Re: [Python-checkins] cpython (3.2): don't use a slot wrapper from a different special method (closes #14658)

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Re: [Python-checkins] cpython (3.2): don't use a slot wrapper from a different special method (closes #14658)

Mark Shannon-3
I'm not happy with this fix.

Admittedly code like:

class S(str):
    __getattr__ = str.__add__
s = S('a')
print(S.b)

is a little weird.
But I think it should work (ie print 'ab') properly.

This works without the patch.

class S(str):
    __getattribute__ = str.__add__
s = S('a')
print(S.b)

(Prints 'ab')

Also "slot wrapper" is a low-level implementation detail and
shouldn't impact the language semantics.

dict.__getitem__ is a slot wrapper; dict.__getitem__ is not.
str.__getitem__ is a slot wrapper; list.__getitem__ is not.
If any of these change then the semantics of the language changes.

Cheers,
Mark

benjamin.peterson wrote:

> http://hg.python.org/cpython/rev/971865f12377
> changeset:   76518:971865f12377
> branch:      3.2
> parent:      76506:f7b002e5cac7
> user:        Benjamin Peterson <[hidden email]>
> date:        Tue Apr 24 11:06:25 2012 -0400
> summary:
>   don't use a slot wrapper from a different special method (closes #14658)
>
> This also alters the fix to #11603. Specifically, setting __repr__ to
> object.__str__ now raises a recursion RuntimeError when str() or repr() is
> called instead of silently bypassing the recursion. I believe this behavior is
> more correct.
>
> files:
>   Lib/test/test_descr.py |  10 +++++++++-
>   Misc/NEWS              |   6 ++++++
>   Objects/typeobject.c   |   5 +++--
>   3 files changed, 18 insertions(+), 3 deletions(-)
>
>
> diff --git a/Lib/test/test_descr.py b/Lib/test/test_descr.py
> --- a/Lib/test/test_descr.py
> +++ b/Lib/test/test_descr.py
> @@ -4430,7 +4430,15 @@
>              pass
>          Foo.__repr__ = Foo.__str__
>          foo = Foo()
> -        str(foo)
> +        self.assertRaises(RuntimeError, str, foo)
> +        self.assertRaises(RuntimeError, repr, foo)
> +
> +    def test_mixing_slot_wrappers(self):
> +        class X(dict):
> +            __setattr__ = dict.__setitem__
> +        x = X()
> +        x.y = 42
> +        self.assertEqual(x["y"], 42)
>  
>      def test_cycle_through_dict(self):
>          # See bug #1469629
> diff --git a/Misc/NEWS b/Misc/NEWS
> --- a/Misc/NEWS
> +++ b/Misc/NEWS
> @@ -10,6 +10,12 @@
>  Core and Builtins
>  -----------------
>  
> +- Issue #11603 (again): Setting __repr__ to __str__ now raises a RuntimeError
> +  when repr() or str() is called on such an object.
> +
> +- Issue #14658: Fix binding a special method to a builtin implementation of a
> +  special method with a different name.
> +
>  - Issue #14630: Fix a memory access bug for instances of a subclass of int
>    with value 0.
>  
> diff --git a/Objects/typeobject.c b/Objects/typeobject.c
> --- a/Objects/typeobject.c
> +++ b/Objects/typeobject.c
> @@ -2928,7 +2928,7 @@
>      unaryfunc f;
>  
>      f = Py_TYPE(self)->tp_repr;
> -    if (f == NULL || f == object_str)
> +    if (f == NULL)
>          f = object_repr;
>      return f(self);
>  }
> @@ -5757,7 +5757,8 @@
>              }
>              continue;
>          }
> -        if (Py_TYPE(descr) == &PyWrapperDescr_Type) {
> +        if (Py_TYPE(descr) == &PyWrapperDescr_Type &&
> +            ((PyWrapperDescrObject *)descr)->d_base->name_strobj == p->name_strobj) {
>              void **tptr = resolve_slotdups(type, p->name_strobj);
>              if (tptr == NULL || tptr == ptr)
>                  generic = p->function;
>
>
>
> ------------------------------------------------------------------------
>
> _______________________________________________
> Python-checkins mailing list
> [hidden email]
> http://mail.python.org/mailman/listinfo/python-checkins

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Re: [Python-checkins] cpython (3.2): don't use a slot wrapper from a different special method (closes #14658)

Benjamin Peterson-3
2012/4/24 Mark Shannon <[hidden email]>:
> I'm not happy with this fix.

It's not perfect, but it's an improvement.

>
> Admittedly code like:
>
> class S(str):
>   __getattr__ = str.__add__
> s = S('a')
> print(S.b)
>
> is a little weird.
> But I think it should work (ie print 'ab') properly.
>
> This works without the patch.
>
> class S(str):
>   __getattribute__ = str.__add__
> s = S('a')
> print(S.b)

Does it?

$ cat > x.py
class S(str):
  __getattribute__ = str.__add__
s = S('a')
print(S.b)
$ python3 x.py
Traceback (most recent call last):
  File "x.py", line 4, in <module>
    print(S.b)
AttributeError: type object 'S' has no attribute 'b'

>
> (Prints 'ab')
>
> Also "slot wrapper" is a low-level implementation detail and
> shouldn't impact the language semantics.
>
> dict.__getitem__ is a slot wrapper; dict.__getitem__ is not.
> str.__getitem__ is a slot wrapper; list.__getitem__ is not.
> If any of these change then the semantics of the language changes.



--
Regards,
Benjamin
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Re: [Python-checkins] cpython (3.2): don't use a slot wrapper from a different special method (closes #14658)

Mark Shannon-3
Benjamin Peterson wrote:

> 2012/4/24 Mark Shannon <[hidden email]>:
>> I'm not happy with this fix.
>
> It's not perfect, but it's an improvement.
>
>> Admittedly code like:
>>
>> class S(str):
>>   __getattr__ = str.__add__
>> s = S('a')
>> print(S.b)

My typo, should be:
print(s.b)
(Instance not class)
This doesn't work.

>>
>> is a little weird.
>> But I think it should work (ie print 'ab') properly.
>>
>> This works without the patch.
>>
>> class S(str):
>>   __getattribute__ = str.__add__
>> s = S('a')
>> print(S.b)

Same typo,
this does work (with correct spelling :) )
>
> Does it?
>
> $ cat > x.py
> class S(str):
>   __getattribute__ = str.__add__
> s = S('a')
> print(S.b)

> $ python3 x.py
> Traceback (most recent call last):
>   File "x.py", line 4, in <module>
>     print(S.b)
> AttributeError: type object 'S' has no attribute 'b'
>
>> (Prints 'ab')
>>
>> Also "slot wrapper" is a low-level implementation detail and
>> shouldn't impact the language semantics.
>>
>> dict.__getitem__ is a slot wrapper; dict.__getitem__ is not.
>> str.__getitem__ is a slot wrapper; list.__getitem__ is not.
>> If any of these change then the semantics of the language changes.
>
>
>

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Re: [Python-checkins] cpython (3.2): don't use a slot wrapper from a different special method (closes #14658)

Ethan Furman-2
Mark Shannon wrote:

> Benjamin Peterson wrote:
>> 2012/4/24 Mark Shannon <[hidden email]>:
>>> I'm not happy with this fix.
>>
>> It's not perfect, but it's an improvement.
>>
>>> Admittedly code like:
>>>
>>> class S(str):
>>>   __getattr__ = str.__add__
>>> s = S('a')
>>> print(S.b)
>
> My typo, should be:
> print(s.b)
> (Instance not class)
>
>>> is a little weird.
>>> But I think it should work (ie print 'ab') properly.

I can easily believe I'm missing something, but here are the results
with the patch in place:

{'x': 42} 42
{'x': 42} 42
ab

and here's the code:

class Foo1(dict):
     def __getattr__(self, key): return self[key]
     def __setattr__(self, key, value): self[key] = value

class Foo2(dict):
     __getattr__ = dict.__getitem__
     __setattr__ = dict.__setitem__

o1 = Foo1()
o1.x = 42
print(o1, o1.x)

o2 = Foo2()
o2.x = 42
print(o2, o2.x)

class S(str):
    __getattr__ = str.__add__
s = S('a')
print(s.b)

~Ethan~
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Re: [Python-checkins] cpython (3.2): don't use a slot wrapper from a different special method (closes #14658)

Mark Shannon-3
In reply to this post by Benjamin Peterson-3
Benjamin Peterson wrote:
> 2012/4/24 Mark Shannon <[hidden email]>:
>> I'm not happy with this fix.
>
> It's not perfect, but it's an improvement.
>
Actually, I think it is probably correct.
I've been trying to break it by assigning various unusual
objects to special attributes and it seems OK so far.

I don't really trust all that slot-wrapper stuff,
but rewriting is a lot of work and would introduce new errors,
so I'll just leave it at that.

[snip]

Cheers,
Mark.



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