a type without a __mro__?

[Alex Martelli]

Can anybody suggest where to find (within the standard library) or how
to easily make (e.g. in a C extension) a type without a __mro__, except
for those (such as types.InstanceT ype) which are explicitly recorded in
the dispatch table copy._deepcopy_ dispatch...?

Weird request, I know, so let me explain. There's a bug in the
forthcoming 2.3.5's copy.py which can be tickled only by such a type.

Fixing the bug is trivially easy (I'll just use inspect.getmro( cls)
instead of cls.__mro__), but I also want to add to test_copy.py a unit
test that tickles the bug, rather than just commit an ``untested fix''.

I'm stumped at finding a type that's suitable for reproducing the bug;
I've asked the submitter of the original bug report (which comes up with
Zope and some specific application -- unsuitable for a core python unit
test, sigh), I've asked on python-dev, I've even IM'd a couple of
Python-guru friends, but the friends are equally stumped and I'm getting
no answers on python-dev nor from the submitter of the bug report. So,
I thought I'd turn to the collective ingenuity of comp.lang.pytho n...
thanks in advance for any help!


Alex

[Fredrik Lundh]

Re: a type without a __mro__?

Alex Martelli wrote:
[color=blue]
> Can anybody suggest where to find (within the standard library) or how
> to easily make (e.g. in a C extension) a type without a __mro__, except
> for those (such as types.InstanceT ype) which are explicitly recorded in
> the dispatch table copy._deepcopy_ dispatch...?[/color]

something like this?
[color=blue][color=green][color=darkred]
>>> import re
>>> x = re.compile("")
>>> x[/color][/color][/color]
<_sre.SRE_Patte rn object at 0x00B2F7A0>[color=blue][color=green][color=darkred]
>>> x.__mro__[/color][/color][/color]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
AttributeError: __mro__[color=blue][color=green][color=darkred]
>>> import copy
>>> type(x) in copy._deepcopy_ dispatch[/color][/color][/color]
False

</F>

[Nick Coghlan]

Re: a type without a __mro__?

Fredrik Lundh wrote:[color=blue]
> Alex Martelli wrote:
>
>[color=green]
>>Can anybody suggest where to find (within the standard library) or how
>>to easily make (e.g. in a C extension) a type without a __mro__, except
>>for those (such as types.InstanceT ype) which are explicitly recorded in
>>the dispatch table copy._deepcopy_ dispatch...?[/color]
>
>
> something like this?
>
>[color=green][color=darkred]
>>>>import re
>>>>x = re.compile("")
>>>>x[/color][/color]
>
> <_sre.SRE_Patte rn object at 0x00B2F7A0>
>[color=green][color=darkred]
>>>>x.__mro__[/color][/color]
>
> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> AttributeError: __mro__
>[color=green][color=darkred]
>>>>import copy
>>>>type(x) in copy._deepcopy_ dispatch[/color][/color]
>
> False
>
> </F>[/color]

Unfortunately, it *does* have a __deepcopy__ method, so I don't think it
actually triggers the bug Alex is interested in:

Py> import re
Py> x = re.compile("")
Py> x.__mro__
Traceback (most recent call last):
File "<stdin>", line 1, in ?
AttributeError: __mro__
Py> from copy import deepcopy
Py> deepcopy(x)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "C:\Python24\li b\copy.py", line 172, in deepcopy
y = copier(memo)
TypeError: cannot deepcopy this pattern object
Py> x.__deepcopy__
<built-in method __deepcopy__ of _sre.SRE_Patter n object at 0x00B242C0>

Cheers,
Nick.

--
Nick Coghlan | ncoghlan@email. com | Brisbane, Australia
---------------------------------------------------------------

http://boredomandlaziness.skystorm.net

[Alex Martelli]

Re: a type without a __mro__?

Fredrik Lundh <fredrik@python ware.com> wrote:
[color=blue]
> Alex Martelli wrote:
>[color=green]
> > Can anybody suggest where to find (within the standard library) or how
> > to easily make (e.g. in a C extension) a type without a __mro__, except[/color][/color]
^^^^^^[color=blue][color=green]
> > for those (such as types.InstanceT ype) which are explicitly recorded in
> > the dispatch table copy._deepcopy_ dispatch...?[/color]
>
> something like this?
>[color=green][color=darkred]
> >>> import re
> >>> x = re.compile("")
> >>> x[/color][/color]
> <_sre.SRE_Patte rn object at 0x00B2F7A0>[color=green][color=darkred]
> >>> x.__mro__[/color][/color]
> Traceback (most recent call last):
> File "<stdin>", line 1, in ?
> AttributeError: __mro__[/color]

Alas, no -- wish it were so easy! It does need to be a TYPE, not just
any object, to reproduce the bug that was reported about 2.3.5c1's
copy.py. The relevant code in copy.py does the equivalent of:
[color=blue][color=green][color=darkred]
>>> type(x).__mro__[/color][/color][/color]
(<type '_sre.SRE_Patte rn'>, <type 'object'>)[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

not of just x.__mro__, which would easily be made to fail.

How a type(whatever) can end up without a __mro__ in 2.3.* is rather
murky to me; looks like something strange must be happening wrt the
type's flags, or something. Normal types such as _sre.SRE_Patter n, or
the Copyable type I put in _testcapi, just sprout an __mro__ without
even trying. Ah well, thanks anyway -- guess I'll commit the fix (using
inspect.getmro( cls) rather than cls.__mro__) even though I don't have a
unit test to show it's necessary and sufficient:-(


Alex

[John Lenton]

Re: a type without a __mro__?

On Sat, Feb 05, 2005 at 11:37:10AM +0100, Alex Martelli wrote:[color=blue]
> Can anybody suggest where to find (within the standard library) or how
> to easily make (e.g. in a C extension) a type without a __mro__, except
> for those (such as types.InstanceT ype) which are explicitly recorded in
> the dispatch table copy._deepcopy_ dispatch...?[/color]

would this do what you need?

class C(type):
def __getattribute_ _(self, attr):
if attr == '__mro__':
raise AttributeError, "What, *me*, a __mro__? Nevah!"
return super(C, self).__getattr ibute__(attr)

class D(object):
__metaclass__ = C

instances of D have a type that behaves as if it didn't have a
__mro__. This isn't exactly what you asked for, but it might be
enough.

--
John Lenton (john@grulic.or g.ar) -- Random fortune:
El tiempo cura los dolores y las querellas porque cambiamos. Ya no somos la
misma persona.
-- Blaise Pascal. (1600-1662) Filósofo y escritor francés.

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[Alex Martelli]

Re: a type without a __mro__?

John Lenton <john@grulic.or g.ar> wrote:
[color=blue]
> class C(type):
> def __getattribute_ _(self, attr):
> if attr == '__mro__':
> raise AttributeError, "What, *me*, a __mro__? Nevah!"
> return super(C, self).__getattr ibute__(attr)
>
> class D(object):
> __metaclass__ = C[/color]

Yay -- *exactly*!!! This simple trick reproduces the problem AND shows
that using inspect.getmro fixes it. EXACTLY what we needed. Thanks!!!
Now I can make a proper patch with unittest as well as the fix.


Alex