsdDbShelf", "DbfilenameShelf", "open"]
class _ClosedDict(collections.MutableMapping):
'Marker for a closed dict. Access attempts raise a ValueError.'
def closed(self, *args):
raise ValueError('invalid operation on closed shelf')
__iter__ = __len__ = __getitem__ = __setitem__ = __delitem__ = keys = closed
def __repr__(self):
return '<Closed Dictionary>'
class Shelf(collections.MutableMapping):
"""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,
keyencoding="utf-8"):
self.dict = dict
if protocol is None:
protocol = 3
self._protocol = protocol
self.writeback = writeback
self.cache = {}
self.keyencoding = keyencoding
def __iter__(self):
for k in self.dict.keys():
yield k.decode(self.keyencoding)
def __len__(self):
return len(self.dict)
def __contains__(self, key):
return key.encode(self.keyencoding) in self.dict
def get(self, key, default=None):
if key.encode(self.keyencoding) in self.dict:
return self[key]
return default
def __getitem__(self, key):
try:
value = self.cache[key]
except KeyError:
f = BytesIO(self.dict[key.encode(self.keyencoding)])
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 = BytesIO()
p = Pickler(f, self._protocol)
p.dump(value)
self.dict[key.encode(self.keyencoding)] = f.getvalue()
def __delitem__(self, key):
del self.dict[key.encode(self.keyencoding)]
try:
del self.cache[key]
except KeyError:
pass
def __enter__(self):
return self
def __exit__(self, type, value, traceback):
self.close()
def close(self):
if self.dict is None:
return
try:
self.sync()
try:
self.dict.close()
except AttributeError:
pass
finally:
# Catch errors that may happen when close is called from __del__
# because CPython is in interpreter shutdown.
try:
self.dict = _ClosedDict()
except:
self.dict = None
def __del__(self):
if not hasattr(self, 'writeback'):
# __init__ didn't succeed, so don't bother closing
# see http://bugs.python.org/issue1339007 for details
return
self.close()
def sync(self):
if self.writeback and self.cache:
self.writeback = False
for key, entry in self.cache.items():
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,
keyencoding="utf-8"):
Shelf.__init__(self, dict, protocol, writeback, keyencoding)
def set_location(self, key):
(key, value) = self.dict.set_location(key)
f = BytesIO(value)
return (key.decode(self.keyencoding), Unpickler(f).load())
def next(self):
(key, value) = next(self.dict)
f = BytesIO(value)
return (key.decode(self.keyencoding), Unpickler(f).load())
def previous(self):
(key, value) = self.dict.previous()
f = BytesIO(value)
return (key.decode(self.keyencoding), Unpickler(f).load())
def first(self):
(key, value) = self.dict.first()
f = BytesIO(value)
return (key.decode(self.keyencoding), Unpickler(f).load())
def last(self):
(key, value) = self.dict.last()
f = BytesIO(value)
return (key.decode(self.keyencoding), Unpickler(f).load())
class DbfilenameShelf(Shelf):
"""Shelf implementation using the "dbm" 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 dbm
Shelf.__init__(self, dbm.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
dbm.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)
