13.4 shelve -- Python object persistence
import shelved = shelve.open(filename) # open -- file may get suffix added by low-level
# library
d = data # store data at key (overwrites old data if
# using an existing key)
data = d # retrieve a COPY of data at key (raise KeyError if no
# such key)
del d # 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, 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
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