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author | Benjamin Peterson <benjamin@python.org> | 2008-06-20 21:03:22 (GMT) |
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committer | Benjamin Peterson <benjamin@python.org> | 2008-06-20 21:03:22 (GMT) |
commit | be149d0a0d23626af13205dcc5ba1ba4ea1f5f92 (patch) | |
tree | 6695c33f47d5798dc68e85d640261e95336c8793 /Doc | |
parent | 4fe93b2835bc4f2ed8d237590ace3910a042e3b8 (diff) | |
download | cpython-be149d0a0d23626af13205dcc5ba1ba4ea1f5f92.zip cpython-be149d0a0d23626af13205dcc5ba1ba4ea1f5f92.tar.gz cpython-be149d0a0d23626af13205dcc5ba1ba4ea1f5f92.tar.bz2 |
remove references of cPickle in the pickle docs (uhh. unlabeled footnotes)
Diffstat (limited to 'Doc')
-rw-r--r-- | Doc/library/pickle.rst | 140 |
1 files changed, 23 insertions, 117 deletions
diff --git a/Doc/library/pickle.rst b/Doc/library/pickle.rst index 76248ac..2e6ea48 100644 --- a/Doc/library/pickle.rst +++ b/Doc/library/pickle.rst @@ -20,27 +20,15 @@ process whereby a Python object hierarchy is converted into a byte stream, and "unpickling" is the inverse operation, whereby a byte stream is converted back into an object hierarchy. Pickling (and unpickling) is alternatively known as "serialization", "marshalling," [#]_ or "flattening", however, to avoid -confusion, the terms used here are "pickling" and "unpickling". - -This documentation describes both the :mod:`pickle` module and the -:mod:`cPickle` module. +confusion, the terms used here are "pickling" and "unpickling".. Relationship to other Python modules ------------------------------------ -The :mod:`pickle` module has an optimized cousin called the :mod:`cPickle` -module. As its name implies, :mod:`cPickle` is written in C, so it can be up to -1000 times faster than :mod:`pickle`. However it does not support subclassing -of the :func:`Pickler` and :func:`Unpickler` classes, because in :mod:`cPickle` -these are functions, not classes. Most applications have no need for this -functionality, and can benefit from the improved performance of :mod:`cPickle`. -Other than that, the interfaces of the two modules are nearly identical; the -common interface is described in this manual and differences are pointed out -where necessary. In the following discussions, we use the term "pickle" to -collectively describe the :mod:`pickle` and :mod:`cPickle` modules. - -The data streams the two modules produce are guaranteed to be interchangeable. +The :mod:`pickle` module has an transparent optimizer (:mod:`_pickle`) written +in C. It is used whenever available. Otherwise the pure Python implementation is +used. Python has a more primitive serialization module called :mod:`marshal`, but in general :mod:`pickle` should always be the preferred way to serialize Python @@ -229,7 +217,7 @@ The :mod:`pickle` module also defines three exceptions: necessarily limited to) :exc:`AttributeError`, :exc:`EOFError`, :exc:`ImportError`, and :exc:`IndexError`. -The :mod:`pickle` module also exports two callables [#]_, :class:`Pickler` and +The :mod:`pickle` module also exports two callables, :class:`Pickler` and :class:`Unpickler`: @@ -305,11 +293,6 @@ instance. If the same object is pickled by multiple :meth:`dump` calls, the ids" that may be referenced in a pickle data stream. See section :ref:`pickle-protocol` below for more details. - **Note:** the :meth:`noload` method is currently only available on - :class:`Unpickler` objects created with the :mod:`cPickle` module. - :mod:`pickle` module :class:`Unpickler`\ s do not have the :meth:`noload` - method. - What can be pickled and unpickled? ---------------------------------- @@ -535,7 +518,7 @@ 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 :mod:`pickle` module; it will delegate this resolution to user defined -functions on the pickler and unpickler. [#]_ +functions on the pickler and unpickler. To define external persistent id resolution, you need to set the :attr:`persistent_id` attribute of the pickler object and the @@ -600,15 +583,8 @@ Here's a silly example that *might* shed more light:: j = up.load() print(j) -In the :mod:`cPickle` module, the unpickler's :attr:`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. -[#]_ Setting :attr:`persistent_load` to a list is usually used in conjunction -with the :meth:`noload` method on the Unpickler. -.. BAW: Both pickle and cPickle support something called inst_persistent_id() +.. BAW: pickle supports something called inst_persistent_id() which appears to give unknown types a second shot at producing a persistent id. Since Jim Fulton can't remember why it was added or what it's for, I'm leaving it undocumented. @@ -625,32 +601,22 @@ Subclassing Unpicklers By default, unpickling will import any class that it finds in the pickle data. You can control exactly what gets unpickled and what gets called by customizing -your unpickler. Unfortunately, exactly how you do this is different depending -on whether you're using :mod:`pickle` or :mod:`cPickle`. [#]_ - -In the :mod:`pickle` module, you need to derive a subclass from -:class:`Unpickler`, overriding the :meth:`load_global` method. -:meth:`load_global` should read two lines from the pickle data stream where the -first line will the name of the module containing the class and the second line -will be the name of the instance's class. It then looks up the class, possibly -importing the module and digging out the attribute, then it appends what it -finds to the unpickler's stack. Later on, this class will be assigned to the -:attr:`__class__` attribute of an empty class, as a way of magically creating an -instance without calling its class's :meth:`__init__`. Your job (should you -choose to accept it), would be to have :meth:`load_global` push onto the -unpickler's stack, a known safe version of any class you deem safe to unpickle. -It is up to you to produce such a class. Or you could raise an error if you -want to disallow all unpickling of instances. If this sounds like a hack, -you're right. Refer to the source code to make this work. - -Things are a little cleaner with :mod:`cPickle`, but not by much. To control -what gets unpickled, you can set the unpickler's :attr:`find_global` attribute -to a function or ``None``. If it is ``None`` then any attempts to unpickle -instances will raise an :exc:`UnpicklingError`. If it is a function, then it -should accept a module name and a class name, and return the corresponding class -object. It is responsible for looking up the class and performing any necessary -imports, and it may raise an error to prevent instances of the class from being -unpickled. +your unpickler. + +You need to derive a subclass from :class:`Unpickler`, overriding the +:meth:`load_global` method. :meth:`load_global` should read two lines from the +pickle data stream where the first line will the name of the module containing +the class and the second line will be the name of the instance's class. It then +looks up the class, possibly importing the module and digging out the attribute, +then it appends what it finds to the unpickler's stack. Later on, this class +will be assigned to the :attr:`__class__` attribute of an empty class, as a way +of magically creating an instance without calling its class's +:meth:`__init__`. Your job (should you choose to accept it), would be to have +:meth:`load_global` push onto the unpickler's stack, a known safe version of any +class you deem safe to unpickle. It is up to you to produce such a class. Or +you could raise an error if you want to disallow all unpickling of instances. +If this sounds like a hack, you're right. Refer to the source code to make this +work. The moral of the story is that you should be really careful about the source of the strings your application unpickles. @@ -777,48 +743,10 @@ the same process or a new process. :: High-performance serialization of built-in types. -:mod:`cPickle` --- A faster :mod:`pickle` -========================================= - -.. module:: cPickle - :synopsis: Faster version of pickle, but not subclassable. -.. moduleauthor:: Jim Fulton <jim@zope.com> -.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org> - - -.. index:: module: pickle - -The :mod:`cPickle` module supports serialization and de-serialization of Python -objects, providing an interface and functionality nearly identical to the -:mod:`pickle` module. There are several differences, the most important being -performance and subclassability. - -First, :mod:`cPickle` can be up to 1000 times faster than :mod:`pickle` because -the former is implemented in C. Second, in the :mod:`cPickle` module the -callables :func:`Pickler` and :func:`Unpickler` are functions, not classes. -This means that you cannot use them to derive custom pickling and unpickling -subclasses. Most applications have no need for this functionality and should -benefit from the greatly improved performance of the :mod:`cPickle` module. - -The pickle data stream produced by :mod:`pickle` and :mod:`cPickle` are -identical, so it is possible to use :mod:`pickle` and :mod:`cPickle` -interchangeably with existing pickles. [#]_ - -There are additional minor differences in API between :mod:`cPickle` and -:mod:`pickle`, however for most applications, they are interchangeable. More -documentation is provided in the :mod:`pickle` module documentation, which -includes a list of the documented differences. - .. rubric:: Footnotes .. [#] Don't confuse this with the :mod:`marshal` module -.. [#] In the :mod:`pickle` module these callables are classes, which you could - subclass to customize the behavior. However, in the :mod:`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 - :ref:`pickle-sub` for more details. - .. [#] *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 :class:`Pickler` instance, the object is not @@ -834,25 +762,3 @@ includes a list of the documented differences. .. [#] This protocol is also used by the shallow and deep copying operations defined in the :mod:`copy` module. - -.. [#] The actual mechanism for associating these user defined functions is slightly - different for :mod:`pickle` and :mod:`cPickle`. The description given here - works the same for both implementations. Users of the :mod:`pickle` module - could also use subclassing to effect the same results, overriding the - :meth:`persistent_id` and :meth:`persistent_load` methods in the derived - classes. - -.. [#] We'll leave you with the image of Guido and Jim sitting around sniffing pickles - in their living rooms. - -.. [#] A word of caution: the mechanisms described here use internal attributes and - methods, which are subject to change in future versions of Python. We intend to - someday provide a common interface for controlling this behavior, which will - work in either :mod:`pickle` or :mod:`cPickle`. - -.. [#] Since the pickle data format is actually a tiny stack-oriented programming - language, and some freedom is taken in the encodings of certain objects, it is - possible that the two modules produce different data streams for the same input - objects. However it is guaranteed that they will always be able to read each - other's data streams. - |