Overriding a method at the instance level on a subclass of a builtin type

George Sakkis george.sakkis at gmail.com
Thu Dec 4 18:57:20 CET 2008

On Dec 4, 12:31 pm, Arnaud Delobelle <arno... at googlemail.com> wrote:
> "Zac Burns" <zac... at gmail.com> writes:
> > The class method seems to be the most promising, however I have more
> > 'state' methods to worry about so I might end up building new classes
> > on the fly rather than have a class per permutation of states! Now the
> > code isn't quite as clear as I thought it was going to be.
> > It seems unfortunate to me that methods are always looked up on the
> > class for new style objects. Was this done for speed reasons?
> It's only special methods such as __getitem__, ...
> You can override normal method on a per-object basis just by adding a
> callable attribute with its name to the object:
> >>> class A(object):
> ...     def foo(self): print 'A.foo'
> ...>>> a = A()
> >>> a.foo()
> A.foo
> >>> def a_foo(): print 'a.foo'
> ...
> >>> a.foo = a_foo
> >>> a.foo()

Note that the overriden "method" here is a plain function; it doesn't
take self as the first argument. If you want to bind it to a callable
that expects the first argument to be self, you have to bind
explicitly self to the object:

>>> def a_foo(self): print 'a.foo'
>>> a.foo = a_foo
>>> a.foo()
TypeError: a_foo() takes exactly 1 argument (0 given)
>>> from functools import partial
>>> a.foo = partial(a_foo,a)
>>> a.foo()


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