Improve pickle's documentation.

avassalotti · avassalotti · commit 758bca6e3616 · 2008-10-18T19:25:07.000Z
There is still much to be done, but I am committing my changes
incrementally to avoid losing them again (for a third time now).
diff --git a/Doc/library/pickle.rst b/Doc/library/pickle.rst
@@ -92,11 +92,9 @@ 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 :mod:`pickle` 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
-:mod:`pickle`'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.
+By default, the :mod:`pickle` data format uses a compact binary representation.
+The module :mod:`pickletools` contains tools for analyzing data streams
+generated by :mod:`pickle`.
 
 There are currently 4 different protocols which can be used for pickling.
 
@@ -110,17 +108,15 @@ There are currently 4 different protocols which can be used for pickling.
   efficient pickling of :term:`new-style class`\es.
 
 * Protocol version 3 was added in Python 3.0.  It has explicit support for
-  bytes and cannot be unpickled by Python 2.x pickle modules.
+  bytes and cannot be unpickled by Python 2.x pickle modules. This is
+  the current recommended protocol, use it whenever it is possible.
 
 Refer to :pep:`307` for more information.
 
-If a *protocol* is not specified, protocol 3 is used.  If *protocol* is 
+If a *protocol* is not specified, protocol 3 is used.  If *protocol* is
 specified as a negative value or :const:`HIGHEST_PROTOCOL`, the highest
 protocol version available will be used.
 
-A binary format, which is slightly more efficient, can be chosen by specifying a
-*protocol* version >= 1.
-
 
 Usage
 -----
@@ -146,152 +142,210 @@ an unpickler, then you call the unpickler's :meth:`load` method.  The
    as line terminators and therefore will look "funny" when viewed in Notepad or
    other editors which do not support this format.
 
+.. data:: DEFAULT_PROTOCOL
+
+   The default protocol used for pickling.  May be less than HIGHEST_PROTOCOL.
+   Currently the default protocol is 3; a backward-incompatible protocol
+   designed for Python 3.0.
+
+
 The :mod:`pickle` module provides the following functions to make the pickling
 process more convenient:
 
-
 .. function:: 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)``.
+   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 3 is used.  If *protocol* is
-   specified as a negative value or :const:`HIGHEST_PROTOCOL`, the highest 
-   protocol version will be used.
+   The optional *protocol* argument tells the pickler to use the given protocol;
+   supported protocols are 0, 1, 2, 3.  The default protocol is 3; a
+   backward-incompatible protocol designed for Python 3.0.
 
-   *file* must have a :meth:`write` method that accepts a single string argument.
-   It can thus be a file object opened for writing, a :mod:`StringIO` object, or
-   any other custom object that meets this interface.
+   Specifying a negative protocol version selects the highest protocol version
+   supported.  The higher the protocol used, the more recent the version of
+   Python needed to read the pickle produced.
 
+   The *file* argument must have a write() method that accepts a single bytes
+   argument. It can thus be a file object opened for binary writing, a
+   io.BytesIO instance, or any other custom object that meets this interface.
 
-.. function:: load(file)
+.. function:: dumps(obj[, protocol])
 
-   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()``.
+   Return the pickled representation of the object as a :class:`bytes`
+   object, instead of writing it to a file.
 
-   *file* must have two methods, a :meth:`read` method that takes an integer
-   argument, and a :meth:`readline` method that requires no arguments.  Both
-   methods should return a string.  Thus *file* can be a file object opened for
-   reading, a :mod:`StringIO` object, or any other custom object that meets this
-   interface.
+   The optional *protocol* argument tells the pickler to use the given protocol;
+   supported protocols are 0, 1, 2, 3.  The default protocol is 3; a
+   backward-incompatible protocol designed for Python 3.0.
 
-   This function automatically determines whether the data stream was written in
-   binary mode or not.
+   Specifying a negative protocol version selects the highest protocol version
+   supported.  The higher the protocol used, the more recent the version of
+   Python needed to read the pickle produced.
 
+.. function:: load(file, [\*, encoding="ASCII", errors="strict"])
 
-.. function:: dumps(obj[, protocol])
+   Read a pickled object representation from the open file object *file* and
+   return the reconstituted object hierarchy specified therein.  This is
+   equivalent to ``Unpickler(file).load()``.
 
-   Return the pickled representation of the object as a :class:`bytes`
-   object, instead of writing it to a file.
+   The protocol version of the pickle is detected automatically, so no protocol
+   argument is needed.  Bytes past the pickled object's representation are
+   ignored.
 
-   If the *protocol* parameter is omitted, protocol 3 is used.  If *protocol* 
-   is specified as a negative value or :const:`HIGHEST_PROTOCOL`, the highest 
-   protocol version will be used.
+   The argument *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 bytes.  Thus *file* can be a binary file object opened
+   for reading, a BytesIO object, or any other custom object that meets this
+   interface.
 
+   Optional keyword arguments are encoding and errors, which are used to decode
+   8-bit string instances pickled by Python 2.x.  These default to 'ASCII' and
+   'strict', respectively.
 
-.. function:: loads(bytes_object)
+.. function:: loads(bytes_object, [\*, encoding="ASCII", errors="strict"])
 
-   Read a pickled object hierarchy from a :class:`bytes` object.
-   Bytes past the pickled object's representation are ignored.
+   Read a pickled object hierarchy from a :class:`bytes` object and return the
+   reconstituted object hierarchy specified therein
 
-The :mod:`pickle` module also defines three exceptions:
+   The protocol version of the pickle is detected automatically, so no protocol
+   argument is needed.  Bytes past the pickled object's representation are
+   ignored.
 
+   Optional keyword arguments are encoding and errors, which are used to decode
+   8-bit string instances pickled by Python 2.x.  These default to 'ASCII' and
+   'strict', respectively.
+
+
+The :mod:`pickle` module defines three exceptions:
 
 .. exception:: PickleError
 
-   A common base class for the other exceptions defined below.  This inherits from
+   Common base class for the other pickling exceptions. It inherits
    :exc:`Exception`.
 
-
 .. exception:: PicklingError
 
-   This exception is raised when an unpicklable object is passed to the
-   :meth:`dump` method.
-
+   Error raised when an unpicklable object is encountered by :class:`Pickler`.
+   It inherits :exc:`PickleError`.
 
 .. 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) :exc:`AttributeError`, :exc:`EOFError`,
-   :exc:`ImportError`, and :exc:`IndexError`.
+   Error raised when there a problem unpickling an object, such as a data
+   corruption or a security violation. It inherits :exc:`PickleError`.
 
-The :mod:`pickle` module also exports two callables, :class:`Pickler` and
-:class:`Unpickler`:
+   Note that other exceptions may also be raised during unpickling, including
+   (but not necessarily limited to) AttributeError, EOFError, ImportError, and
+   IndexError.
 
 
-.. class:: Pickler(file[, protocol])
+The :mod:`pickle` module exports two classes, :class:`Pickler` and
+:class:`Unpickler`:
 
-   This takes a file-like object to which it will write a pickle data stream.
+.. class:: Pickler(file[, protocol])
 
-   If the *protocol* parameter is omitted, protocol 3 is used.  If *protocol* is
-   specified as a negative value or :const:`HIGHEST_PROTOCOL`, the highest
-   protocol version will be used.
+   This takes a binary file for writing a pickle data stream.
 
-   *file* must have a :meth:`write` method that accepts a single string argument.
-   It can thus be an open file object, a :mod:`StringIO` object, or any other
-   custom object that meets this interface.
+   The optional *protocol* argument tells the pickler to use the given protocol;
+   supported protocols are 0, 1, 2, 3.  The default protocol is 3; a
+   backward-incompatible protocol designed for Python 3.0.
 
-   :class:`Pickler` objects define one (or two) public methods:
+   Specifying a negative protocol version selects the highest protocol version
+   supported.  The higher the protocol used, the more recent the version of
+   Python needed to read the pickle produced.
 
+   The *file* argument must have a write() method that accepts a single bytes
+   argument. It can thus be a file object opened for binary writing, a
+   io.BytesIO instance, or any other custom object that meets this interface.
 
    .. method:: 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.
+      Write a pickled representation of *obj* to the open file object given in
+      the constructor.
+
+   .. method:: persistent_id(obj)
 
+      Do nothing by default.  This exists so a subclass can override it.
+
+      If :meth:`persistent_id` returns ``None``, *obj* is pickled as usual.  Any
+      other value causes :class:`Pickler` to emit the returned value as a
+      persistent ID for *obj*.  The meaning of this persistent ID should be
+      defined by :meth:`Unpickler.persistent_load`.  Note that the value
+      returned by :meth:`persistent_id` cannot itself have a persistent ID.
+
+      See :ref:`pickle-persistent` for details and examples of uses.
 
    .. method:: 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.
+      Deprecated.  Use the :meth:`clear` method on the :attr:`memo`.  Clear the
+      pickler's memo, useful when reusing picklers.
+
+   .. attribute:: fast
+
+      Enable fast mode if set to a true value.  The fast mode disables the usage
+      of memo, therefore speeding the pickling process by not generating
+      superfluous PUT opcodes.  It should not be used with self-referential
+      objects, doing otherwise will cause :class:`Pickler` to recurse
+      infinitely.
+
+      Use :func:`pickletools.optimize` if you need more compact pickles.
+
+   .. attribute:: memo
+
+      Dictionary holding previously pickled objects to allow shared or
+      recursive objects to pickled by reference as opposed to by value.
 
 
 It is possible to make multiple calls to the :meth:`dump` method of the same
 :class:`Pickler` instance.  These must then be matched to the same number of
 calls to the :meth:`load` method of the corresponding :class:`Unpickler`
 instance.  If the same object is pickled by multiple :meth:`dump` calls, the
-:meth:`load` will all yield references to the same object. [#]_
+:meth:`load` will all yield references to the same object.
 
-:class:`Unpickler` objects are defined as:
+Please note, 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
+pickled again --- a reference to it is pickled and the :class:`Unpickler` will
+return the old value, not the modified one.
 
 
-.. class:: Unpickler(file)
+.. class:: Unpickler(file, [\*, encoding="ASCII", errors="strict"])
 
-   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 :class:`Pickler`
-   factory.
+   This takes a binary file for reading a pickle data stream.
 
-   *file* must have two methods, a :meth:`read` method that takes an integer
-   argument, and a :meth:`readline` method that requires no arguments.  Both
-   methods should return a string.  Thus *file* can be a file object opened for
-   reading, a :mod:`StringIO` object, or any other custom object that meets this
-   interface.
+   The protocol version of the pickle is detected automatically, so no
+   protocol argument is needed.
 
-   :class:`Unpickler` objects have one (or two) public methods:
+   The argument *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 bytes.  Thus *file* can be a binary file object opened
+   for reading, a BytesIO object, or any other custom object that meets this
+   interface.
 
+   Optional keyword arguments are encoding and errors, which are used to decode
+   8-bit string instances pickled by Python 2.x.  These default to 'ASCII' and
+   'strict', respectively.
 
    .. method:: load()
 
       Read a pickled object representation from the open file object given in
       the constructor, and return the reconstituted object hierarchy specified
-      therein.
+      therein. Bytes past the pickled object's representation are ignored.
 
-      This method automatically determines whether the data stream was written
-      in binary mode or not.
+   .. method:: persistent_load(pid)
 
+      Raise an :exc:`UnpickingError` by default.
 
-   .. method:: noload()
+      If defined, :meth:`persistent_load` should return the object specified by
+      the persistent ID *pid*. On errors, such as if an invalid persistent ID is
+      encountered, an :exc:`UnpickingError` should be raised.
 
-      This is just like :meth:`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
-      :ref:`pickle-protocol` below for more details.
+      See :ref:`pickle-persistent` for details and examples of uses.
+
+   .. method:: find_class(module, name)
+
+      Import *module* if necessary and return the object called *name* from it.
+      Subclasses may override this to gain control over what type of objects can
+      be loaded, potentially reducing security risks.
 
 
 What can be pickled and unpickled?
@@ -506,6 +560,8 @@ The registered constructor is deemed a "safe constructor" for purposes of
 unpickling as described above.
 
 
+.. _pickle-persistent:
+
 Pickling and unpickling external objects
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -747,14 +803,6 @@ the same process or a new process. ::
 
 .. [#] Don't confuse this with the :mod:`marshal` module
 
-.. [#] *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
-   pickled again --- a reference to it is pickled and the :class:`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.
-
 .. [#] The exception raised will likely be an :exc:`ImportError` or an
    :exc:`AttributeError` but it could be something else.
 
