498 lines
16 KiB
Plaintext
498 lines
16 KiB
Plaintext
PEP: 318
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Title: Decorators for Functions and Methods
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Version: $Revision$
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Last-Modified: $Date$
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Author: Kevin D. Smith <Kevin.Smith@theMorgue.org>,
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Jim Jewett <jimjjewett@users.sourceforge.net>,
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Skip Montanaro <skip@pobox.com>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 05-Jun-2003
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Python-Version: 2.4
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Post-History: 09-Jun-2003, 10-Jun-2003, 27-Feb-2004, 23-Mar-2004
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Abstract
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========
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The current method for declaring class and static methods is awkward
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and can lead to code that is difficult to understand. Ideally, these
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transformations should be made at the same point in the code where the
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declaration itself is made. This PEP introduces new syntax for
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transformations of a declaration.
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Motivation
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==========
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The current method of applying a transformation to a function or
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method places the actual translation after the function body. For
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large functions this separates a key component of the function's
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behavior from the definition of the rest of the function's external
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interface. For example::
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def foo(self):
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perform method operation
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foo = classmethod(foo)
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This becomes less readable with longer methods. It also seems less
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than pythonic to name the function three times for what is
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conceptually a single declaration. A solution to this problem is to
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move the transformation of the method closer to the method's own
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declaration. While the new syntax is not yet final, the intent is to
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replace::
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def foo(cls):
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pass
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foo = synchronized(lock)(foo)
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foo = classmethod(foo)
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with an alternative that places the decoration in the function's
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declaration::
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@classmethod
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@synchronized(lock)
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def foo(cls):
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pass
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Modifying classes in this fashion is also possible, though the
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benefits are not as immediately apparent. Almost certainly, anything
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which could be done with class decorators could be done using
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metaclasses, but using metaclasses is sufficiently obscure that there
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is some attraction to having an easier way to make simple
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modifications to classes. For Python 2.4, only function decorators
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are being added.
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Background
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==========
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There is general agreement that syntactic support is desirable to the
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current state of affairs. Guido mentioned `syntactic support for
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decorators`_ in his DevDay keynote presentation at the `10th Python
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Conference`_, though `he later said`_ it was only one of several
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extensions he proposed there "semi-jokingly". `Michael Hudson raised
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the topic`_ on ``python-dev`` shortly after the conference,
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attributing the bracketed syntax to an earlier proposal on
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``comp.lang.python`` by `Gareth McCaughan`_.
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.. _syntactic support for decorators:
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http://www.python.org/doc/essays/ppt/python10/py10keynote.pdf
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.. _10th python conference:
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http://www.python.org/workshops/2002-02/
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.. _michael hudson raised the topic:
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http://mail.python.org/pipermail/python-dev/2002-February/020005.html
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.. _he later said:
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http://mail.python.org/pipermail/python-dev/2002-February/020017.html
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.. _gareth mccaughan:
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http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&selm=slrna40k88.2h9o.Gareth.McCaughan%40g.local
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Class decorations seem like an obvious next step because class
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definition and function definition are syntactically similar.
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The discussion continued on and off on python-dev from February 2002
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through July 2004. Many hundreds of posts were made, with people
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proposing many possible syntax variations. Guido took a list of
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proposals to `EuroPython 2004`_, where a discussion took place.
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Subsequent to this, he decided that for 2.4a2 we'd have the Java-style
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@decorator syntax. Barry Warsaw named this the 'pie-decorator'
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syntax, in honor of the Pie-thon Parrot shootout which was announced
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about the same time as the decorator syntax, and because the @ looks a
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little like a pie. Guido `outlined his case`_ on Python-dev,
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including `this piece`_ on the various rejected forms.
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.. _EuroPython 2004:
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http://www.python.org/doc/essays/ppt/euro2004/euro2004.pdf
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.. _outlined his case:
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http://mail.python.org/pipermail/python-dev/2004-August/author.html
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.. _this piece:
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http://mail.python.org/pipermail/python-dev/2004-August/046672.html
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Design Goals
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============
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The new syntax should
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* work for arbitrary wrappers, including user-defined callables and
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the existing builtins ``classmethod()`` and ``staticmethod()``
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* work with multiple wrappers per definition
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* make it obvious what is happening; at the very least it should be
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obvious that new users can safely ignore it when writing their own
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code
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* not make future extensions more difficult
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* be easy to type; programs that use it are expected to use it very
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frequently
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* not make it more difficult to scan through code quickly. It should
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still be easy to search for all definitions, a particular
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definition, or the arguments that a function accepts
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* not needlessly complicate secondary support tools such as
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language-sensitive editors and other "`toy parser tools out
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there`_"
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.. _toy parser tools out there:
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http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&selm=mailman.1010809396.32158.python-list%40python.org
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Andrew Kuchling has links to a bunch of the discussions about motivations
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`in his blog`_.
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.. _in his blog:
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http://www.amk.ca/diary/archives/cat_python.html#003255
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Proposed Syntax
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===============
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The current syntax for function decorators as implemented in Python
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2.4a2 is::
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@dec2
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@dec1
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def func(arg1, arg2, ...):
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pass
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This is equivalent to::
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def func(arg1, arg2, ...):
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pass
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func = dec2(dec1(func))
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without the intermediate assignment to the variable ``func``. The
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decorators are near the function declaration. The @ sign makes it
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clear that something new is going on here.
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The decorator statement is limited in what it can accept - arbitrary
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expressions will not work. Guido preferred this because of a `gut feeling`_
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.. _gut feeling:
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http://mail.python.org/pipermail/python-dev/2004-August/046711.html
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Alternate Proposals
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===================
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Several other syntaxes have been proposed::
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def func(arg1, arg2, ...) as dec1, dec2, ...:
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pass
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The absence of brackets makes it cumbersome to break long lists of
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decorators across multiple lines, and the keyword "as" doesn't have
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the same meaning as its use in the ``import`` statement. Plenty of
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`alternatives to "as"`_ have been proposed. :-)
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.. _alternatives to "as":
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http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=mailman.236.1079968472.742.python-list%40python.org&rnum=2&prev=/groups%3Fq%3Dpython%2Bpep%2B318%26hl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26selm%3Dmailman.236.1079968472.742.python-list%2540python.org%26rnum%3D2
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::
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def [dec1, dec2, ...] func(arg1, arg2, ...):
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pass
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This form has the disadvantage that the decorators visually assume
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higher priority than the function name and argument list.
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::
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def func [dec1, dec2, ...] (arg1, arg2, ...):
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pass
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Quixote's `Python Template Language`_ uses this form, but only supports a
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single decorator chosen from a restricted set. For short lists it
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works okay, but for long list it separates the argument list from the
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function name.
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.. _Python Template Language:
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http://www.mems-exchange.org/software/quixote/doc/PTL.html
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::
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using:
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dec1
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dec2
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...
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def foo(arg1, arg2, ...):
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pass
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The function definition is not nested within the using: block making
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it impossible to tell which objects following the block will be
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decorated. Nesting the function definition within the using: block
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suggests nesting of namespaces that doesn't exist. The name ``foo``
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would actually exist at the same scope as the using: block. Finally,
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it would require the introduction of a new keyword.
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The obvious alternative that nests the function within the block
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::
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using:
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dec1
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dec2
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...
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def foo(arg1, arg2, ...):
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pass
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has its own set of drawbacks. Having the minimal indent level be
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three deep for methods is painful for those using limited-width
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windows. The inconsistent indentation between methods of the same
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class with and without decorators would be a readability problem.
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Finally, adding or removing decorators would require reindenting the
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entire function/method body.
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Guido proposed and implemented a patch to support interpretation of
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a `list of decorators`_ as a prefix to function definitions ::
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[dec1, dec2, ...]
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def foo(arg1, arg2, ...):
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pass
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For a while this was Guido's preferred solution, but negative sentiment ran
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high, mostly because that syntax, though useless except for side
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effects of the list, is already legal and thus creates a special case.
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.. _list of decorators:
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http://python.org/sf/926860
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Another variant on the list syntax that was initially favored was::
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def func(arg1, arg2, ...) [dec1, dec2]:
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pass
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Guido decided `he preferred`_ having the decorators on the line before
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the 'def', because it was felt that a long argument list would mean
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that the decorators would be 'hidden'
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.. _he preferred:
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http://mail.python.org/pipermail/python-dev/2004-March/043756.html
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Phillip Eby and Jp Calderone both proposed variants that required
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no new syntax, but instead used some fairly advanced introspection
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to provide decorator-like behavoiur, but Guido was unimpressed by
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these, stating::
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Using functions with "action-at-a-distance" through
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sys.settraceback may be okay for an obscure feature that can't be
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had any other way yet doesn't merit changes to the language, but
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that's not the situation for decorators. The widely held view
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here is that decorators need to be added as a syntactic feature to
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avoid the problems with the postfix notation used in 2.2 and 2.3.
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Decorators are slated to be an important new language feature and
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their design needs to be forward-looking, not constrained by what
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can be implemented in 2.3.
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A `page on the Python Wiki`_ was created to summarize a number of the
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proposals. Once it stabilizes perhaps someone would care to
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incorporate its content into this PEP (hint, hint).
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.. _page on the Python Wiki:
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http://www.python.org/moin/PythonDecorators
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Why @?
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------
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There is some history in Java using @ initially as a marker in
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`Javadoc comments`_ and later in ... mumble mumble ... The fact that
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@ was previously unused as a token in Python also means it's clear
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there is no possibility of such code being parsed by an earlier
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version of Python, leading to possibly subtle semantic bugs. That
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said, @ is still a fairly arbitrary choice. Some have suggested using
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| instead.
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For syntax options which use a list-like syntax (no matter where it
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appears) to specify the decorators a few alternatives were proposed:
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``[|...|]``, ``*[...]*``, and ``<...>``. None gained much traction.
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The alternatives which involve square brackets only serve to make it
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obvious that the decorator construct is not a list. They do nothing
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to make parsing any easier. The '<...>' alternative presents parsing
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problems because '<' and '>' already parse as un-paired. They present
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a further parsing ambiguity because a right angle bracket might be a
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greater than symbol instead of a closer for the decorators.
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.. _Javadoc comments:
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http://java.sun.com/j2se/javadoc/writingdoccomments/
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Current Implementation
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======================
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Guido asked for a voluteer to implement his preferred syntax, and Mark
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Russell stepped up and posted a `patch`_ to SF. The syntax accepted
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for 2.4a2 is::
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@dec2
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@dec1
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def func(arg1, arg2, ...):
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pass
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is equivalent to::
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def func(arg1, arg2, ...):
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pass
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func = dec2(dec1(func))
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though without the intermediate creation of a variable named ``func``.
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.. _patch: http://www.python.org/sf/979728
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A `previous patch`_ from Michael Hudson which implements the
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list-after-def syntax is also still kicking around.
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.. _previous patch: http://starship.python.net/crew/mwh/hacks/meth-syntax-sugar-3.diff
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Examples
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========
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Much of the discussion on ``comp.lang.python`` and the ``python-dev``
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mailing list focuses on the use of decorators as a cleaner way to use
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the ``staticmethod()`` and ``classmethod()`` builtins. This
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capability is much more powerful than that. This section presents
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some examples of use.
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1. Define a function to be executed at exit. Note that the function
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isn't actually "wrapped" in the usual sense.
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::
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def onexit(f):
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import atexit
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atexit.register(f)
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return f
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@onexit
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def func():
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...
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2. Define a class with a singleton instance. Note that once the class
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disappears enterprising programmers would have to be more creative
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to create more instances. (From Shane Hathaway on ``python-dev``.)
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::
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def singleton(cls):
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instances = {}
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def getinstance():
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if cls not in instances:
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instances[cls] = cls()
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return instances[cls]
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return getinstance
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@singleton
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class MyClass:
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...
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3. Add attributes to a function. (Based on an example posted by
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Anders Munch on ``python-dev``.)
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::
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def attrs(**kwds):
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def decorate(f):
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for k in kwds:
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setattr(f, k, kwds[k])
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return f
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return decorate
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@attrs(versionadded="2.2",
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author="Guido van Rossum")
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def mymethod(f):
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...
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4. Enforce function argument and return types. (Note that this is not
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exactly correct, as the returned new_f doesn't have "func" as its
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func_name attribute.)
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::
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def accepts(*types):
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def check_accepts(f):
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assert len(types) == f.func_code.co_argcount
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def new_f(*args, **kwds):
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for (a, t) in zip(args, types):
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assert isinstance(a, t), \
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"arg %r does not match %s" % (a,t)
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return f(*args, **kwds)
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return new_f
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return check_accepts
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def returns(rtype):
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def check_returns(f):
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def new_f(*args, **kwds):
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result = f(*args, **kwds)
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assert isinstance(result, rtype), \
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"return value %r does not match %s" % (result,rtype)
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return result
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return new_f
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return check_returns
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@accepts(int, (int,float))
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@returns((int,float))
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def func(arg1, arg2):
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return arg1 * arg2
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5. Declare that a class implements a particular (set of) interface(s).
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This is from a posting by Bob Ippolito on ``python-dev`` based on
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experience with `PyProtocols`_.
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.. _PyProtocols: http://peak.telecommunity.com/PyProtocols.html
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::
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def provides(*interfaces):
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"""
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An actual, working, implementation of provides for
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the current implementation of PyProtocols. Not
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particularly important for the PEP text.
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"""
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def provides(typ):
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declareImplementation(typ, instancesProvide=interfaces)
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return typ
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return provides
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class IBar(Interface):
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"""Declare something about IBar here"""
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@provides(IBar)
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class Foo(object):
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"""Implement something here..."""
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Of course, all these examples are possible today, though without
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syntactic support.
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Open Issues
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===========
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1. It's not yet certain that class decorators will be incorporated
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into the language at this point. Guido expressed skepticism about
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the concept, but various people have made some `strong arguments`_
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(search for ``PEP 318 - posting draft``) on their behalf in
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``python-dev``.
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.. _strong arguments:
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http://mail.python.org/pipermail/python-dev/2004-March/thread.html
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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End:
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