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Document extended __reduce__ API.

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Guido van Rossum 2003-01-31 21:58:34 +00:00
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@ -106,6 +106,84 @@ Security issues
unauthenticated source *** unauthenticated source ***
Extended __reduce__ API
There are several APIs that a class can use to control pickling.
Perhaps the most popular of these are __getstate__ and
__setstate__; but the most powerful one is __reduce__. (There's
also __getinitargs__, and we're adding __getnewargs__ below.)
There are two ways to provide __reduce__ functionality: a class
can implement a __reduce__ method, or a reduce function can be
declared in copy_reg (copy_reg.dispatch_table maps classes to
functions). The return values are interpreted exactly the same,
though, and we'll refer to these collectively as __reduce__.
__reduce__ must return either a string or a tuple. If it returns
a string, this is an object whose state is not to be pickled, but
instead a reference to an equivalent object referenced by name.
Surprisingly, the string returned by __reduce__ should be the
object's local name (relative to its module); the pickle module
searches the module namespace to determine the object's module.
The rest of this section is concerned with the tuple returned by
__reduce__. It is a variable length tuple. Only the first two
items (function and arguments) are required. The remaining items
may be None or left off from the end. The last two items are new
in this PEP. The items are, in order:
function A callable object (not necessarily a function) called
to create the initial version of the object; state may
be added to the object later to fully reconstruct the
pickled state. This function must itself be
picklable.
arguments A tuple giving the argument list for the function.
As a special case, designed for Zope 2's
ExtensionClass, this may be None; in that case,
function should be a class or type, and
function.__basicnew__() is called to create the
initial version of the object. This exception is
deprecated.
state Additional state. If this is not None, the state is
pickled, and obj.__setstate__(state) will called when
unpickling. If no __setstate__ method is defined, a
default implementation is provided, which assumes
that state is a dictionary mapping instance variable
names to their values, and calls
obj.__dict__.update(state) or "for k, v in
state.items(): obj[k] = v", if update() call fails.
listitems New in this PEP. If this is not None, it should be
an iterator (not a sequence!) yielding successive
list items. These list items will be pickled, and
appended to the object using either obj.append(item)
or obj.extend(list_of_items). This is primarily used
for list subclasses, but may be used by other classes
as long as they have append() and extend() methods
with the appropriate signature. (Whether append() or
extend() is used depend on which pickle protocol
version is used as well as the number of items to
append, so both must be supported.)
dictitems New in this PEP. If this is not None, it should be
an iterator (not a sequence!) yielding successive
dictionary items, which should be tuples of the form
(key, value). These items will be pickled, and
stored to the object using obj[key] = value. This is
primarily used for dict subclasses, but may be used
by other classes as long as they implement
__settitem__.
Note: in Python 2.2 and before, when using cPickle, state would be
pickled if present even if it is None; the only safe way to avoid
the __setstate__ call was to return a two-tuple from __reduce__.
(But pickle.py would not pickle state if it was None.) In Python
2.3, __setstate__ will never be called when __reduce__ returns a
state with value None.
Copyright Copyright
This document has been placed in the public domain. This document has been placed in the public domain.