shelve
— Python 对象持久性
¶
源代码： Lib/shelve.py
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.
shelve.
open
(
filename
,
flag='c'
,
protocol=None
,
writeback=False
)
¶
Open a persistent dictionary. The filename specified 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. By default, the underlying database file is opened for reading and writing. The optional
flag
parameter has the same interpretation as the
flag
parameter of
dbm.open()
.
By default, version 3 pickles are used to serialize values. The version of the pickle protocol can be specified with the protocol 参数。
Because of Python semantics, a shelf cannot know when a mutable persistent-dictionary entry is modified. By default modified objects are written
only
when assigned to the shelf (see
范例
). If the optional
writeback
parameter is set to
True
, all entries accessed are also cached in memory, and written back on
sync()
and
close()
; this can make it handier to mutate mutable entries in the persistent dictionary, but, if many entries are accessed, it can consume vast amounts of memory for the cache, and it can make the close operation very slow since all accessed entries are written back (there is no way to determine which accessed entries are mutable, nor which ones were actually mutated).
注意
Do not rely on the shelf being closed automatically; always call
close()
explicitly when you don’t need it any more, or use
shelve.open()
as a context manager:
with shelve.open('spam') as db:
db['eggs'] = 'eggs'
警告
由于
shelve
module is backed by
pickle
, it is insecure to load a shelf from an untrusted source. Like with pickle, loading a shelf can execute arbitrary code.
Shelf objects support all methods supported by dictionaries. This eases the transition from dictionary based scripts to those requiring persistent storage.
Two additional methods are supported:
Shelf.
sync
(
)
¶
Write back all entries in the cache if the shelf was opened with
writeback
设为
True
. Also empty the cache and synchronize the persistent dictionary on disk, if feasible. This is called automatically when the shelf is closed with
close()
.
Shelf.
close
(
)
¶
Synchronize and close the persistent
dict
object. Operations on a closed shelf will fail with a
ValueError
.
另请参阅
Persistent dictionary recipe with widely supported storage formats and having the speed of native dictionaries.
dbm.ndbm
or
dbm.gnu
) depends on which interface is available. Therefore it is not safe to open the database directly using
dbm
. The database is also (unfortunately) subject to the limitations of
dbm
, if it is used — this means that (the pickled representation of) the objects stored in the database should be fairly small, and in rare cases key collisions may cause the database to refuse updates.
shelve
module does not support
concurrent
read/write access to shelved objects. (Multiple simultaneous read accesses are safe.) When a program has a shelf open for writing, no other program should have it open for reading or writing. Unix file locking can be used to solve this, but this differs across Unix versions and requires knowledge about the database implementation used.
shelve.
Shelf
(
dict
,
protocol=None
,
writeback=False
,
keyencoding='utf-8'
)
¶
子类化的
collections.abc.MutableMapping
which stores pickled values in the
dict
对象。
By default, version 3 pickles are used to serialize values. The version of the pickle protocol can be specified with the
protocol
parameter. See the
pickle
documentation for a discussion of the pickle protocols.
若
writeback
参数为
True
, the object will hold a cache of all entries accessed and write them back to the
dict
at sync and close times. This allows natural operations on mutable entries, but can consume much more memory and make sync and close take a long time.
keyencoding parameter is the encoding used to encode keys before they are used with the underlying dict.
A
Shelf
object can also be used as a context manager, in which case it will be automatically closed when the
with
block ends.
3.2 版改变： 添加 keyencoding parameter; previously, keys were always encoded in UTF-8.
3.4 版改变： 添加上下文管理器支持。
shelve.
BsdDbShelf
(
dict
,
protocol=None
,
writeback=False
,
keyencoding='utf-8'
)
¶
子类化的
Shelf
which exposes
first()
,
next()
,
previous()
,
last()
and
set_location()
which are available in the third-party
bsddb
module from
pybsddb
but not in other database modules. The
dict
object passed to the constructor must support those methods. This is generally accomplished by calling one of
bsddb.hashopen()
,
bsddb.btopen()
or
bsddb.rnopen()
. The optional
protocol
,
writeback
，和
keyencoding
parameters have the same interpretation as for the
Shelf
类。
shelve.
DbfilenameShelf
(
filename
,
flag='c'
,
protocol=None
,
writeback=False
)
¶
子类化的
Shelf
which accepts a
filename
instead of a dict-like object. The underlying file will be opened using
dbm.open()
. By default, the file will be created and opened for both read and write. The optional
flag
parameter has the same interpretation as for the
open()
function. The optional
protocol
and
writeback
parameters have the same interpretation as for the
Shelf
类。
To summarize the interface (
key
is a string,
data
is an arbitrary object):
import shelve
d = shelve.open(filename) # open -- file may get suffix added by low-level
# library
d[key] = data # store data at key (overwrites old data if
# using an existing key)
data = d[key] # retrieve a COPY of data at key (raise KeyError
# if no such key)
del d[key] # delete data stored at key (raises KeyError
# if no such key)
flag = key in d # true if the key exists
klist = list(d.keys()) # a list of all existing keys (slow!)
# as d was opened WITHOUT writeback=True, beware:
d['xx'] = [0, 1, 2] # this works as expected, but...
d['xx'].append(3) # *this doesn't!* -- d['xx'] is STILL [0, 1, 2]!
# having opened d without writeback=True, you need to code carefully:
temp = d['xx'] # extracts the copy
temp.append(5) # mutates the copy
d['xx'] = temp # stores the copy right back, to persist it
# or, d=shelve.open(filename,writeback=True) would let you just code
# d['xx'].append(5) and have it work as expected, BUT it would also
# consume more memory and make the d.close() operation slower.
d.close() # close it