234 lines
7.6 KiB
ReStructuredText
234 lines
7.6 KiB
ReStructuredText
PEP: 3135
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Title: New Super
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Author: Calvin Spealman <ironfroggy@gmail.com>,
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Tim Delaney <timothy.c.delaney@gmail.com>,
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Lie Ryan <lie.1296@gmail.com>
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Status: Final
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 28-Apr-2007
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Python-Version: 3.0
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Post-History: `28-Apr-2007 <https://mail.python.org/pipermail/python-dev/2007-April/072807.html>`__,
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`29-Apr-2007 <https://mail.python.org/pipermail/python-dev/2007-April/072835.html>`__,
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`29-Apr-2007 <https://mail.python.org/pipermail/python-dev/2007-April/072858.html>`__,
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`14-May-2007 <https://mail.python.org/pipermail/python-dev/2007-May/073127.html>`__,
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`12-Mar-2009 <https://mail.python.org/pipermail/python-bugs-list/2009-March/072665.html>`__
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Numbering Note
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==============
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This PEP started its life as :pep:`367`. Since it is now targeted
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for Python 3000, it has been moved into the 3xxx space.
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Abstract
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========
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This PEP proposes syntactic sugar for use of the ``super`` type to automatically
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construct instances of the super type binding to the class that a method was
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defined in, and the instance (or class object for classmethods) that the method
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is currently acting upon.
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The premise of the new super usage suggested is as follows::
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super().foo(1, 2)
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to replace the old::
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super(Foo, self).foo(1, 2)
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Rationale
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=========
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The current usage of super requires an explicit passing of both the class and
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instance it must operate from, requiring a breaking of the DRY (Don't Repeat
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Yourself) rule. This hinders any change in class name, and is often considered
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a wart by many.
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Specification
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=============
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Within the specification section, some special terminology will be used to
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distinguish similar and closely related concepts. "super class" will refer to
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the actual builtin class named "super". A "super instance" is simply an
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instance of the super class, which is associated with another class and
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possibly with an instance of that class.
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The new ``super`` semantics are only available in Python 3.0.
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Replacing the old usage of super, calls to the next class in the MRO (method
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resolution order) can be made without explicitly passing the class object
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(although doing so will still be supported). Every function
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will have a cell named ``__class__`` that contains the class object that the
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function is defined in.
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The new syntax::
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super()
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is equivalent to::
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super(__class__, <firstarg>)
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where ``__class__`` is the class that the method was defined in, and
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``<firstarg>`` is the first parameter of the method (normally ``self``
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for instance methods, and ``cls`` for class methods). For functions
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defined outside a class body, ``__class__`` is not defined, and will
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result in runtime ``SystemError``.
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While ``super`` is not a reserved word, the parser recognizes the use
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of ``super`` in a method definition and only passes in the
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``__class__`` cell when this is found. Thus, calling a global alias
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of ``super`` without arguments will not necessarily work.
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Closed Issues
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=============
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Determining the class object to use
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-----------------------------------
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The class object is taken from a cell named ``__class__``.
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Should ``super`` actually become a keyword?
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-------------------------------------------
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No. It is not necessary for super to become a keyword.
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super used with __call__ attributes
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-----------------------------------
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It was considered that it might be a problem that instantiating super instances
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the classic way, because calling it would lookup the __call__ attribute and
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thus try to perform an automatic super lookup to the next class in the MRO.
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However, this was found to be false, because calling an object only looks up
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the __call__ method directly on the object's type. The following example shows
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this in action.
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.. code:: python
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class A(object):
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def __call__(self):
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return '__call__'
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def __getattribute__(self, attr):
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if attr == '__call__':
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return lambda: '__getattribute__'
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a = A()
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assert a() == '__call__'
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assert a.__call__() == '__getattribute__'
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In any case, this issue goes away entirely because classic calls to
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``super(<class>, <instance>)`` are still supported with the same meaning.
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Alternative Proposals
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=====================
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No Changes
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----------
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Although its always attractive to just keep things how they are, people have
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sought a change in the usage of super calling for some time, and for good
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reason, all mentioned previously.
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- Decoupling from the class name (which might not even be bound to the
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right class anymore!)
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- Simpler looking, cleaner super calls would be better
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Dynamic attribute on super type
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-------------------------------
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The proposal adds a dynamic attribute lookup to the super type, which will
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automatically determine the proper class and instance parameters. Each super
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attribute lookup identifies these parameters and performs the super lookup on
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the instance, as the current super implementation does with the explicit
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invocation of a super instance upon a class and instance.
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This proposal relies on sys._getframe(), which is not appropriate for anything
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except a prototype implementation.
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self.__super__.foo(\*args)
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--------------------------
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The __super__ attribute is mentioned in this PEP in several places, and could
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be a candidate for the complete solution, actually using it explicitly instead
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of any super usage directly. However, double-underscore names are usually an
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internal detail, and attempted to be kept out of everyday code.
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super(self, \*args) or __super__(self, \*args)
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----------------------------------------------
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This solution only solves the problem of the type indication, does not handle
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differently named super methods, and is explicit about the name of the
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instance. It is less flexible without being able to enacted on other method
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names, in cases where that is needed. One use case this fails is where a
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base-class has a factory classmethod and a subclass has two factory
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classmethods,both of which needing to properly make super calls to the one
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in the base-class.
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super.foo(self, \*args)
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-----------------------
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This variation actually eliminates the problems with locating the proper
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instance, and if any of the alternatives were pushed into the spotlight, I
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would want it to be this one.
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super(\*p, \*\*kw)
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------------------
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There has been the proposal that directly calling ``super(*p, **kw)`` would
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be equivalent to calling the method on the ``super`` object with the same name
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as the method currently being executed i.e. the following two methods would be
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equivalent:
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.. code:: python
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def f(self, *p, **kw):
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super.f(*p, **kw)
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.. code:: python
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def f(self, *p, **kw):
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super(*p, **kw)
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There is strong sentiment for and against this, but implementation and style
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concerns are obvious. Guido has suggested that this should be excluded from
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this PEP on the principle of KISS (Keep It Simple Stupid).
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History
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=======
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29-Apr-2007
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- Changed title from "Super As A Keyword" to "New Super"
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- Updated much of the language and added a terminology section
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for clarification in confusing places.
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- Added reference implementation and history sections.
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06-May-2007
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- Updated by Tim Delaney to reflect discussions on the python-3000
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and python-dev mailing lists.
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12-Mar-2009
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- Updated to reflect the current state of implementation.
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References
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==========
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[1] Fixing super anyone?
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\ (https://mail.python.org/pipermail/python-3000/2007-April/006667.html)
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[2] PEP 3130: Access to Module/Class/Function Currently Being Defined (this)
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\ (https://mail.python.org/pipermail/python-ideas/2007-April/000542.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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