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Document __reduce_ex__.

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Guido van Rossum 2003-02-19 01:59:59 +00:00
parent d378d4b7c3
commit 10b912421d
1 changed files with 31 additions and 11 deletions
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pep-0307.txt
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@ -141,18 +141,19 @@ Extended __reduce__ API
__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__.
There are several ways to provide __reduce__ functionality: a
class can implement a __reduce__ method or a __reduce_ex__ method
(see next section), 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__.
IMPORTANT: while a classic class can implement a __reduce__()
method, pickling its instances ignores the method, so that a classic
class cannot provide __reduce__ functionality in the sense intended
here. (The copy_reg dispatch table is not consulted for classic
classes either.) A classic class must use __getinitargs__ and/or
__gestate__ to customize pickling. These are described below.
IMPORTANT: pickling of classic class instances does not look for a
__reduce__ or __reduce_ex__ method or a reduce function in the
copy_reg dispatch table, so that a classic class cannot provide
__reduce__ functionality in the sense intended here. A classic
class must use __getinitargs__ and/or __gestate__ to customize
pickling. These are described below.
__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
@ -245,6 +246,22 @@ Extended __reduce__ API
state (how this is done is up to the application).
The __reduce_ex__ API
It is sometimes useful to know the protocol version when
implementing __reduce__. This can be done by implementing a
method named __reduce_ex__ instead of __reduce__. __reduce_ex__,
when it exists, is called in preference over __reduce__ (you may
still provide __reduce__ for backwards compatibility). The
__reduce_ex__ method will be called with a single integer
argument, the protocol version.
The 'object' class implements both __reduce__ and __reduce_ex__;
however, if a subclass overrides __reduce__ but not __reduce_ex__,
the __reduce_ex__ implementation detects this and calls
__reduce__.
Customizing pickling absent a __reduce__ implementation
If no __reduce__ implementation is available for a particular
@ -662,6 +679,9 @@ The copy module
In Python 2.3, several changes are made to the copy module:
- __reduce_ex__ is supported (and always called with 2 as the
protocol version argument).
- The four- and five-argument return values of __reduce__ are
supported.