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13.4.2 Example

13.4.2 Example

To summarize the interface (key is a string, data is an arbitrary object):

import shelve

d = # 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 = d.has_key(key)   # true if the key exists
klist = d.keys() # a list of all existing keys (slow!)

# as d was opened WITHOUT writeback=True, beware:
d['xx'] = range(4)  # this works as expected, but...
d['xx'].append(5)   # *this doesn't!* -- d['xx'] is STILL range(4)!!!

# 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,,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 anydbm:
Generic interface to dbm-style databases.
Module bsddb:
BSD db database interface.
Module dbhash:
Thin layer around the bsddb which provides an open function like the other database modules.
Module dbm:
Standard Unix database interface.
Module dumbdbm:
Portable implementation of the dbm interface.
Module gdbm:
GNU database interface, based on the dbm interface.
Module pickle:
Object serialization used by shelve.
Module cPickle:
High-performance version of pickle.
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