[Python-Dev] PEP 367: New Super
Tim Delaney
tcdelaney at optusnet.com.au
Mon May 14 09:23:52 CEST 2007
Here is my modified version of PEP 367. The reference implementation in it
is pretty long, and should probably be split out to somewhere else (esp.
since it can't fully implement the semantics).
Cheers,
Tim Delaney
PEP: 367
Title: New Super
Version: $Revision$
Last-Modified: $Date$
Author: Calvin Spealman <ironfroggy at gmail.com>
Author: Tim Delaney <timothy.c.delaney at gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 28-Apr-2007
Python-Version: 2.6
Post-History: 28-Apr-2007, 29-Apr-2007 (1), 29-Apr-2007 (2), 14-May-2007
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 callvar '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
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)
--------------------------
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 flexable 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
inheritence 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?
(http://mail.python.org/pipermail/python-3000/2007-April/006667.html)
.. [2] PEP 3130: Access to Module/Class/Function Currently Being Defined
(this)
(http://mail.python.org/pipermail/python-ideas/2007-April/000542.html)
Copyright
=========
This document has been placed in the public domain.
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