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Kirill Smelkov
cpython
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c8148265
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c8148265
authored
Oct 17, 2010
by
Georg Brandl
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Port changes to pickle docs apparently lost in py3k.
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c8148265
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@@ -427,33 +427,38 @@ implementation of this behaviour::
obj.__dict__.update(attributes)
return obj
.. index:: single: __getnewargs__() (copy protocol)
Classes can alter the default behaviour by providing one or several special
methods. In protocol 2 and newer, classes that implements the
:meth:`__getnewargs__` method can dictate the values passed to the
:meth:`__new__` method upon unpickling. This is often needed for classes
whose :meth:`__new__` method requires arguments.
methods:
..
index:: single: __getstate__() (copy protocol
)
..
method:: object.__getnewargs__(
)
Classes can further influence how their instances are pickled; if the class
defines the method :meth:`__getstate__`, it is called and the returned object is
pickled as the contents for the instance, instead of the contents of the
instance's dictionary. If the :meth:`__getstate__` method is absent, the
instance's :attr:`__dict__` is pickled as usual.
In protocol 2 and newer, classes that implements the :meth:`__getnewargs__`
method can dictate the values passed to the :meth:`__new__` method upon
unpickling. This is often needed for classes whose :meth:`__new__` method
requires arguments.
.. index:: single: __setstate__() (copy protocol)
Upon unpickling, if the class defines :meth:`__setstate__`, it is called with
the unpickled state. In that case, there is no requirement for the state object
to be a dictionary. Otherwise, the pickled state must be a dictionary and its
items are assigned to the new instance's dictionary.
.. method:: object.__getstate__()
.. note::
Classes can further influence how their instances are pickled; if the class
defines the method :meth:`__getstate__`, it is called and the returned object
is pickled as the contents for the instance, instead of the contents of the
instance's dictionary. If the :meth:`__getstate__` method is absent, the
instance's :attr:`__dict__` is pickled as usual.
.. method:: object.__setstate__(state)
Upon unpickling, if the class defines :meth:`__setstate__`, it is called with
the unpickled state. In that case, there is no requirement for the state
object to be a dictionary. Otherwise, the pickled state must be a dictionary
and its items are assigned to the new instance's dictionary.
.. note::
If :meth:`__getstate__` returns a false value, the :meth:`__setstate__`
method will not be called upon unpickling.
If :meth:`__getstate__` returns a false value, the :meth:`__setstate__`
method will not be called.
Refer to the section :ref:`pickle-state` for more information about how to use
the methods :meth:`__getstate__` and :meth:`__setstate__`.
...
...
@@ -462,14 +467,12 @@ the methods :meth:`__getstate__` and :meth:`__setstate__`.
At unpickling time, some methods like :meth:`__getattr__`,
:meth:`__getattribute__`, or :meth:`__setattr__` may be called upon the
instance. In case those methods rely on some internal invariant being
t
rue, the type should implement either :meth:`__getinitargs__` or
:meth:`__getnewargs__` to establish such an invariant; otherwise, neither
:meth:`__new__` nor :meth:`__init__` will be
called.
instance. In case those methods rely on some internal invariant being
true,
t
he type should implement :meth:`__getnewargs__` to establish such an
invariant; otherwise, neither :meth:`__new__` nor :meth:`__init__` will be
called.
.. index::
pair: copy; protocol
single: __reduce__() (copy protocol)
.. index:: pair: copy; protocol
As we shall see, pickle does not use directly the methods described above. In
fact, these methods are part of the copy protocol which implements the
...
...
@@ -480,58 +483,61 @@ objects. [#]_
Although powerful, implementing :meth:`__reduce__` directly in your classes is
error prone. For this reason, class designers should use the high-level
interface (i.e., :meth:`__getnewargs__`, :meth:`__getstate__` and
:meth:`__setstate__`) whenever possible. We will show, however, cases where using
:meth:`__reduce__` is the only option or leads to more efficient pickling or
both.
The interface is currently defined as follows. The :meth:`__reduce__` method
takes no argument and shall return either a string or preferably a tuple (the
returned object is often referred to as the "reduce value").
If a string is returned, the string should be interpreted as the name of a
global variable. It should be the object's local name relative to its module;
the pickle module searches the module namespace to determine the object's
module. This behaviour is typically useful for singletons.
When a tuple is returned, it must be between two and five items long. Optional
items can either be omitted, or ``None`` can be provided as their value. The
semantics of each item are in order:
.. XXX Mention __newobj__ special-case?
* A callable object that will be called to create the initial version of the
object.
* A tuple of arguments for the callable object. An empty tuple must be given if
the callable does not accept any argument.
* Optionally, the object's state, which will be passed to the object's
:meth:`__setstate__` method as previously described. If the object has no
such method then, the value must be a dictionary and it will be added to the
object's :attr:`__dict__` attribute.
* Optionally, an iterator (and not a sequence) yielding successive items. These
items will be appended to the object either using ``obj.append(item)`` or, in
batch, using ``obj.extend(list_of_items)``. This is primarily used for list
subclasses, but may be used by other classes as long as they have
:meth:`append` and :meth:`extend` methods with the appropriate signature.
(Whether :meth:`append` or :meth:`extend` is used depends on which pickle
protocol version is used as well as the number of items to append, so both
must be supported.)
* Optionally, an iterator (not a sequence) yielding successive key-value pairs.
These items will be stored to the object using ``obj[key] = value``. This is
primarily used for dictionary subclasses, but may be used by other classes as
long as they implement :meth:`__setitem__`.
.. index:: single: __reduce_ex__() (copy protocol)
Alternatively, a :meth:`__reduce_ex__` method may be defined. The only
difference is this method should take a single integer argument, the protocol
version. When defined, pickle will prefer it over the :meth:`__reduce__`
method. In addition, :meth:`__reduce__` automatically becomes a synonym for the
extended version. The main use for this method is to provide
backwards-compatible reduce values for older Python releases.
:meth:`__setstate__`) whenever possible. We will show, however, cases where
using :meth:`__reduce__` is the only option or leads to more efficient pickling
or both.
.. method:: object.__reduce__()
The interface is currently defined as follows. The :meth:`__reduce__` method
takes no argument and shall return either a string or preferably a tuple (the
returned object is often referred to as the "reduce value").
If a string is returned, the string should be interpreted as the name of a
global variable. It should be the object's local name relative to its
module; the pickle module searches the module namespace to determine the
object's module. This behaviour is typically useful for singletons.
When a tuple is returned, it must be between two and five items long.
Optional items can either be omitted, or ``None`` can be provided as their
value. The semantics of each item are in order:
.. XXX Mention __newobj__ special-case?
* A callable object that will be called to create the initial version of the
object.
* A tuple of arguments for the callable object. An empty tuple must be given
if the callable does not accept any argument.
* Optionally, the object's state, which will be passed to the object's
:meth:`__setstate__` method as previously described. If the object has no
such method then, the value must be a dictionary and it will be added to
the object's :attr:`__dict__` attribute.
* Optionally, an iterator (and not a sequence) yielding successive items.
These items will be appended to the object either using
``obj.append(item)`` or, in batch, using ``obj.extend(list_of_items)``.
This is primarily used for list subclasses, but may be used by other
classes as long as they have :meth:`append` and :meth:`extend` methods with
the appropriate signature. (Whether :meth:`append` or :meth:`extend` is
used depends on which pickle protocol version is used as well as the number
of items to append, so both must be supported.)
* Optionally, an iterator (not a sequence) yielding successive key-value
pairs. These items will be stored to the object using ``obj[key] =
value``. This is primarily used for dictionary subclasses, but may be used
by other classes as long as they implement :meth:`__setitem__`.
.. method:: object.__reduce_ex__(protocol)
Alternatively, a :meth:`__reduce_ex__` method may be defined. The only
difference is this method should take a single integer argument, the protocol
version. When defined, pickle will prefer it over the :meth:`__reduce__`
method. In addition, :meth:`__reduce__` automatically becomes a synonym for
the extended version. The main use for this method is to provide
backwards-compatible reduce values for older Python releases.
.. _pickle-persistent:
...
...
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