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# 12.3. `shelve` — Python object persistence

Source code: 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 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).

  Note

  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'

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 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 set to `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`.

See also

Persistent dictionary recipe with widely supported storage formats and having the speed of native dictionaries.

## 12.3.1. Restrictions

- 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.

class `shelve.``Shelf`(dict, protocol=None, writeback=False, keyencoding='utf-8')

  A subclass of `collections.abc.MutableMapping` which stores pickled values in the dict object.

  By default, version 0 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.

  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.

  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.

class `shelve.``BsdDbShelf`(dict, protocol=None, writeback=False, keyencoding='utf-8')

  A subclass of `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, and keyencoding parameters have the same interpretation as for the `Shelf` class.

class `shelve.``DbfilenameShelf`(filename, flag='c', protocol=None, writeback=False)

  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 and writeback parameters have the same interpretation as for the `Shelf` class.

## 12.3.2. Example

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

See also

Module `dbm`
  Generic interface to `dbm`-style databases.
Module `pickle`
  Object serialization used by `shelve`.

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