Source

python-peps / pep-0367.txt

Full commit
  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
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, 29-Apr-2007 (1), 29-Apr-2007 (2), 14-May-2007

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 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
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?
   (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.



..
   Local Variables:
   mode: indented-text
   indent-tabs-mode: nil
   sentence-end-double-space: t
   fill-column: 70
   coding: utf-8
   End: