python-peps/peps/pep-0367.rst

PEP: 367
Title: New Super
Version: $Revision$
Last-Modified: $Date$
Author: Calvin Spealman <ironfroggy@gmail.com>,
        Tim Delaney <timothy.c.delaney@gmail.com>
Status: Superseded
Type: Standards Track
Content-Type: text/x-rst
Created: 28-Apr-2007
Python-Version: 2.6
Post-History: `28-Apr-2007 <https://mail.python.org/pipermail/python-dev/2007-April/072807.html>`__,
              `29-Apr-2007 <https://mail.python.org/pipermail/python-dev/2007-April/072835.html>`__,
              `29-Apr-2007 <https://mail.python.org/pipermail/python-dev/2007-April/072858.html>`__,
              `14-May-2007 <https://mail.python.org/pipermail/python-dev/2007-May/073127.html>`__

Numbering Note
==============

This PEP has been renumbered to :pep:`3135`.  The text below is the last
version submitted under the old number.

Abstract
========

This PEP proposes syntactic sugar for use of the ``super`` type to automatically
construct instances of the super type binding to the class that a method was
defined in, and the instance (or class object for classmethods) that the method
is currently acting upon.

The premise of the new super usage suggested is as follows::

    super.foo(1, 2)

to replace the old::

    super(Foo, self).foo(1, 2)

and the current ``__builtin__.super`` be aliased to ``__builtin__.__super__``
(with ``__builtin__.super`` to be removed in Python 3.0).

It is further proposed that assignment to ``super`` become a ``SyntaxError``,
similar to the behaviour of ``None``.


Rationale
=========

The current usage of super requires an explicit passing of both the class and
instance it must operate from, requiring a breaking of the DRY (Don't Repeat
Yourself) rule. This hinders any change in class name, and is often considered
a wart by many.


Specification
=============

Within the specification section, some special terminology will be used to
distinguish similar and closely related concepts. "super type" will refer to
the actual builtin type named "super". A "super instance" is simply an instance
of the super type, which is associated with a class and possibly with an
instance of that class.

Because the new ``super`` semantics are not backwards compatible with Python
2.5, the new semantics will require a ``__future__`` import::

    from __future__ import new_super

The current ``__builtin__.super`` will be aliased to ``__builtin__.__super__``.
This will occur regardless of whether the new ``super`` semantics are active.
It is not possible to simply rename ``__builtin__.super``, as that would affect
modules that do not use the new ``super`` semantics. In Python 3.0 it is
proposed that the name ``__builtin__.super`` will be removed.

Replacing the old usage of super, calls to the next class in the MRO (method
resolution order) can be made without explicitly creating a ``super``
instance (although doing so will still be supported via ``__super__``). Every
function will have an implicit local named ``super``. This name behaves
identically to a normal local, including use by inner functions via a cell,
with the following exceptions:

1. Assigning to the name ``super`` will raise a ``SyntaxError`` at compile time;

2. Calling a static method or normal function that accesses the name ``super``
   will raise a ``TypeError`` at runtime.

Every function that uses the name ``super``, or has an inner function that
uses the name ``super``, will include a preamble that performs the equivalent
of::

    super = __builtin__.__super__(<class>, <instance>)

where ``<class>`` is the class that the method was defined in, and
``<instance>`` is the first parameter of the method (normally ``self`` for
instance methods, and ``cls`` for class methods). For static methods and normal
functions, ``<class>`` will be ``None``, resulting in a ``TypeError`` being
raised during the preamble.

Note: The relationship between ``super`` and ``__super__`` is similar to that
between ``import`` and ``__import__``.

Much of this was discussed in the thread of the python-dev list, "Fixing super
anyone?" [1]_.


Open Issues
-----------


Determining the class object to use
'''''''''''''''''''''''''''''''''''

The exact mechanism for associating the method with the defining class is not
specified in this PEP, and should be chosen for maximum performance. For
CPython, it is suggested that the class instance be held in a C-level variable
on the function object which is bound to one of ``NULL`` (not part of a class),
``Py_None`` (static method) or a class object (instance or class method).


Should ``super`` actually become a keyword?
'''''''''''''''''''''''''''''''''''''''''''

With this proposal, ``super`` would become a keyword to the same extent that
``None`` is a keyword. It is possible that further restricting the ``super``
name may simplify implementation, however some are against the actual
keyword-ization of super. The simplest solution is often the correct solution
and the simplest solution may well not be adding additional keywords to the
language when they are not needed. Still, it may solve other open issues.


Closed Issues
-------------

super used with __call__ attributes
'''''''''''''''''''''''''''''''''''

It was considered that it might be a problem that instantiating super instances
the classic way, because calling it would lookup the __call__ attribute and
thus try to perform an automatic super lookup to the next class in the MRO.
However, this was found to be false, because calling an object only looks up
the __call__ method directly on the object's type. The following example shows
this in action.

::

    class A(object):
        def __call__(self):
            return '__call__'
        def __getattribute__(self, attr):
            if attr == '__call__':
                return lambda: '__getattribute__'
    a = A()
    assert a() == '__call__'
    assert a.__call__() == '__getattribute__'

In any case, with the renaming of ``__builtin__.super`` to
``__builtin__.__super__`` this issue goes away entirely.


Reference Implementation
========================

It is impossible to implement the above specification entirely in Python. This
reference implementation has the following differences to the specification:

1. New ``super`` semantics are implemented using bytecode hacking.

2. Assignment to ``super`` is not a ``SyntaxError``. Also see point #4.

3. Classes must either use the metaclass ``autosuper_meta`` or inherit from
   the base class ``autosuper`` to acquire the new ``super`` semantics.

4. ``super`` is not an implicit local variable. In particular, for inner
   functions to be able to use the super instance, there must be an assignment
   of the form ``super = super`` in the method.

The reference implementation assumes that it is being run on Python 2.5+.

::

    #!/usr/bin/env python
    #
    # autosuper.py

    from array import array
    import dis
    import new
    import types
    import __builtin__
    __builtin__.__super__ = __builtin__.super
    del __builtin__.super

    # We need these for modifying bytecode
    from opcode import opmap, HAVE_ARGUMENT, EXTENDED_ARG

    LOAD_GLOBAL = opmap['LOAD_GLOBAL']
    LOAD_NAME = opmap['LOAD_NAME']
    LOAD_CONST = opmap['LOAD_CONST']
    LOAD_FAST = opmap['LOAD_FAST']
    LOAD_ATTR = opmap['LOAD_ATTR']
    STORE_FAST = opmap['STORE_FAST']
    LOAD_DEREF = opmap['LOAD_DEREF']
    STORE_DEREF = opmap['STORE_DEREF']
    CALL_FUNCTION = opmap['CALL_FUNCTION']
    STORE_GLOBAL = opmap['STORE_GLOBAL']
    DUP_TOP = opmap['DUP_TOP']
    POP_TOP = opmap['POP_TOP']
    NOP = opmap['NOP']
    JUMP_FORWARD = opmap['JUMP_FORWARD']
    ABSOLUTE_TARGET = dis.hasjabs

    def _oparg(code, opcode_pos):
        return code[opcode_pos+1] + (code[opcode_pos+2] << 8)

    def _bind_autosuper(func, cls):
        co = func.func_code
        name = func.func_name
        newcode = array('B', co.co_code)
        codelen = len(newcode)
        newconsts = list(co.co_consts)
        newvarnames = list(co.co_varnames)

        # Check if the global 'super' keyword is already present
        try:
            sn_pos = list(co.co_names).index('super')
        except ValueError:
            sn_pos = None

        # Check if the varname 'super' keyword is already present
        try:
            sv_pos = newvarnames.index('super')
        except ValueError:
            sv_pos = None

        # Check if the cellvar 'super' keyword is already present
        try:
            sc_pos = list(co.co_cellvars).index('super')
        except ValueError:
            sc_pos = None

        # If 'super' isn't used anywhere in the function, we don't have anything to do
        if sn_pos is None and sv_pos is None and sc_pos is None:
            return func

        c_pos = None
        s_pos = None
        n_pos = None

        # Check if the 'cls_name' and 'super' objects are already in the constants
        for pos, o in enumerate(newconsts):
            if o is cls:
                c_pos = pos

            if o is __super__:
                s_pos = pos

            if o == name:
                n_pos = pos

        # Add in any missing objects to constants and varnames
        if c_pos is None:
            c_pos = len(newconsts)
            newconsts.append(cls)

        if n_pos is None:
            n_pos = len(newconsts)
            newconsts.append(name)

        if s_pos is None:
            s_pos = len(newconsts)
            newconsts.append(__super__)

        if sv_pos is None:
            sv_pos = len(newvarnames)
            newvarnames.append('super')

        # This goes at the start of the function. It is:
        #
        #   super = __super__(cls, self)
        #
        # If 'super' is a cell variable, we store to both the
        # local and cell variables (i.e. STORE_FAST and STORE_DEREF).
        #
        preamble = [
            LOAD_CONST, s_pos & 0xFF, s_pos >> 8,
            LOAD_CONST, c_pos & 0xFF, c_pos >> 8,
            LOAD_FAST, 0, 0,
            CALL_FUNCTION, 2, 0,
        ]

        if sc_pos is None:
            # 'super' is not a cell variable - we can just use the local variable
            preamble += [
                STORE_FAST, sv_pos & 0xFF, sv_pos >> 8,
            ]
        else:
            # If 'super' is a cell variable, we need to handle LOAD_DEREF.
            preamble += [
                DUP_TOP,
                STORE_FAST, sv_pos & 0xFF, sv_pos >> 8,
                STORE_DEREF, sc_pos & 0xFF, sc_pos >> 8,
            ]

        preamble = array('B', preamble)

        # Bytecode for loading the local 'super' variable.
        load_super = array('B', [
            LOAD_FAST, sv_pos & 0xFF, sv_pos >> 8,
        ])

        preamble_len = len(preamble)
        need_preamble = False
        i = 0

        while i < codelen:
            opcode = newcode[i]
            need_load = False
            remove_store = False

            if opcode == EXTENDED_ARG:
                raise TypeError("Cannot use 'super' in function with EXTENDED_ARG opcode")

            # If the opcode is an absolute target it needs to be adjusted
            # to take into account the preamble.
            elif opcode in ABSOLUTE_TARGET:
                oparg = _oparg(newcode, i) + preamble_len
                newcode[i+1] = oparg & 0xFF
                newcode[i+2] = oparg >> 8

            # If LOAD_GLOBAL(super) or LOAD_NAME(super) then we want to change it into
            # LOAD_FAST(super)
            elif (opcode == LOAD_GLOBAL or opcode == LOAD_NAME) and _oparg(newcode, i) == sn_pos:
                need_preamble = need_load = True

            # If LOAD_FAST(super) then we just need to add the preamble
            elif opcode == LOAD_FAST and _oparg(newcode, i) == sv_pos:
                need_preamble = need_load = True

            # If LOAD_DEREF(super) then we change it into LOAD_FAST(super) because
            # it's slightly faster.
            elif opcode == LOAD_DEREF and _oparg(newcode, i) == sc_pos:
                need_preamble = need_load = True

            if need_load:
                newcode[i:i+3] = load_super

            i += 1

            if opcode >= HAVE_ARGUMENT:
                i += 2

        # No changes needed - get out.
        if not need_preamble:
            return func

        # Our preamble will have 3 things on the stack
        co_stacksize = max(3, co.co_stacksize)

        # Conceptually, our preamble is on the `def` line.
        co_lnotab = array('B', co.co_lnotab)

        if co_lnotab:
            co_lnotab[0] += preamble_len

        co_lnotab = co_lnotab.tostring()

        # Our code consists of the preamble and the modified code.
        codestr = (preamble + newcode).tostring()

        codeobj = new.code(co.co_argcount, len(newvarnames), co_stacksize,
                           co.co_flags, codestr, tuple(newconsts), co.co_names,
                           tuple(newvarnames), co.co_filename, co.co_name,
                           co.co_firstlineno, co_lnotab, co.co_freevars,
                           co.co_cellvars)

        func.func_code = codeobj
        func.func_class = cls
        return func

    class autosuper_meta(type):
        def __init__(cls, name, bases, clsdict):
            UnboundMethodType = types.UnboundMethodType

            for v in vars(cls):
                o = getattr(cls, v)
                if isinstance(o, UnboundMethodType):
                    _bind_autosuper(o.im_func, cls)

    class autosuper(object):
        __metaclass__ = autosuper_meta

    if __name__ == '__main__':
        class A(autosuper):
            def f(self):
                return 'A'

        class B(A):
            def f(self):
                return 'B' + super.f()

        class C(A):
            def f(self):
                def inner():
                    return 'C' + super.f()

                # Needed to put 'super' into a cell
                super = super
                return inner()

        class D(B, C):
            def f(self, arg=None):
                var = None
                return 'D' + super.f()

        assert D().f() == 'DBCA'

Disassembly of B.f and C.f reveals the different preambles used when ``super``
is simply a local variable compared to when it is used by an inner function.

::

    >>> dis.dis(B.f)

    214           0 LOAD_CONST               4 (<type 'super'>)
                  3 LOAD_CONST               2 (<class '__main__.B'>)
                  6 LOAD_FAST                0 (self)
                  9 CALL_FUNCTION            2
                 12 STORE_FAST               1 (super)

    215          15 LOAD_CONST               1 ('B')
                 18 LOAD_FAST                1 (super)
                 21 LOAD_ATTR                1 (f)
                 24 CALL_FUNCTION            0
                 27 BINARY_ADD
                 28 RETURN_VALUE

::

    >>> dis.dis(C.f)

    218           0 LOAD_CONST               4 (<type 'super'>)
                  3 LOAD_CONST               2 (<class '__main__.C'>)
                  6 LOAD_FAST                0 (self)
                  9 CALL_FUNCTION            2
                 12 DUP_TOP
                 13 STORE_FAST               1 (super)
                 16 STORE_DEREF              0 (super)

    219          19 LOAD_CLOSURE             0 (super)
                 22 LOAD_CONST               1 (<code object inner at 00C160A0, file "autosuper.py", line 219>)
                 25 MAKE_CLOSURE             0
                 28 STORE_FAST               2 (inner)

    223          31 LOAD_FAST                1 (super)
                 34 STORE_DEREF              0 (super)

    224          37 LOAD_FAST                2 (inner)
                 40 CALL_FUNCTION            0
                 43 RETURN_VALUE

Note that in the final implementation, the preamble would not be part of the
bytecode of the method, but would occur immediately following unpacking of
parameters.


Alternative Proposals
=====================

No Changes
----------

Although its always attractive to just keep things how they are, people have
sought a change in the usage of super calling for some time, and for good
reason, all mentioned previously.

- Decoupling from the class name (which might not even be bound to the
  right class anymore!)
- Simpler looking, cleaner super calls would be better

Dynamic attribute on super type
-------------------------------

The proposal adds a dynamic attribute lookup to the super type, which will
automatically determine the proper class and instance parameters. Each super
attribute lookup identifies these parameters and performs the super lookup on
the instance, as the current super implementation does with the explicit
invocation 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)
--------------------------

The __super__ attribute is mentioned in this PEP in several places, and could
be a candidate for the complete solution, actually using it explicitly instead
of any super usage directly. However, double-underscore names are usually an
internal detail, and attempted to be kept out of everyday code.

super(self, \*args) or __super__(self, \*args)
----------------------------------------------

This solution only solves the problem of the type indication, does not handle
differently named super methods, and is explicit about the name of the
instance. It is less flexible without being able to enacted on other method
names, in cases where that is needed. One use case this fails is where a
base-class has a factory classmethod and a subclass has two factory
classmethods, both of which needing to properly make super calls to the one
in the base-class.

super.foo(self, \*args)
-----------------------

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)
------------------

There has been the proposal that directly calling ``super(*p, **kw)`` would
be equivalent to calling the method on the ``super`` object with the same name
as the method currently being executed i.e. the following two methods would be
equivalent:

::

    def f(self, *p, **kw):
        super.f(*p, **kw)

::

    def f(self, *p, **kw):
        super(*p, **kw)

There is strong sentiment for and against this, but implementation and style
concerns are obvious. Guido has suggested that this should be excluded from
this PEP on the principle of KISS (Keep It Simple Stupid).



History
=======
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.
            - Added reference implementation and history sections.

06-May-2007 - Updated by Tim Delaney to reflect discussions on the python-3000
              and python-dev mailing lists.

References
==========

.. [1] Fixing super anyone?
   (https://mail.python.org/pipermail/python-3000/2007-April/006667.html)

.. [2] PEP 3130: Access to Module/Class/Function Currently Being Defined (this)
   (https://mail.python.org/pipermail/python-ideas/2007-April/000542.html)


Copyright
=========

This document has been placed in the public domain.



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