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13.1.3 Usage

13.1.3 Usage

To serialize an object hierarchy, you first create a pickler, then you call the pickler's dump() method. To de-serialize a data stream, you first create an unpickler, then you call the unpickler's load() method. The pickle module provides the following constant:

HIGHEST_PROTOCOL
The highest protocol version available. This value can be passed as a protocol value. New in version 2.3.

Note: Be sure to always open pickle files created with protocols >= 1 in binary mode. For the old ASCII-based pickle protocol 0 you can use either text mode or binary mode as long as you stay consistent.

A pickle file written with protocol 0 in binary mode will contain lone linefeeds as line terminators and therefore will look ``funny'' when viewed in Notepad or other editors which do not support this format.

The pickle module provides the following functions to make the pickling process more convenient:

dump( obj, file[, protocol])
Write a pickled representation of obj to the open file object file. This is equivalent to Pickler(file, protocol).dump(obj).

If the protocol parameter is omitted, protocol 0 is used. If protocol is specified as a negative value or HIGHEST_PROTOCOL, the highest protocol version will be used.

Changed in version 2.3: Introduced the protocol parameter.

file must have a write() method that accepts a single string argument. It can thus be a file object opened for writing, a StringIO object, or any other custom object that meets this interface.

load( file)
Read a string from the open file object file and interpret it as a pickle data stream, reconstructing and returning the original object hierarchy. This is equivalent to Unpickler(file).load().

file must have two methods, a read() method that takes an integer argument, and a readline() method that requires no arguments. Both methods should return a string. Thus file can be a file object opened for reading, a StringIO object, or any other custom object that meets this interface.

This function automatically determines whether the data stream was written in binary mode or not.

dumps( obj[, protocol])
Return the pickled representation of the object as a string, instead of writing it to a file.

If the protocol parameter is omitted, protocol 0 is used. If protocol is specified as a negative value or HIGHEST_PROTOCOL, the highest protocol version will be used.

Changed in version 2.3: The protocol parameter was added.

loads( string)
Read a pickled object hierarchy from a string. Characters in the string past the pickled object's representation are ignored.

The pickle module also defines three exceptions:

exception PickleError
A common base class for the other exceptions defined below. This inherits from Exception.

exception PicklingError
This exception is raised when an unpicklable object is passed to the dump() method.

exception UnpicklingError
This exception is raised when there is a problem unpickling an object. Note that other exceptions may also be raised during unpickling, including (but not necessarily limited to) AttributeError, EOFError, ImportError, and IndexError.

The pickle module also exports two callables13.2, Pickler and Unpickler:

class Pickler( file[, protocol])
This takes a file-like object to which it will write a pickle data stream.

If the protocol parameter is omitted, protocol 0 is used. If protocol is specified as a negative value, the highest protocol version will be used.

Changed in version 2.3: Introduced the protocol parameter.

file must have a write() method that accepts a single string argument. It can thus be an open file object, a StringIO object, or any other custom object that meets this interface.

Pickler objects define one (or two) public methods:

dump( obj)
Write a pickled representation of obj to the open file object given in the constructor. Either the binary or ASCII format will be used, depending on the value of the protocol argument passed to the constructor.

clear_memo( )
Clears the pickler's ``memo''. The memo is the data structure that remembers which objects the pickler has already seen, so that shared or recursive objects pickled by reference and not by value. This method is useful when re-using picklers.

Note: Prior to Python 2.3, clear_memo() was only available on the picklers created by cPickle. In the pickle module, picklers have an instance variable called memo which is a Python dictionary. So to clear the memo for a pickle module pickler, you could do the following:

mypickler.memo.clear()

Code that does not need to support older versions of Python should simply use clear_memo().

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.13.3

Unpickler objects are defined as:

class Unpickler( file)
This takes a file-like object from which it will read a pickle data stream. This class automatically determines whether the data stream was written in binary mode or not, so it does not need a flag as in the Pickler factory.

file must have two methods, a read() method that takes an integer argument, and a readline() method that requires no arguments. Both methods should return a string. Thus file can be a file object opened for reading, a StringIO object, or any other custom object that meets this interface.

Unpickler objects have one (or two) public methods:

load( )
Read a pickled object representation from the open file object given in the constructor, and return the reconstituted object hierarchy specified therein.

noload( )
This is just like load() except that it doesn't actually create any objects. This is useful primarily for finding what's called ``persistent ids'' that may be referenced in a pickle data stream. See section 13.1.5 below for more details.

Note: the noload() method is currently only available on Unpickler objects created with the cPickle module. pickle module Unpicklers do not have the noload() method.



Footnotes

... callables13.2
In the pickle module these callables are classes, which you could subclass to customize the behavior. However, in the cPickle module these callables are factory functions and so cannot be subclassed. One common reason to subclass is to control what objects can actually be unpickled. See section 13.1.6 for more details.
... object.13.3
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: (1) detecting changes, and (2) marshalling a minimal set of changes. Garbage Collection may also become a problem here.
See About this document... for information on suggesting changes.
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