8. The pickle module

Status:in progress

Python provides the pickle module to store any Python object in a file and get it back later on. This is called storing the object persistently.In fact there is also a module called cPickle, which is simply faster and works the same way as pickle.

Note

in other languages, pickling may be called with other names such as serializing or marshalling.

8.1. Quick start

>>> import cPickle as p
>>> a = ['test', 1, {'key1':[1,2,3,4]}]
>>> f = file('/tmp/data', 'w')
>>> p.dump(a, f)
>>> f.close()

>>> del a
>>> f = file('/tmp/data')
>>> new_a = p.load(f)
>>> print new_a
['test', 1, {'key1':[1,2,3,4]}]

You can pickle, None, numbers, strings, list, tuples, dictionaries, functions defined at top level, built-in functions, calsses defined at top-levels, instances of classes defined at top level of a module.

Pickler and UnPickler are classes available in the pickle ,module. Note, however that they can be subclasses whereas the in cPickle they are functions.

8.2. Remarks

from [Norton]

• Not all Python objects can be pickled. For instance, you can pickle a class instance, but the class itself must be available when you unpickle the object. This isn’t a problem for instances of classes in standard Python modules, but if you pickle an instance of a class you wrote, make sure that the module containing that class is available when you unpickle. The class itself isn’t pickled. You don’t have to import the module containing the class. Python does this for you, as long as the module is available.
• Other types of objects may or may not be pickleable. An instance of an extension type (see Chapter 17) generally cannot be pickled, unless specialized pickling functions are available for that extension type. This includes some types in the standard Python modules.
• Pickles are portable between operating systems and architectures. For example, you can create a pickle on a Windows or GNU/Linux PC and unpickle it on a Mac, or even a Sun workstation. This enables you to move pickle files and transfer pickles over a network between different types of computers.
• Pickles are Python-specific. There’s no easy way to access the contents of a pickle with programs written in other languages.

from [Fehily]

• if you pickle a container (e.g. list) that contains aliases that refer to the same object, the items in the unpickeled object will still be aliases.

8.3. todo

pickle.dispatch_table
pickle.HIGHEST_PROTOCOL
pickle.StringIO
pickle.StringTypes
pickle.struct
TracebackType
pickle.encode_long
PicklingError
pickle.loads
pickle.re
pickle.classmap
pickle.marshal
pickle.UnpicklingError
pickle.format_version
pickle.decode_long
pickle.whichmodule
pickle.mloads

8.4. cpickle todo

cPickle.dump
cPickle.dumps
cPickle.load
cPickle.PicklingError
cPickle.compatible_formats
cPickle.loads
cPickle.UnpickleableError
cPickle.format_version