Entry

Changing instance methods

Jul 5th, 2000 10:00
Nathan Wallace, Hans Nowak, Snippet 106, Tim Peters

"""
Packages: oop
"""
"""
> I don't understand how to change instance methods.
That's good, because Python doesn't have such a thing <wink>.  Direct
attributes of instances are never methods without extreme trickery.  A
function becomes a method by virtue of being found in an instance's
*class's* dict.  Python was designed to allow overriding methods at the
class level, but not at the instance level.
> For example:
"""
# This part copied here for later use -- PSST
class Foo:
    def __init__(self):
        self.data = 42
    def m(self):
        print "Foo.m"
        print dir(self)
def m2(self):
    print "m2"
    print dir(self)
f = Foo()
f.m()
"""
> # this fails
> # f.m = m2
> # f.m()
Right, a direct attribute of an instance is never a method.  Except that
this "works":
"""
import new
f.m = new.instancemethod(m2, f, Foo)
f.m()
"""
This sets f.m to a *bound* instance method that refers to f, which Python
treats as an ordinary function when later referenced via f.m.  Without
using the "new" module, you can get the same effect via e.g.
"""
old_Foo_m = Foo.m
Foo.m = m2
f.m = f.m  # heh heh <wink>
Foo.m = old_Foo_m
"""
In either case, though, the value of f.m now contains a hidden reference
to f, so you've created a piece of immortal cyclic trash.
subclassing-is-a-lot-easier-ly y'rs  - tim
"""