cPickle (Jython API documentation)
Date: 2012-08-13

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## org.python.modules
Class cPickle

    java.lang.Object
      org.python.modules.cPickle

All Implemented Interfaces:
  ClassDictInit

    public class cPickle

extends Object
implements ClassDictInit

From the python documentation:

The `cPickle.java` module implements a basic but powerful algorithm for ``pickling'' (a.k.a. serializing, marshalling or flattening) nearly arbitrary Python objects. This is the act of converting objects to a stream of bytes (and back: ``unpickling''). This is a more primitive notion than persistency -- although `cPickle.java` reads and writes file objects, it does not handle the issue of naming persistent objects, nor the (even more complicated) area of concurrent access to persistent objects. The `cPickle.java` module can transform a complex object into a byte stream and it can transform the byte stream into an object with the same internal structure. The most obvious thing to do with these byte streams is to write them onto a file, but it is also conceivable to send them across a network or store them in a database. The module `shelve` provides a simple interface to pickle and unpickle objects on ``dbm''-style database files.

Note: The `cPickle.java` have the same interface as the standard module `pickle`except that `Pickler` and `Unpickler` are factory functions, not classes (so they cannot be used as base classes for inheritance). This limitation is similar for the original cPickle.c version.

Unlike the built-in module `marshal`, `cPickle.java` handles the following correctly:

- recursive objects (objects containing references to themselves)

- object sharing (references to the same object in different places)

- user-defined classes and their instances

The data format used by `cPickle.java` is Python-specific. This has the advantage that there are no restrictions imposed by external standards such as XDR (which can't represent pointer sharing); however it means that non-Python programs may not be able to reconstruct pickled Python objects.

By default, the `cPickle.java` data format uses a printable ASCII representation. This is slightly more voluminous than a binary representation. The big advantage of using printable ASCII (and of some other characteristics of `cPickle.java`'s representation) is that for debugging or recovery purposes it is possible for a human to read the pickled file with a standard text editor.

A binary format, which is slightly more efficient, can be chosen by specifying a nonzero (true) value for the bin argument to the `Pickler` constructor or the `dump()` and `dumps()` functions. The binary format is not the default because of backwards compatibility with the Python 1.4 pickle module. In a future version, the default may change to binary.

The `cPickle.java` module doesn't handle code objects.

For the benefit of persistency modules written using `cPickle.java`, it supports the notion of a reference to an object outside the pickled data stream. Such objects are referenced by a name, which is an arbitrary string of printable ASCII characters. The resolution of such names is not defined by the `cPickle.java` module -- the persistent object module will have to implement a method `persistent_load()`. To write references to persistent objects, the persistent module must define a method `persistent_id()` which returns either `None` or the persistent ID of the object.

There are some restrictions on the pickling of class instances.

First of all, the class must be defined at the top level in a module. Furthermore, all its instance variables must be picklable.

When a pickled class instance is unpickled, its `__init__()` method is normally not invoked. Note: This is a deviation from previous versions of this module; the change was introduced in Python 1.5b2. The reason for the change is that in many cases it is desirable to have a constructor that requires arguments; it is a (minor) nuisance to have to provide a `__getinitargs__()` method.

If it is desirable that the `__init__()` method be called on unpickling, a class can define a method `__getinitargs__()`, which should return a tuple containing the arguments to be passed to the class constructor (`__init__()`). This method is called at pickle time; the tuple it returns is incorporated in the pickle for the instance.

Classes can further influence how their instances are pickled -- if the class defines the method `__getstate__()`, it is called and the return state is pickled as the contents for the instance, and if the class defines the method `__setstate__()`, it is called with the unpickled state. (Note that these methods can also be used to implement copying class instances.) If there is no `__getstate__()` method, the instance's `__dict__` is pickled. If there is no `__setstate__()` method, the pickled object must be a dictionary and its items are assigned to the new instance's dictionary. (If a class defines both `__getstate__()` and `__setstate__()`, the state object needn't be a dictionary -- these methods can do what they want.) This protocol is also used by the shallow and deep copying operations defined in the `copy` module.

Note that when class instances are pickled, their class's code and data are not pickled along with them. Only the instance data are pickled. This is done on purpose, so you can fix bugs in a class or add methods and still load objects that were created with an earlier version of the class. If you plan to have long-lived objects that will see many versions of a class, it may be worthwhile to put a version number in the objects so that suitable conversions can be made by the class's `__setstate__()` method.

When a class itself is pickled, only its name is pickled -- the class definition is not pickled, but re-imported by the unpickling process. Therefore, the restriction that the class must be defined at the top level in a module applies to pickled classes as well.

The interface can be summarized as follows.

To pickle an object `x` onto a file `f`, open for writing:

       p = pickle.Pickler(f)
       p.dump(x)

A shorthand for this is:

       pickle.dump(x, f)

To unpickle an object `x` from a file `f`, open for reading:

       u = pickle.Unpickler(f)
       x = u.load()

A shorthand is:

       x = pickle.load(f)

The `Pickler` class only calls the method `f.write()` with a string argument. The `Unpickler` calls the methods `f.read()` (with an integer argument) and `f.readline()` (without argument), both returning a string. It is explicitly allowed to pass non-file objects here, as long as they have the right methods.

The constructor for the `Pickler` class has an optional second argument, bin. If this is present and nonzero, the binary pickle format is used; if it is zero or absent, the (less efficient, but backwards compatible) text pickle format is used. The `Unpickler` class does not have an argument to distinguish between binary and text pickle formats; it accepts either format.

The following types can be pickled:

- `None`

- integers, long integers, floating point numbers

- strings

- tuples, lists and dictionaries containing only picklable objects

- classes that are defined at the top level in a module

- instances of such classes whose `__dict__` or `__setstate__()` is picklable

Attempts to pickle unpicklable objects will raise the `PicklingError` exception; when this happens, an unspecified number of bytes may have been written to the file.

It is possible to make multiple calls to the `dump()` method of the same `Pickler` instance. These must then be matched to the same number of calls to the `load()` method of the corresponding `Unpickler` instance. If the same object is pickled by multiple `dump()` calls, the `load()` will all yield references to the same object. Warning: this is intended for pickling multiple objects without intervening modifications to the objects or their parts. If you modify an object and then pickle it again using the same `Pickler` instance, the object is not pickled again -- a reference to it is pickled and the `Unpickler` will return the old value, not the modified one. (There are two problems here: (a) detecting changes, and (b) marshalling a minimal set of changes. I have no answers. Garbage Collection may also become a problem here.)

Apart from the `Pickler` and `Unpickler` classes, the module defines the following functions, and an exception:

`dump` (object, file[, bin])
  Write a pickled representation of obect to the open file object file. This is equivalent to "`Pickler(file, bin).dump(object)`". If the optional bin argument is present and nonzero, the binary pickle format is used; if it is zero or absent, the (less efficient) text pickle format is used.

`load` (file)
  Read a pickled object from the open file object file. This is equivalent to "`Unpickler(file).load()`".

`dumps` (object[, bin])
  Return the pickled representation of the object as a string, instead of writing it to a file. If the optional bin argument is present and nonzero, the binary pickle format is used; if it is zero or absent, the (less efficient) text pickle format is used.

`loads` (string)
  Read a pickled object from a string instead of a file. Characters in the string past the pickled object's representation are ignored.

`PicklingError`
  This exception is raised when an unpicklable object is passed to `Pickler.dump()`.

For the complete documentation on the pickle module, please see the "Python Library Reference"

The module is based on both original pickle.py and the cPickle.c version, except that all mistakes and errors are my own.

Version:
  cPickle.java,v 1.30 1999/05/15 17:40:12 fb Exp
Author:
  Finn Bock, bckfnn@pipmail.dknet.dk

Nested Class Summary
`static class` | `cPickle.Pickler` The Pickler object
`static class` | `cPickle.Unpickler` The Unpickler object.

Field Summary
`static String[]` | `__depends__`
`static String` | `__doc__` The doc string
`static String` | `__version__` The program version.
`static PyObject` | `BadPickleGet`
`static String[]` | `compatible_formats` Old format versions we can read.
`static String` | `format_version` File format version we write.
`static int` | `HIGHEST_PROTOCOL` Highest protocol version supported.
`static PyObject` | `PickleError`
`static PyObject` | `PicklingError`
`static PyObject` | `UnpickleableError`
`static PyObject` | `UnpicklingError`

Constructor Summary
`cPickle()`

Method Summary
`static PyString` | `_PickleError__str__(PyObject self, PyObject[] args, String[] kwargs)`
`static PyObject` | `_PickleError()`
`static PyString` | `_UnpickleableError__str__(PyObject self, PyObject[] args, String[] kwargs)`
`static PyObject` | `_UnpickleableError()`
`static void` | `classDictInit(PyObject dict)` Initialization when module is imported.
`static void` | `dump(PyObject object, PyObject file)` Shorthand function which pickles the object on the file.
`static void` | `dump(PyObject object, PyObject file, int protocol)` Shorthand function which pickles the object on the file.
`static PyString` | `dumps(PyObject object)` Shorthand function which pickles and returns the string representation.
`static PyString` | `dumps(PyObject object, int protocol)` Shorthand function which pickles and returns the string representation.
`static PyObject` | `exceptionNamespace()`
`static Object` | `load(PyObject file)` Shorthand function which unpickles a object from the file and returns the new object.
`static Object` | `loads(PyObject str)` Shorthand function which unpickles a object from the string and returns the new object.
`static cPickle.Pickler` | `Pickler(PyObject file)`
`static cPickle.Pickler` | `Pickler(PyObject file, int protocol)`
`static cPickle.Unpickler` | `Unpickler(PyObject file)` Returns a unpickler instance.

Methods inherited from class java.lang.Object
`equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Field Detail

### __doc__

    public static String __doc__

  The doc string

### __version__

    public static String __version__

  The program version.

### format_version

    public static final String format_version

  File format version we write.

  See Also:
    Constant Field Values

### compatible_formats

    public static final String[] compatible_formats

  Old format versions we can read.

### HIGHEST_PROTOCOL

    public static final int HIGHEST_PROTOCOL

  Highest protocol version supported.

  See Also:
    Constant Field Values

### __depends__

    public static String[] __depends__

### PickleError

    public static PyObject PickleError

### PicklingError

    public static PyObject PicklingError

### UnpickleableError

    public static PyObject UnpickleableError

### UnpicklingError

    public static PyObject UnpicklingError

### BadPickleGet

    public static PyObject BadPickleGet

Constructor Detail

### cPickle

    public cPickle()

Method Detail

### classDictInit

    public static void classDictInit(PyObject dict)

  Initialization when module is imported.

### exceptionNamespace

    public static PyObject exceptionNamespace()

### _PickleError

    public static PyObject _PickleError()

### _PickleError__str__

    public static PyString _PickleError__str__(PyObject self,
                                               PyObject[] args,
                                               String[] kwargs)

### _UnpickleableError

    public static PyObject _UnpickleableError()

### _UnpickleableError__str__

    public static PyString _UnpickleableError__str__(PyObject self,
                                                     PyObject[] args,
                                                     String[] kwargs)

### Pickler

    public static cPickle.Pickler Pickler(PyObject file)

  Parameters:
    `file` - a file-like object, can be a cStringIO.StringIO, a PyFile or any python object which implements a write method. The data will be written as text.
  Returns:
    a new Pickler instance.

### Pickler

    public static cPickle.Pickler Pickler(PyObject file,
                                          int protocol)

  Parameters:
    `file` - a file-like object, can be a cStringIO.StringIO, a PyFile or any python object which implements a write method.
    `protocol` - pickle protocol version (0 - text, 1 - pre-2.3 binary, 2 - 2.3)
  Returns:
    a new Pickler instance.

### Unpickler

    public static cPickle.Unpickler Unpickler(PyObject file)

  Returns a unpickler instance.

  Parameters:
    `file` - a file-like object, can be a cStringIO.StringIO, a PyFile or any python object which implements a read and readline method.
  Returns:
    a new Unpickler instance.

### dump

    public static void dump(PyObject object,
                            PyObject file)

  Shorthand function which pickles the object on the file.

  Parameters:
    `object` - a data object which should be pickled.
    `file` - a file-like object, can be a cStringIO.StringIO, a PyFile or any python object which implements a write method. The data will be written as text.

### dump

    public static void dump(PyObject object,
                            PyObject file,
                            int protocol)

  Shorthand function which pickles the object on the file.

  Parameters:
    `object` - a data object which should be pickled.
    `file` - a file-like object, can be a cStringIO.StringIO, a PyFile or any python object which implements a write method.
    `protocol` - pickle protocol version (0 - text, 1 - pre-2.3 binary, 2 - 2.3)

### dumps

    public static PyString dumps(PyObject object)

  Shorthand function which pickles and returns the string representation.

  Parameters:
    `object` - a data object which should be pickled.
  Returns:
    a string representing the pickled object.

### dumps

    public static PyString dumps(PyObject object,
                                 int protocol)

  Shorthand function which pickles and returns the string representation.

  Parameters:
    `object` - a data object which should be pickled.
    `protocol` - pickle protocol version (0 - text, 1 - pre-2.3 binary, 2 - 2.3)
  Returns:
    a string representing the pickled object.

### load

    public static Object load(PyObject file)

  Shorthand function which unpickles a object from the file and returns the new object.

  Parameters:
    `file` - a file-like object, can be a cStringIO.StringIO, a PyFile or any python object which implements a read and readline method.
  Returns:
    a new object.

### loads

    public static Object loads(PyObject str)

  Shorthand function which unpickles a object from the string and returns the new object.

  Parameters:
    `str` - a strings which must contain a pickled object representation.
  Returns:
    a new object.

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