`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 参数对于 dbm.open() .

By default, pickles created with pickle.DEFAULT_PROTOCOL 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).

3.10 版改变： pickle.DEFAULT_PROTOCOL is now used as the default pickle protocol.

3.11 版改变：接受像路径对象 for filename.

限定 ¶

The choice of which database package will be used (such as 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.

The 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.

在 macOS dbm.ndbm can silently corrupt the database file on updates, which can cause hard crashes when trying to read from the database.

class 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, pickles created with pickle.DEFAULT_PROTOCOL 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.

The 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。

3.10 版改变： pickle.DEFAULT_PROTOCOL is now used as the default pickle protocol.

范例 ¶

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

`shelve` — Python 对象持久性 ¶

限定 ¶

范例 ¶

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shelve — Python 对象持久性 ¶

限定 ¶

范例 ¶

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`shelve` — Python 对象持久性 ¶