Making special method names, work with __getattr__

Antoon Pardon apardon at forel.vub.ac.be
Mon Apr 26 12:57:58 CEST 2010


Op 2010-04-23, Chris Rebert schreef <clp2 at rebertia.com>:
> On Fri, Apr 23, 2010 at 2:41 AM, Antoon Pardon <apardon at forel.vub.ac.be> wrote:
>> test()
>>
>> --------------------------------------------------------------------------
>>
>> The result I get is:
>>
>> 5
>> 8
>> 15
>> Traceback (most recent call last):
>>  File "Symbolics", line 54, in <module>
>>    test()
>>  File "Symbolics", line 51, in test
>>    product = val1 * 7
>> TypeError: unsupported operand type(s) for *: 'Symbol' and 'int'
>>
>> What I had hoped for was, that the line:
>>
>>  product = val1 * 7
>>
>> would be translated into something like
>>
>>  product = val1.__mul__(7)
>
> That's basically correct.
>
>> which would then be treated by the __getattr__ of the Expression superclass.
>>
>> That doesn't seem to happen.
>
> Indeed it doesn't. The lookup of fouble-underscore special methods
> bypasses __getattribute__() and friends. For details, see
> http://docs.python.org/reference/datamodel.html#special-method-lookup-for-new-style-classes

This doesn't seem to be the whole picture.

If I replace the line:

  product = val1 * 7

with

  product = val1.__mul__(7)


The code works.

So the problem is not that the lookup for special methods is different.
It seems that the lookup for special methods differ depending on wether they are
called implicitly or explicitly.

-- 
Antoon Pardon



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