A class method which behaves differently when called as an instance method?

Question

I'm wondering if it's possible to make a method which behaves differently when called as a class method than when called as an instance method.

For example, as a skills-improvement project, I'm writing a Matrix class (yes, I know there are perfectly good matrix classes already out there). I've created a class method for it called identity which returns an identity matrix of a specified size.

Now, when called on an instance of Matrix, it seems logical that the size shouldn't need to be specified; it should return an identity matrix of the same size as the Matrix it's called on.

In other words, I'd like to define a method which can determine whether it was called via an instance and, if so, access that instance's attributes. Unfortunately, even after digging through the documentation and a few Google searches, I haven't found anything which suggests this is possible. Does anyone know differently?

Edit:

Wow! Clearly, I'm still not quite used to first-class functions. Here's what I ended up with — thanks to Unknown for providing the key!

class Foo(object):     def __init__(self, bar):         self.baz = bar         self.bar = MethodType(lambda self: self.__class__.bar(self.baz), self, self.__class__)      @classmethod     def bar(cls, baz):         return 5 * baz  Foo.bar(3) # returns 15  foo = Foo(7) foo.bar() # returns 35 

Edit 2:

Just a quick note — this technique (and most of those presented below) won't work on classes which define __slots__, as you cannot reassign the method.

Answer 1

Questionably useful Python hacks are my forte.

from types import *  class Foo(object):     def __init__(self):         self.bar = methodize(bar, self)         self.baz = 999      @classmethod     def bar(cls, baz):         return 2 * baz   def methodize(func, instance):     return MethodType(func, instance, instance.__class__)  def bar(self):     return 4*self.baz   >>> Foo.bar(5) 10 >>> a=Foo() >>> a.bar() 3996