Why can't you pickle instancemethods?

[mdsteele at gmail.com]

Chris wrote:
> Why can pickle serialize references to functions, but not methods?
>
> Pickling a function serializes the function name, but pickling a
> staticmethod, classmethod, or instancemethod generates an error. In
> these cases, pickle knows the instance or class, and the method, so
> what's the problem? Pickle doesn't serialize code objects, so why can't
> it serialize the name as it does for functions? Is this one of those
> features that's feasible, but not useful, so no one's ever gotten
> around to implementing it?

I have often wondered this myself.  I'm convinced that it would in fact
be useful -- more than once I've written a program that has lots of
objects with function pointers, and where it was inconvenient that the
method pointers could not be pickled.  One compromise that I have used
before is to write a class such as:

class InstanceMethodSet(object):
    def __init__(self,methods):
        self.methods = set(methods)
    def __getstate__(self):
        return [(method.im_self, method.im_func.func_name)
                for method in self.method]
    def __setstate__(self,state):
        self.methods = set(getattr(obj,name) for obj,name in state)

Obviously, this particular example is crude and not terribly robust,
but it seems to do the job -- it effectively lets you pickle a set of
instance method pointers.  I don't know of any reason why instance
methods (or class or static methods) couldn't be pickled directly,
unless perhaps there exists some kind of pathological corner case that
would create Badness?

-Matt