Lib/shelve.py

   1 """Manage shelves of pickled objects.
   2
   3 A "shelf" is a persistent, dictionary-like object.  The difference
   4 with dbm databases is that the values (not the keys!) in a shelf can
   5 be essentially arbitrary Python objects -- anything that the "pickle"
   6 module can handle.  This includes most class instances, recursive data
   7 types, and objects containing lots of shared sub-objects.  The keys
   8 are ordinary strings.
   9
  10 To summarize the interface (key is a string, data is an arbitrary
  11 object):
  12
  13         import shelve
  14         d = shelve.open(filename) # open, with (g)dbm filename -- no suffix
  15
  16         d[key] = data   # store data at key (overwrites old data if
  17                         # using an existing key)
  18         data = d[key]   # retrieve data at key (raise KeyError if no
  19                         # such key)
  20         del d[key]      # delete data stored at key (raises KeyError
  21                         # if no such key)
  22         flag = d.has_key(key)   # true if the key exists
  23         list = d.keys() # a list of all existing keys (slow!)
  24
  25         d.close()       # close it
  26
  27 Dependent on the implementation, closing a persistent dictionary may
  28 or may not be necessary to flush changes to disk.
  29 """
  30
  31 # Try using cPickle and cStringIO if available.
  32
  33 try:
  34         from cPickle import Pickler, Unpickler
  35 except ImportError:
  36         from pickle import Pickler, Unpickler
  37
  38 try:
  39         from cStringIO import StringIO
  40 except ImportError:
  41         from StringIO import StringIO
  42
  43
  44 class Shelf:
  45         """Base class for shelf implementations.
  46
  47         This is initialized with a dictionary-like object.
  48         See the module's __doc__ string for an overview of the interface.
  49         """
  50
  51         def __init__(self, dict):
  52                 self.dict = dict
  53
  54         def keys(self):
  55                 return self.dict.keys()
  56
  57         def __len__(self):
  58                 return len(self.dict)
  59
  60         def has_key(self, key):
  61                 return self.dict.has_key(key)
  62
  63         def get(self, key, default=None):
  64                 if self.dict.has_key(key):
  65                         return self[key]
  66                 return default
  67
  68         def __getitem__(self, key):
  69                 f = StringIO(self.dict[key])
  70                 return Unpickler(f).load()
  71
  72         def __setitem__(self, key, value):
  73                 f = StringIO()
  74                 p = Pickler(f)
  75                 p.dump(value)
  76                 self.dict[key] = f.getvalue()
  77
  78         def __delitem__(self, key):
  79                 del self.dict[key]
  80
  81         def close(self):
  82                 try:
  83                         self.dict.close()
  84                 except:
  85                         pass
  86                 self.dict = 0
  87
  88         def __del__(self):
  89                 self.close()
  90
  91         def sync(self):
  92                 if hasattr(self.dict, 'sync'):
  93                         self.dict.sync()
  94
  95
  96 class BsdDbShelf(Shelf):
  97         """Shelf implementation using the "BSD" db interface.
  98
  99         This adds methods first(), next(), previous(), last() and
 100         set_location() that have no counterpart in [g]dbm databases.
 101
 102         The actual database must be opened using one of the "bsddb"
 103         modules "open" routines (i.e. bsddb.hashopen, bsddb.btopen or
 104         bsddb.rnopen) and passed to the constructor.
 105
 106         See the module's __doc__ string for an overview of the interface.
 107         """
 108
 109         def __init__(self, dict):
 110             Shelf.__init__(self, dict)
 111
 112         def set_location(self, key):
 113              (key, value) = self.dict.set_location(key)
 114              f = StringIO(value)
 115              return (key, Unpickler(f).load())
 116
 117         def next(self):
 118              (key, value) = self.dict.next()
 119              f = StringIO(value)
 120              return (key, Unpickler(f).load())
 121
 122         def previous(self):
 123              (key, value) = self.dict.previous()
 124              f = StringIO(value)
 125              return (key, Unpickler(f).load())
 126
 127         def first(self):
 128              (key, value) = self.dict.first()
 129              f = StringIO(value)
 130              return (key, Unpickler(f).load())
 131
 132         def last(self):
 133              (key, value) = self.dict.last()
 134              f = StringIO(value)
 135              return (key, Unpickler(f).load())
 136
 137
 138 class DbfilenameShelf(Shelf):
 139         """Shelf implementation using the "anydbm" generic dbm interface.
 140
 141         This is initialized with the filename for the dbm database.
 142         See the module's __doc__ string for an overview of the interface.
 143         """
 144
 145         def __init__(self, filename, flag='c'):
 146                 import anydbm
 147                 Shelf.__init__(self, anydbm.open(filename, flag))
 148
 149
 150 def open(filename, flag='c'):
 151         """Open a persistent dictionary for reading and writing.
 152
 153         Argument is the filename for the dbm database.
 154         See the module's __doc__ string for an overview of the interface.
 155         """
 156
 157         return DbfilenameShelf(filename, flag)