[Python-Dev] Getting rid of unbound methods: patch available

[Nick Coghlan]

M.-A. Lemburg wrote:
> Nick Coghlan wrote:
> 
>> Guido van Rossum wrote:
>>
>>> What do people think? (My main motivation for this, as stated before,
>>> is that it adds complexity without much benefit.)
>>
>>
>>
>> I'm in favour, since it removes the "an unbound method is almost like 
>> a bare function, only not quite as useful" distinction. It would allow 
>> things like str.join(sep, seq) to work correctly for a Unicode separator. 
> 
> 
> This won't work. Strings and Unicode are two different types,
> not subclasses of one another.

My comment was based on misremembering how str.join actually works. It 
automatically flips to Unicode when it finds a Unicode string in the sequence - 
however, it doesn't do that for the separator, since that should already have 
been determined to be a string by the method lookup machinery.

However, looking at the code for string_join suggests another possible issue 
with removing unbound methods. The function doesn't check the type of the first 
argument - it just assumes it is a PyStringObject.

PyString_Join adds the typecheck that is normally performed by the method 
wrapper when str.join is invoked from Python.

The issue is that, if the unbound method wrapper is removed, 
str.join(some_unicode_str, seq) will lead to PyString_AS_STRING being invoked on 
a PyUnicode object. Ditto for getting the arguments out of order, as in 
str.join(seq, separator). At the moment, the unbound method wrapper takes care 
of raising a TypeError in both of these cases. Without it, we get an unsafe 
PyString macro being applied to an arbitrary type.

I wonder how many other C methods make the same assumption, and skip type 
checking on the 'self' argument? It certainly seems to be the standard approach 
in stringobject.c.

Regards,
Nick.

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