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shelve.py
"""Manage shelves of pickled objects. 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. To summarize the interface (key is a string, data is an arbitrary object): import shelve d = shelve.open(filename) # open, with (g)dbm filename -- no suffix d[key] = data # store data at key (overwrites old data if # using an existing key) data = d[key] # retrieve a COPY of the data at key (raise # KeyError if no such key) -- NOTE that this # access returns a *copy* of the entry! del d[key] # delete data stored at key (raises KeyError # if no such key) flag = key in d # true if the key exists list = d.keys() # a list of all existing keys (slow!) d.close() # close it Dependent on the implementation, closing a persistent dictionary may or may not be necessary to flush changes to disk. Normally, d[key] returns a COPY of the entry. This needs care when mutable entries are mutated: for example, if d[key] is a list, d[key].append(anitem) does NOT modify the entry d[key] itself, as stored in the persistent mapping -- it only modifies the copy, which is then immediately discarded, so that the append has NO effect whatsoever. To append an item to d[key] in a way that will affect the persistent mapping, use: data = d[key] data.append(anitem) d[key] = data To avoid the problem with mutable entries, you may pass the keyword argument writeback=True in the call to shelve.open. When you use: d = shelve.open(filename, writeback=True) then d keeps a cache of all entries you access, and writes them all back to the persistent mapping when you call d.close(). This ensures that such usage as d[key].append(anitem) works as intended. However, using keyword argument writeback=True may consume vast amount of memory for the cache, and it may make d.close() very slow, if you access many of d's entries after opening it in this way: d has no way to check which of the entries you access are mutable and/or which ones you actually mutate, so it must cache, and write back at close, all of the entries that you access. You can call d.sync() to write back all the entries in the cache, and empty the cache (d.sync() also synchronizes the persistent dictionary on disk, if feasible). """ from pickle import Pickler, Unpickler from io import BytesIO import collections.abc __all__ = ["Shelf", "BsdDbShelf", "DbfilenameShelf", "open"] class _ClosedDict(collections.abc.MutableMapping): 'Marker for a closed dict. Access attempts raise a ValueError.' def closed(self, *args): raise ValueError('invalid operation on closed shelf') __iter__ = __len__ = __getitem__ = __setitem__ = __delitem__ = keys = closed def __repr__(self): return '<Closed Dictionary>' class Shelf(collections.abc.MutableMapping): """Base class for shelf implementations. This is initialized with a dictionary-like object. See the module's __doc__ string for an overview of the interface. """ def __init__(self, dict, protocol=None, writeback=False, keyencoding="utf-8"): self.dict = dict if protocol is None: protocol = 3 self._protocol = protocol self.writeback = writeback self.cache = {} self.keyencoding = keyencoding def __iter__(self): for k in self.dict.keys(): yield k.decode(self.keyencoding) def __len__(self): return len(self.dict) def __contains__(self, key): return key.encode(self.keyencoding) in self.dict def get(self, key, default=None): if key.encode(self.keyencoding) in self.dict: return self[key] return default def __getitem__(self, key): try: value = self.cache[key] except KeyError: f = BytesIO(self.dict[key.encode(self.keyencoding)]) value = Unpickler(f).load() if self.writeback: self.cache[key] = value return value def __setitem__(self, key, value): if self.writeback: self.cache[key] = value f = BytesIO() p = Pickler(f, self._protocol) p.dump(value) self.dict[key.encode(self.keyencoding)] = f.getvalue() def __delitem__(self, key): del self.dict[key.encode(self.keyencoding)] try: del self.cache[key] except KeyError: pass def __enter__(self): return self def __exit__(self, type, value, traceback): self.close() def close(self): if self.dict is None: return try: self.sync() try: self.dict.close() except AttributeError: pass finally: # Catch errors that may happen when close is called from __del__ # because CPython is in interpreter shutdown. try: self.dict = _ClosedDict() except: self.dict = None def __del__(self): if not hasattr(self, 'writeback'): # __init__ didn't succeed, so don't bother closing # see http://bugs.python.org/issue1339007 for details return self.close() def sync(self): if self.writeback and self.cache: self.writeback = False for key, entry in self.cache.items(): self[key] = entry self.writeback = True self.cache = {} if hasattr(self.dict, 'sync'): self.dict.sync() class BsdDbShelf(Shelf): """Shelf implementation using the "BSD" db interface. This adds methods first(), next(), previous(), last() and set_location() that have no counterpart in [g]dbm databases. The actual database must be opened using one of the "bsddb" modules "open" routines (i.e. bsddb.hashopen, bsddb.btopen or bsddb.rnopen) and passed to the constructor. See the module's __doc__ string for an overview of the interface. """ def __init__(self, dict, protocol=None, writeback=False, keyencoding="utf-8"): Shelf.__init__(self, dict, protocol, writeback, keyencoding) def set_location(self, key): (key, value) = self.dict.set_location(key) f = BytesIO(value) return (key.decode(self.keyencoding), Unpickler(f).load()) def next(self): (key, value) = next(self.dict) f = BytesIO(value) return (key.decode(self.keyencoding), Unpickler(f).load()) def previous(self): (key, value) = self.dict.previous() f = BytesIO(value) return (key.decode(self.keyencoding), Unpickler(f).load()) def first(self): (key, value) = self.dict.first() f = BytesIO(value) return (key.decode(self.keyencoding), Unpickler(f).load()) def last(self): (key, value) = self.dict.last() f = BytesIO(value) return (key.decode(self.keyencoding), Unpickler(f).load()) class DbfilenameShelf(Shelf): """Shelf implementation using the "dbm" generic dbm interface. This is initialized with the filename for the dbm database. See the module's __doc__ string for an overview of the interface. """ def __init__(self, filename, flag='c', protocol=None, writeback=False): import dbm Shelf.__init__(self, dbm.open(filename, flag), protocol, writeback) def open(filename, flag='c', protocol=None, writeback=False): """Open a persistent dictionary for reading and writing. The filename parameter 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. The optional flag parameter has the same interpretation as the flag parameter of dbm.open(). The optional protocol parameter specifies the version of the pickle protocol. See the module's __doc__ string for an overview of the interface. """ return DbfilenameShelf(filename, flag, protocol, writeback)
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