[Python-Dev] [python] Should we do away with unbound methods in Py3k?

Thomas Lee tom at vector-seven.com
Thu Nov 22 01:07:12 CET 2007

Michael Foord wrote:
> Guido van Rossum wrote:
>> I'm asking a Py3k question on python-dev because I'd like to have
>> opinions from people who haven't thought about Py3k much yet. Consider
>> the following example:
>>   class C:
>>       def foo(self): pass
>>   C.foo(42)
>> This currently fails with this error message:
>>   TypeError: unbound method foo() must be called with C instance as
>> first argument (got int instance instead)
>> This message is called when isinstance(self, C) returns False, where
>> self is the first argument passed to the unbound method.
>> That's nice, but there is a cost associated with this: the expression
>> "C.foo" can't just return the function object "foo", it has to wrap it
>> in an unbound method object. In Py3k the cost of calling an unbound
>> method object goes up, because the isinstance() check may be
>> overloaded. This typically happens when the class C uses the special
>> metaclass (abc.ABCMeta) used for virtual inheritance (see PEP 3119).
>> in Py3k the I/O stream classes are perhaps the most common use case.
>> Given that the error is of limited value and that otherwise the
>> unbound method behaves exactly the same as the original function
>> object, I'd like to see if there are strenuous objections against
>> dropping unbound method objects altogether (or at least not using them
>> in this case), so that explicit super calls (via the unbound method)
>> may go a little faster. Also, it would make it easier to fix this
>> issue: http://bugs.python.org/issue1109
> On occasions I've found it a drag that you *can't* call unbound methods 
> with a different type. Python normally allows duck typing and this is 
> one place it actually places type restrictions...
> I'd be happy to see this restriction go. :-)
> Michael Foord
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+1 to getting rid of unbound methods!

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