Python: Binding method

Question

In following example I am trying to bind a method object via types.MethodType(...). It does not seem to work. Any suggestions? Thanks in advance.

import types
class Base:
 def payload(self, *args): 
 print "Base:payload"
class Drvd(Base):
 def iter(self, func):
 derived_func = types.MethodType(func, self, Drvd) # bind
 print "drvd_func:", derived_func 
 derived_func() # result: calls Base:payload 
 # expected:calls Drvd:payload; why???
 def payload(self, *args):
 print "Drvd:payload"
derived = Drvd() 
base_func = Base.payload 
print "base_func:", base_func
derived.iter(base_func) # pass unbound method object

The output shows:

base_func: <unbound method Base.payload>
drvd_func: <bound method Drvd.payload of <main.Drvd instance at 0x00B51648>>
Base:payload

• S.Lott: Since the subclass inherits the superclass function, why are you doing this? What's the point?
• pi.: Alex has pointed out, that the original unbound method Base.payload cannot be converted to another classes member function with the same name. Even with inherited classes. I am currently setting up a tree iteration. Looks like I should use a Visitor/Multimethod approach or have IDs per payload handler with a dispatcher per node. It will be used to store XML output from a Sax parser-
• S.Lott: Use Visitor whenever possible. Avoid magic.

Answer 1

You're specifically requesting the use of the underlying function (im_func) of Base.payload, with a fake im_class of Drvd. Add after the existing print in iter:

 print "w/class:", derived_func.im_class
 print "w/func:", derived_func.im_func

and you'll see the total output as:

$ python bou.py 
base_func: <unbound method Base.payload>
drvd_func: <bound method Drvd.payload of <__main__.Drvd instance at 0x24a918>>
w/class: __main__.Drvd
w/func: <unbound method Base.payload>
Base:payload

i.e., as you've asked, the underlying code being used is indeed Base.payload -- which is also what you observe in the call.

From Base.payload there is no direct way to get to Drvd.payload except by getting the name and using it to do a getattr on self (a Drvd instance), or equivalent.

• pi.: Thanks for your clarification. The name of the passed unbound function is indeed the only link amongst Base and Drvd implementations.

Answer 2

Because you passed Base's payload (not Drvd's) to iter!

Hint:

print "%s: Base:payload"%repr(self)

Kudos for creating an accursed chimera of a thing, though!

Answer 3

I don't really understand what you're trying to do - but to me it's working as I'd expect. You are binding Base.payload to Drvd - from your print out you can see it is binding correctly to your Drvd instance. I'm not sure why you are then expecting it to call Drvd.payload - you have bound the function object Base.payload. It is always going to be that function.

[edit: just to add - it might be helpful if you say what your trying to do in general terms. What you're doing at the moment doesn't make much sense - Python has inheritence so if you want to override the payload method you can just do it - you don't need to do any manual binding.]

• pi.: I am trying to reduce coding effort on tarversing a treelike structure of nodes compared to a visitor-pattern or multimethod approach. It seems to become a hack though on bending pythons underlying method signatures.