13.1.5.3 Pickling and unpickling external objectsFor the benefit of object persistence, the pickle module supports the notion of a reference to an object outside the pickled data stream. Such objects are referenced by a ``persistent id'', which is just an arbitrary string of printable ASCII characters. The resolution of such names is not defined by the pickle module; it will delegate this resolution to user defined functions on the pickler and unpickler.13.7 To define external persistent id resolution, you need to set the persistent_id attribute of the pickler object and the persistent_load attribute of the unpickler object.
To pickle objects that have an external persistent id, the pickler
must have a custom persistent_id() method that takes an
object as an argument and returns either To unpickle external objects, the unpickler must have a custom persistent_load() function that takes a persistent id string and returns the referenced object. Here's a silly example that might shed more light:
import pickle from cStringIO import StringIO src = StringIO() p = pickle.Pickler(src) def persistent_id(obj): if hasattr(obj, 'x'): return 'the value %d' % obj.x else: return None p.persistent_id = persistent_id class Integer: def __init__(self, x): self.x = x def __str__(self): return 'My name is integer %d' % self.x i = Integer(7) print i p.dump(i) datastream = src.getvalue() print repr(datastream) dst = StringIO(datastream) up = pickle.Unpickler(dst) class FancyInteger(Integer): def __str__(self): return 'I am the integer %d' % self.x def persistent_load(persid): if persid.startswith('the value '): value = int(persid.split()[2]) return FancyInteger(value) else: raise pickle.UnpicklingError, 'Invalid persistent id' up.persistent_load = persistent_load j = up.load() print j In the cPickle module, the unpickler's persistent_load attribute can also be set to a Python list, in which case, when the unpickler reaches a persistent id, the persistent id string will simply be appended to this list. This functionality exists so that a pickle data stream can be ``sniffed'' for object references without actually instantiating all the objects in a pickle.13.8 Setting persistent_load to a list is usually used in conjunction with the noload() method on the Unpickler.
Footnotes
|