mirror of
https://github.com/python/cpython.git
synced 2024-12-15 21:04:00 +08:00
240 lines
8.2 KiB
Python
240 lines
8.2 KiB
Python
"""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):
|
|
self.sync()
|
|
try:
|
|
self.dict.close()
|
|
except AttributeError:
|
|
pass
|
|
# Catch errors that may happen when close is called from __del__
|
|
# because CPython is in interpreter shutdown.
|
|
try:
|
|
self.dict = _ClosedDict()
|
|
except (NameError, TypeError):
|
|
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)
|