Finally update the super() PEP. Lie Ryan submitted a draft which I updated
a bit more. I think this is good enough now.
This commit is contained in:
Guido van Rossum
parent
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pep-3135.txt
439
pep-3135.txt
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@ -3,13 +3,14 @@ Title: New Super
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Version: $Revision$
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Last-Modified: $Date$
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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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Status: Draft
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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: Accepted
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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, 29-Apr-2007 (1), 29-Apr-2007 (2), 14-May-2007
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Post-History: 28-Apr-2007, 29-Apr-2007 (1), 29-Apr-2007 (2), 14-May-2007, 12-Mar-2009
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Numbering Note
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==============
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@ -17,11 +18,6 @@ Numbering Note
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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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NOTE: This PEP needs to be rewritten to match reality. The actual
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implementation is quite different than what is described here;
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effectively, super() equals to super(C, self) where C is the current
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class and self is the first argument of the current function.
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Abstract
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========
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@ -32,19 +28,12 @@ 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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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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and the current ``__builtin__.super`` be aliased to ``__builtin__.__super__``
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(with ``__builtin__.super`` to be removed in Python 3.0).
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It is further proposed that assignment to ``super`` become a ``SyntaxError``,
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similar to the behaviour of ``None``.
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Rationale
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=========
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@ -58,83 +47,54 @@ 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 type" will refer to
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the actual builtin type named "super". A "super instance" is simply an instance
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of the super type, which is associated with a class and possibly with an
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instance of that class.
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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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Because the new ``super`` semantics are not backwards compatible with Python
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2.5, the new semantics will require a ``__future__`` import::
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from __future__ import new_super
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The current ``__builtin__.super`` will be aliased to ``__builtin__.__super__``.
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This will occur regardless of whether the new ``super`` semantics are active.
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It is not possible to simply rename ``__builtin__.super``, as that would affect
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modules that do not use the new ``super`` semantics. In Python 3.0 it is
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proposed that the name ``__builtin__.super`` will be removed.
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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 creating a ``super``
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instance (although doing so will still be supported via ``__super__``). Every
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function will have an implicit local named ``super``. This name behaves
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identically to a normal local, including use by inner functions via a cell,
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with the following exceptions:
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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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1. Assigning to the name ``super`` will raise a ``SyntaxError`` at compile time;
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The new syntax::
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2. Calling a static method or normal function that accesses the name ``super``
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will raise a ``TypeError`` at runtime.
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super()
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Every function that uses the name ``super``, or has an inner function that
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uses the name ``super``, will include a preamble that performs the equivalent
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of::
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is equivalent to::
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super = __builtin__.__super__(<class>, <instance>)
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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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``<instance>`` is the first parameter of the method (normally ``self`` for
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instance methods, and ``cls`` for class methods). For static methods and normal
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functions, ``<class>`` will be ``None``, resulting in a ``TypeError`` being
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raised during the preamble.
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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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Note: The relationship between ``super`` and ``__super__`` is similar to that
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between ``import`` and ``__import__``.
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Much of this was discussed in the thread of the python-dev list, "Fixing super
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anyone?" [1]_.
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Open Issues
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-----------
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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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-----------------------------------
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The exact mechanism for associating the method with the defining class is not
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specified in this PEP, and should be chosen for maximum performance. For
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CPython, it is suggested that the class instance be held in a C-level variable
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on the function object which is bound to one of ``NULL`` (not part of a class),
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``Py_None`` (static method) or a class object (instance or class method).
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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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-------------------------------------------
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With this proposal, ``super`` would become a keyword to the same extent that
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``None`` is a keyword. It is possible that further restricting the ``super``
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name may simplify implementation, however some are against the actual keyword-
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ization of super. The simplest solution is often the correct solution and the
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simplest solution may well not be adding additional keywords to the language
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when they are not needed. Still, it may solve other open issues.
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Closed Issues
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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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-----------------------------------
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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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@ -155,315 +115,8 @@ this in action.
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assert a() == '__call__'
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assert a.__call__() == '__getattribute__'
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In any case, with the renaming of ``__builtin__.super`` to
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``__builtin__.__super__`` this issue goes away entirely.
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Reference Implementation
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========================
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It is impossible to implement the above specification entirely in Python. This
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reference implementation has the following differences to the specification:
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1. New ``super`` semantics are implemented using bytecode hacking.
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2. Assignment to ``super`` is not a ``SyntaxError``. Also see point #4.
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3. Classes must either use the metaclass ``autosuper_meta`` or inherit from
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the base class ``autosuper`` to acquire the new ``super`` semantics.
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4. ``super`` is not an implicit local variable. In particular, for inner
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functions to be able to use the super instance, there must be an assignment
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of the form ``super = super`` in the method.
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The reference implementation assumes that it is being run on Python 2.5+.
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::
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#!/usr/bin/env python
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#
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# autosuper.py
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from array import array
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import dis
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import new
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import types
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import __builtin__
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__builtin__.__super__ = __builtin__.super
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del __builtin__.super
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# We need these for modifying bytecode
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from opcode import opmap, HAVE_ARGUMENT, EXTENDED_ARG
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LOAD_GLOBAL = opmap['LOAD_GLOBAL']
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LOAD_NAME = opmap['LOAD_NAME']
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LOAD_CONST = opmap['LOAD_CONST']
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LOAD_FAST = opmap['LOAD_FAST']
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LOAD_ATTR = opmap['LOAD_ATTR']
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STORE_FAST = opmap['STORE_FAST']
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LOAD_DEREF = opmap['LOAD_DEREF']
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STORE_DEREF = opmap['STORE_DEREF']
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CALL_FUNCTION = opmap['CALL_FUNCTION']
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STORE_GLOBAL = opmap['STORE_GLOBAL']
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DUP_TOP = opmap['DUP_TOP']
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POP_TOP = opmap['POP_TOP']
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NOP = opmap['NOP']
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JUMP_FORWARD = opmap['JUMP_FORWARD']
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ABSOLUTE_TARGET = dis.hasjabs
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def _oparg(code, opcode_pos):
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return code[opcode_pos+1] + (code[opcode_pos+2] << 8)
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def _bind_autosuper(func, cls):
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co = func.func_code
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name = func.func_name
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newcode = array('B', co.co_code)
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codelen = len(newcode)
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newconsts = list(co.co_consts)
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newvarnames = list(co.co_varnames)
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# Check if the global 'super' keyword is already present
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try:
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sn_pos = list(co.co_names).index('super')
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except ValueError:
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sn_pos = None
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# Check if the varname 'super' keyword is already present
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try:
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sv_pos = newvarnames.index('super')
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except ValueError:
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sv_pos = None
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# Check if the callvar 'super' keyword is already present
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try:
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sc_pos = list(co.co_cellvars).index('super')
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except ValueError:
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sc_pos = None
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# If 'super' isn't used anywhere in the function, we don't have anything to do
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if sn_pos is None and sv_pos is None and sc_pos is None:
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return func
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c_pos = None
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s_pos = None
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n_pos = None
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# Check if the 'cls_name' and 'super' objects are already in the constants
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for pos, o in enumerate(newconsts):
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if o is cls:
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c_pos = pos
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if o is __super__:
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s_pos = pos
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if o == name:
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n_pos = pos
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# Add in any missing objects to constants and varnames
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if c_pos is None:
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c_pos = len(newconsts)
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newconsts.append(cls)
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if n_pos is None:
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n_pos = len(newconsts)
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newconsts.append(name)
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if s_pos is None:
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s_pos = len(newconsts)
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newconsts.append(__super__)
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if sv_pos is None:
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sv_pos = len(newvarnames)
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newvarnames.append('super')
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# This goes at the start of the function. It is:
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#
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# super = __super__(cls, self)
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#
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# If 'super' is a cell variable, we store to both the
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# local and cell variables (i.e. STORE_FAST and STORE_DEREF).
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#
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preamble = [
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LOAD_CONST, s_pos & 0xFF, s_pos >> 8,
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LOAD_CONST, c_pos & 0xFF, c_pos >> 8,
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LOAD_FAST, 0, 0,
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CALL_FUNCTION, 2, 0,
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]
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if sc_pos is None:
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# 'super' is not a cell variable - we can just use the local variable
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preamble += [
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STORE_FAST, sv_pos & 0xFF, sv_pos >> 8,
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]
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else:
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# If 'super' is a cell variable, we need to handle LOAD_DEREF.
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preamble += [
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DUP_TOP,
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STORE_FAST, sv_pos & 0xFF, sv_pos >> 8,
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STORE_DEREF, sc_pos & 0xFF, sc_pos >> 8,
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]
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preamble = array('B', preamble)
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# Bytecode for loading the local 'super' variable.
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load_super = array('B', [
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LOAD_FAST, sv_pos & 0xFF, sv_pos >> 8,
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])
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preamble_len = len(preamble)
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need_preamble = False
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i = 0
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while i < codelen:
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opcode = newcode[i]
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need_load = False
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remove_store = False
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if opcode == EXTENDED_ARG:
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raise TypeError("Cannot use 'super' in function with EXTENDED_ARG opcode")
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# If the opcode is an absolute target it needs to be adjusted
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# to take into account the preamble.
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elif opcode in ABSOLUTE_TARGET:
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oparg = _oparg(newcode, i) + preamble_len
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newcode[i+1] = oparg & 0xFF
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newcode[i+2] = oparg >> 8
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# If LOAD_GLOBAL(super) or LOAD_NAME(super) then we want to change it into
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# LOAD_FAST(super)
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elif (opcode == LOAD_GLOBAL or opcode == LOAD_NAME) and _oparg(newcode, i) == sn_pos:
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need_preamble = need_load = True
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# If LOAD_FAST(super) then we just need to add the preamble
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elif opcode == LOAD_FAST and _oparg(newcode, i) == sv_pos:
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need_preamble = need_load = True
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# If LOAD_DEREF(super) then we change it into LOAD_FAST(super) because
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# it's slightly faster.
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elif opcode == LOAD_DEREF and _oparg(newcode, i) == sc_pos:
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need_preamble = need_load = True
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if need_load:
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newcode[i:i+3] = load_super
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i += 1
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if opcode >= HAVE_ARGUMENT:
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i += 2
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# No changes needed - get out.
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if not need_preamble:
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return func
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# Our preamble will have 3 things on the stack
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co_stacksize = max(3, co.co_stacksize)
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# Conceptually, our preamble is on the `def` line.
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co_lnotab = array('B', co.co_lnotab)
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if co_lnotab:
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co_lnotab[0] += preamble_len
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co_lnotab = co_lnotab.tostring()
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# Our code consists of the preamble and the modified code.
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codestr = (preamble + newcode).tostring()
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codeobj = new.code(co.co_argcount, len(newvarnames), co_stacksize,
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co.co_flags, codestr, tuple(newconsts), co.co_names,
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tuple(newvarnames), co.co_filename, co.co_name,
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co.co_firstlineno, co_lnotab, co.co_freevars,
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co.co_cellvars)
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func.func_code = codeobj
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func.func_class = cls
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return func
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class autosuper_meta(type):
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def __init__(cls, name, bases, clsdict):
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UnboundMethodType = types.UnboundMethodType
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for v in vars(cls):
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o = getattr(cls, v)
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if isinstance(o, UnboundMethodType):
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_bind_autosuper(o.im_func, cls)
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class autosuper(object):
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__metaclass__ = autosuper_meta
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if __name__ == '__main__':
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class A(autosuper):
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def f(self):
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return 'A'
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class B(A):
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def f(self):
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return 'B' + super.f()
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class C(A):
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def f(self):
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def inner():
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return 'C' + super.f()
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# Needed to put 'super' into a cell
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super = super
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return inner()
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class D(B, C):
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def f(self, arg=None):
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var = None
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return 'D' + super.f()
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assert D().f() == 'DBCA'
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Disassembly of B.f and C.f reveals the different preambles used when ``super``
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is simply a local variable compared to when it is used by an inner function.
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::
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>>> dis.dis(B.f)
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214 0 LOAD_CONST 4 (<type 'super'>)
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3 LOAD_CONST 2 (<class '__main__.B'>)
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6 LOAD_FAST 0 (self)
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9 CALL_FUNCTION 2
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12 STORE_FAST 1 (super)
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215 15 LOAD_CONST 1 ('B')
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18 LOAD_FAST 1 (super)
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21 LOAD_ATTR 1 (f)
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24 CALL_FUNCTION 0
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27 BINARY_ADD
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28 RETURN_VALUE
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::
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>>> dis.dis(C.f)
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218 0 LOAD_CONST 4 (<type 'super'>)
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3 LOAD_CONST 2 (<class '__main__.C'>)
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6 LOAD_FAST 0 (self)
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9 CALL_FUNCTION 2
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12 DUP_TOP
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13 STORE_FAST 1 (super)
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16 STORE_DEREF 0 (super)
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219 19 LOAD_CLOSURE 0 (super)
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22 LOAD_CONST 1 (<code object inner at 00C160A0, file "autosuper.py", line 219>)
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25 MAKE_CLOSURE 0
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28 STORE_FAST 2 (inner)
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223 31 LOAD_FAST 1 (super)
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34 STORE_DEREF 0 (super)
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224 37 LOAD_FAST 2 (inner)
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40 CALL_FUNCTION 0
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43 RETURN_VALUE
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Note that in the final implementation, the preamble would not be part of the
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bytecode of the method, but would occur immediately following unpacking of
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parameters.
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||||
|
||||
In any case, this issue goes away entirely because classic calls to
|
||||
``super(<class>, <instance>)`` are still supported with the same meaning.
|
||||
|
||||
Alternative Proposals
|
||||
=====================
|
||||
|
|
@ -491,16 +144,6 @@ invokation of a super instance upon a class and instance.
|
|||
This proposal relies on sys._getframe(), which is not appropriate for anything
|
||||
except a prototype implementation.
|
||||
|
||||
|
||||
super(__this_class__, self)
|
||||
---------------------------
|
||||
|
||||
This is nearly an anti-proposal, as it basically relies on the acceptance of
|
||||
the __this_class__ PEP, which proposes a special name that would always be
|
||||
bound to the class within which it is used. If that is accepted, __this_class__
|
||||
could simply be used instead of the class' name explicitly, solving the name
|
||||
binding issues [2]_.
|
||||
|
||||
self.__super__.foo(\*args)
|
||||
--------------------------
|
||||
|
||||
|
|
@ -527,14 +170,6 @@ This variation actually eliminates the problems with locating the proper
|
|||
instance, and if any of the alternatives were pushed into the spotlight, I
|
||||
would want it to be this one.
|
||||
|
||||
super or super()
|
||||
----------------
|
||||
|
||||
This proposal leaves no room for different names, signatures, or application
|
||||
to other classes, or instances. A way to allow some similar use alongside the
|
||||
normal proposal would be favorable, encouraging good design of multiple
|
||||
inheritance trees and compatible methods.
|
||||
|
||||
super(\*p, \*\*kw)
|
||||
------------------
|
||||
|
||||
|
|
@ -561,6 +196,8 @@ this PEP on the principle of KISS (Keep It Simple Stupid).
|
|||
|
||||
History
|
||||
=======
|
||||
12-Mar-2009 - Updated to reflect the current state of implementation.
|
||||
|
||||
29-Apr-2007 - Changed title from "Super As A Keyword" to "New Super"
|
||||
- Updated much of the language and added a terminology section
|
||||
for clarification in confusing places.
|
||||
|
|
|
|||
Loading…
Reference in New Issue