shelve â Python object persistence¶
Source code: Lib/shelve.py
A âshelfâ is a persistent, dictionary-like object. The difference with âdbmâ databases is that the values (not the keys!) in a shelf can be essentially arbitrary Python objects â 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.
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, pickles created with pickle.DEFAULT_PROTOCOL are used to serialize values. The version of the pickle protocol can be specified with the protocol parameter.
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 Example). 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).
By default, shelve uses pickle.dumps() and pickle.loads() for serializing and deserializing. This can be changed by supplying serializer and deserializer, respectively.
The serializer argument must be a callable which takes an object obj and the protocol as inputs and returns the representation obj as a bytes-like object; the protocol value may be ignored by the serializer.
The deserializer argument must be callable which takes a serialized object given as a bytes object and returns the corresponding object.
A ShelveError is raised if serializer is given but deserializer is not, or vice-versa.
Changed in version 3.10: pickle.DEFAULT_PROTOCOL is now used as the default pickle protocol.
Changed in version 3.11: Accepts path-like object for filename.
Changed in version 3.15.0a0 (unreleased): Accepts custom serializer and deserializer functions in place of pickle.dumps() and pickle.loads().
Note
Do not rely on the shelf being closed automatically; always call close() explicitly when you donât need it any more, or use shelve.open() as a context manager:
with shelve.open('spam') as db:
db['eggs'] = 'eggs'
Warning
Because the 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 most of methods and operations supported by dictionaries (except copying, constructors and operators | and |=). This eases the transition from dictionary based scripts to those requiring persistent storage.
Two additional methods are supported:
Write back all entries in the cache if the shelf was opened with writeback set to True. Also empty the cache and synchronize the persistent dictionary on disk, if feasible. This is called automatically when reorganize() is called or the shelf is closed with close().
Calls sync() and attempts to shrink space used on disk by removing empty space resulting from deletions.
Added in version 3.15.0a0 (unreleased).
Synchronize and close the persistent dict object. Operations on a closed shelf will fail with a ValueError.
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 â 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.
On macOS dbm.ndbm can silently corrupt the database file on updates, which can cause hard crashes when trying to read from the database.
Shelf.reorganize() may not be available for all database packages and may temporarily increase resource usage (especially disk space) when called. Additionally, it will never run automatically and instead needs to be called explicitly.
A subclass of collections.abc.MutableMapping which stores pickled values in the dict object.
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.
If the writeback parameter is 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.
The serializer and deserializer parameters have the same interpretation as in open().
A Shelf object can also be used as a context manager, in which case it will be automatically closed when the with block ends.
Changed in version 3.2: Added the keyencoding parameter; previously, keys were always encoded in UTF-8.
Changed in version 3.4: Added context manager support.
Changed in version 3.10: pickle.DEFAULT_PROTOCOL is now used as the default pickle protocol.
Changed in version 3.15.0a0 (unreleased): Added the serializer and deserializer parameters.
A subclass of Shelf which exposes first(), next(), previous(), last() and set_location() methods. These 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, keyencoding, serializer and deserializer parameters have the same interpretation as in open().
Changed in version 3.15.0a0 (unreleased): Added the serializer and deserializer parameters.
A subclass of 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, writeback, serializer and deserializer parameters have the same interpretation as in open().
Changed in version 3.15.0a0 (unreleased): Added the serializer and deserializer parameters.
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
Exceptions¶
Exception raised when one of the arguments deserializer and serializer is missing in the open(), Shelf, BsdDbShelf and DbfilenameShelf.
The deserializer and serializer arguments must be given together.
Added in version 3.15.0a0 (unreleased).
See also
dbm
Generic interface to dbm-style databases.
pickle
Object serialization used by shelve.
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