Thursday, May 29. 2008

Python Brain-Teaser

I'm working on pulling some functionality out of one object and putting it in another, and I came across this interesting problem:

class Foo:

me = "foo"

class Bar:

me = "bar"

def get_me(self):

return self.me

Foo.get_me = Bar.get_me

x = Foo()

print x.get_me()

What does this print?

And, next question, why is that?

After lunch I'll post my thoughts

Until now, I had assumed that the semi-magic self variable was set on method calls (ie, when x.get_me() is called)... But apparently it's set when the object is instantiated (which makes perfect sense, otherwise getattr(Bar(), 'get_me')() would not work).

So I can only presume that something equivalent to this happens when an object is instantiated:

from functools import partial
new_obj = Class()
for (key, val) in new_obj.__dict__.items():
    if not callable(val): continue
    setattr(new_obj, key, partial(val, new_obj))

Posted by David Wolever in Play, Python at 11:14 | Comments (3) | Trackbacks (0)

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x.get_me.im_class

For me he expects that x is class Bar and not from class Foo : )

a good way to do it is:

def get_me(self): return self.me

class Foo: me = "foo"

class Bar: me = "bar" def get_me(self): return self.me

Foo.get_me = get_me

and maybe

Bar.get_me = get_me

For keep everything the same.

x = Foo()

print x.get_me()

#1 Rafael on 2008-05-30 15:08 (Reply)

Huh????

#2 Johnny (Homepage) on 2008-07-10 17:32 (Reply)

The error produced by this code comes from the differences between bound methods, unbound methods and functions.

Bar.get_me is an instancemethod; such objects know the class they’re defined in. You cannot assign a method from a class to another: A method knows which objets it accepts, and since Foo is not a subclass of Bar in your example, Bar.get_me can’t work with Foo instances.

However, you can dynamically assign methods to a class, using function objects:

>>> Foo.get_me = Bar.get_me.im_func >>> Foo().get_me() 'foo'

The “self” name is not magic at all, you understood right. “getattr(Bar(), 'get_me')()” (the same thing as “Bar().get_me()”, really) works because “thing.method()” is equivalent to “ThingClass.method(thing)”.

Hope this helps

Cheers

#3 Merwok on 2010-01-21 23:58 (Reply)

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