[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

I'm currently planning to postpone fixing this until 3.4. However, if someone 
else wants to pick it up for 3.3, go ahead.

--
assignee: ncoghlan -> 
versions: +Python 3.4 -Python 3.3

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

Heh, rereading the issue comments, I noticed that my latest idea is quite 
similar to what Terry suggested last year, just using delegation to adjust the 
signatures appropriately rather than copying the function pointers directly 
over.

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

And, back on topic...

I've been pondering this problem and the approach I adopted in my branch and 
decided it's the *wrong* way to go about it. It takes an already complex piece 
of code and makes it even more complicated.

A completely different approach that I'm considering is to instead make types 
defined in C behave more like their counterparts defined in Python. The reason 
sequences implemented in Python don't have this problem is because their nb_add 
and nb_mul slots get filled in with functions that call up into the Python 
__[ri]add__ and __[ri]mul__ implementations.

So my thought is that, when the type construction machinery is filling in the 
type slots, we could actually do something similar at the C level: define a 
standard _PySequenceAsNumber variable and add a pointer to that in to the 
tp_as_number slot. The nb_add and nb_mul slots in this structure would 
reference functions that delegated the relevant operations to sq_concat and 
sq_repeat.

I haven't actually tried this approach, so there may be practical issues with 
it that haven't occurred to me as yet, but it's definitely appealing as it has 
the potential to *simplify* the dispatch code in abstract.c instead of making 
it even more complicated.

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[issue11477] Bug in code dispatching based on internal slots

[Eric Snow]

Changes by Eric Snow :


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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Added file: http://bugs.python.org/file21386/8bd713a823b5.diff

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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Removed file: http://bugs.python.org/file21384/8bd713a823b5.diff

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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Added file: http://bugs.python.org/file21384/8bd713a823b5.diff

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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Removed file: http://bugs.python.org/file21383/8bd713a823b5.diff

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

Martin and I are still experimenting with this issue as a test case for the 
remote repository diff calculator, so apologies for the noise as we add and 
remove patch files.

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Changes by Nick Coghlan :


Added file: http://bugs.python.org/file21383/8bd713a823b5.diff

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[issue11477] Bug in code dispatching based on internal slots

[Raymond Hettinger]

Changes by Raymond Hettinger :


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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Added file: http://bugs.python.org/file21368/650549138a3d.diff

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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Removed file: http://bugs.python.org/file21367/650549138a3d.diff

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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Removed file: http://bugs.python.org/file21258/f1bd5468dae6.diff

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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Added file: http://bugs.python.org/file21367/650549138a3d.diff

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[issue11477] Bug in code dispatching based on internal slots

[Alex Gaynor]

Changes by Alex Gaynor :


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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

Well, there and in the named diff file on here, of course.

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

The draft code is now only on the "respect_LHS_precedence" branch of my sandbox 
repository.

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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Added file: http://bugs.python.org/file21258/f1bd5468dae6.diff

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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Removed file: http://bugs.python.org/file21257/f1bd5468dae6.diff

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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Added file: http://bugs.python.org/file21257/f1bd5468dae6.diff

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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Removed file: http://bugs.python.org/file21256/f1bd5468dae6.diff

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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Added file: http://bugs.python.org/file21256/f1bd5468dae6.diff

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[issue11477] Bug in code dispatching based on internal slots

[Martin v . Löwis]

Changes by Martin v. Löwis :


Removed file: http://bugs.python.org/file21217/b9b7d4c10bc4.diff

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[issue11477] Bug in code dispatching based on internal slots

[Armin Rigo]

Changes by Armin Rigo :


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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

We know, I left it there to help MvL debug the issues in the diff generator.

The version I created with the rdiff extension is correct though, and Martin 
fixed the Reitveld integration to handle the extra lines at the start of the 
diff.

I wouldn't actually commit the change as it stands (if nothing else, 
PyErr_Matches() calls are needed in the unicode methods), but it gives a clear 
idea of the magnitude of the changes needed.

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[issue11477] Bug in code dispatching based on internal slots

[Terry J. Reedy]

Terry J. Reedy  added the comment:

b9b7d4c10bc4.diff is a huge compilation of all commits from the last few days, 
with the abstract.c diff buried about 3/4ths of the way through.

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

MvL says the review creation script should be able to cope with the rdiff 
output within the next day or so.

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[issue11477] Bug in code dispatching based on internal slots

[Benjamin Peterson]

Benjamin Peterson  added the comment:

Is that a patch you can put on Rietveld then? (/me wanting to review)

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

I think the roundup/Hg integration may be getting confused by the merges of the 
cpython main line of development with my sandbox.

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Changes by Nick Coghlan :


Removed file: http://bugs.python.org/file21220/2f5db44c98f2.diff

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Changes by Nick Coghlan :


Added file: http://bugs.python.org/file21220/2f5db44c98f2.diff

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

I generated a patch between my sandbox and the main repository using the rdiff 
extension immediately after syncing with the main line of development. ("hg 
diff --reverse cpython" where cpython is aliased to the main repository)

This is the output I was hoping to get from the automated branch checking.

--
Added file: http://bugs.python.org/file21219/issue11477_LHS_prededence.diff

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Changes by Nick Coghlan :


Added file: http://bugs.python.org/file21217/b9b7d4c10bc4.diff

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Changes by Nick Coghlan :


Removed file: http://bugs.python.org/file21216/85d7c99fd31e.diff

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Changes by Nick Coghlan :


--
keywords: +patch
Added file: http://bugs.python.org/file21216/85d7c99fd31e.diff

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

Trying out the hg integration MvL added to Roundup.

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

My work in progress is on the respect_LHS_precedence branch in 
http://hg.python.org/sandbox/ncoghlan

Current status is that I have tests for correct handling of sq_concat and 
sq_repeat, and am close to having sq_concat and sq_inplace_concat behaving 
correctly.

I'm not happy with the current duplication of checks in abstract.c, but I'll 
look for ways to make the code prettier after it is working properly.

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

Armin: yeah, I learned better in the course of trying to fix this misbehaviour 
in CPython. I've adjusted assorted sq_concat methods to return NotImplemented 
in the sandbox where I'm working on this, along with modifying abstract.c to 
correctly cope with that.

Terry: the slot signatures vary, so copying function pointers around isn't 
really viable. I'm just fixing abstract.c to call the slots in the right order.

The fun part is that historically, CPython didn't check for NotImplemented 
returns from sq_concat and sq_repeat, so those methods are all written to raise 
TypeError explicitly, which breaks delegation to __radd__ methods (i.e. exactly 
the same thing Armin fixed in PyPy).

As far as 2.7 and 3.2 go, I'm thinking a Py3k warning in the next 2.7 release 
and a CompatibilityWarning (once we have it) in the next 3.2 will be a 
possibility. However, I want to finish the patch and see the magnitude of the 
change before deciding what we do with the maintenance versions.

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[issue11477] Bug in code dispatching based on internal slots

[Terry J. Reedy]

Terry J. Reedy  added the comment:

I think Nick's point, and one I agree with, is (or amounts to):
  'somelist += ob' == 'somelist.__iadd__(ob)' ==
  'somelist.extend(ob)' == 'somelist[len(somelist):len(somelist)]=ob'
is defined and should be implemented for all somelist,ob pairs. If ob is an 
iterable, add the items at the end; if not, raise TypeError.

CPython currently has a bug that breaks the middle equality in a peculiar case. 
We recognize that and hope to fix it.

The proper, future-proof fix for Greg Price & Co. is for them to 
  1. use somelist.append(ob) to append a single object, iterable or not, to 
somelist. If they insist on (mis)using += for appending, 
  2. add the trivial __iter__ yielding just the instance to all classes whose 
instances are ever a target of 'somelist += instance'. Or
  3. wrap each non-iterable instance in an iterable, such as a tuple:
 "somelist += (non-iterable,).
Any of these (1. actually) should have been done in the first place, as has 
always been documented.

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[issue11477] Bug in code dispatching based on internal slots

[Armin Rigo]

Armin Rigo  added the comment:

Nick: we get a TypeError anyway if we do unsupported things like "lst += None". 
 It seems to me that you are confusing levels, unless you can point out a 
specific place in the documentation that would say "never return NotImplemented 
from __iadd__(), __imul__(), etc."

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[issue11477] Bug in code dispatching based on internal slots

[Terry J. Reedy]

Terry J. Reedy  added the comment:

And if num-add is present and seq-add not, copy the other way, even if it were 
recommended to only use the former.

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[issue11477] Bug in code dispatching based on internal slots

[Terry J. Reedy]

Terry J. Reedy  added the comment:

It seems to me that the underlying (design) flaw is having duplicate slots in 
the C type structure*.  I presume that having two different functions in 
num-add and seq-add (concat) (I know, not quite the proper names), etc, is an 
error. I also assume that changing the structure is out, whether frozen in the 
ABI or not, as disabling every extention type.

But could we change how the slots are handled? For instance, when class is 
created, if nun-add is absent and seq-add is present, copy seq-add to num-add 
and thereafter only use num-add and treat seq-add as a dummy left for back 
compatibility.  In other words, merge the duplicate slots in their effect, so 
there is a proper 1-1 relationship between syntax operators and methods, as 
there is for Python-coded classes.

*I am guessing that this was for convenience -- making a number? fill in num 
slots; making a sequence? fill in seq slots. Or perhaps Guido once had some 
idea of possibly separating the operators/functions at the Python level. Does 
not matter at present.

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Changes by Nick Coghlan :


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assignee:  -> ncoghlan

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[issue11477] Bug in code dispatching based on internal slots

[Nick Coghlan]

Nick Coghlan  added the comment:

Armin, I'm not sure returning NotImplemented from __iadd__ is a good idea in 
this case. It means "+=" on a mutable object may silently fail to mutate 
in-place - enabling that seems rather questionable.

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[issue11477] Bug in code dispatching based on internal slots

[Carl Friedrich Bolz]

Changes by Carl Friedrich Bolz :


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[issue11477] Bug in code dispatching based on internal slots

[Meador Inge]

Changes by Meador Inge :


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[issue11477] Bug in code dispatching based on internal slots

[Armin Rigo]

Armin Rigo  added the comment:

Note that I "fixed" one case in PyPy: if the class C has no __iter__() but only 
__radd__(), and we call "somelist += C()".  This was done simply by having 
somelist.__iadd__(x) return NotImplemented in case x is not iterable, instead 
of propagating the TypeError.

This fix doesn't change the outcome in the case reported here: if there are two 
possible ways to get a valid answer, then PyPy will systematically prefer the 
way implemented by the LHS method over the way implemented by the RHS method, 
whereas CPython might not.

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[issue11477] Bug in code dispatching based on internal slots

[Daniel Urban]

Changes by Daniel Urban :


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[issue11477] Bug in code dispatching based on internal slots

[Terry J. Reedy]

Terry J. Reedy  added the comment:

Second link to pydev should be
http://mail.python.org/pipermail/python-dev/2011-March/109130.html

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[issue11477] Bug in code dispatching based on internal slots

[Éric Araujo]

Changes by Éric Araujo :


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[issue11477] Bug in code dispatching based on internal slots

[Andreas Stührk]

Changes by Andreas Stührk :


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[issue11477] Bug in code dispatching based on internal slots

[Terry J. Reedy]

New submission from Terry J. Reedy :

Example (which can serve as testcase with buggy output corrected).

class C(object):
def __iter__(self):
yield 'yes!'
def __radd__(self, other):
other.append('bug!')
return other
def __rmul__(self, other):
other *= 2
return other
def __index__(self):
  return 3

class L(list):
def __iadd__(self, other):
list.__iadd__(self,other)
return self
def __mul__(self, other):
return L(list.__imul__(self,other))

z1, z2, c = [], L(), C()
z1 += c
z2 += c
print(z1, z2, [1]*c, L([1])*c)
>>> 
['bug!'] ['yes!'] [1, 1] [1, 1, 1] # PyPy prints ['yes!'], [1, 1, 1]

Cause was diagnosed by Greg Price in
http://codespeak.net/pipermail/pypy-dev/2011q1/006958.html
as checking forward and reverse numeric slots before checking sequence slots 
for C-coded classes. Such a difference does not exist in Python itself.

In "About raising NotPortableWarning for CPython specific code"
pydev thread starting at
http://codespeak.net/pipermail/pypy-dev/2011q1/006958.html

Nick Coghlin identified the fix as "When a given operation has multiple C level 
slots, shouldn't we be checking all the LHS slots before checking any of the 
RHS slots?"

Guido replied "Yeah, indeed, on everything you said. The code dispatching based 
on internal slots is horribly ad-hoc and likely wrong in subtle ways."

I personally think fix should be backported to 2.7 and 3.2, but did not select 
them because that may be controversial.

--
components: Interpreter Core
messages: 130698
nosy: terry.reedy
priority: normal
severity: normal
stage: needs patch
status: open
title: Bug in code dispatching based on internal slots
type: behavior
versions: Python 3.3

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