[Python-Dev] Should we do away with unbound methods in Py3k? (original) (raw)

Guido van Rossum guido at python.org
Thu Nov 22 00🔞47 CET 2007

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

To illustrate the cost of the isinstance() overloading, step through this simple program with Py3k:

import abc, pdb

class Base(metaclass=abc.ABCMeta): @abc.abstractmethod def foo(self): pass

class C(Base): def foo(self): Base.foo(self)

c = C()

pdb.run("c.foo()")

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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