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

Guido van Rossum guido at python.org
Thu Nov 22 00:18:47 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


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):
  def foo(self):

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

c = C()


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

More information about the Python-Dev mailing list