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Python 3 Instance Attributes as Methods

From: "andrew cooke" <andrew@...>

Date: Sun, 8 Feb 2009 19:40:35 -0300 (CLST)

Python can be quite frustrating/opaque when you try to be "really"
dynamic.  The handling of instance (rather than class) attributes as
methods seems to be particularly poorly documented (there are a lot of
exceptions that make the case hard - instance attributes cannot be
descriptors, for example).

Anyway, it turns out that you can create a method using the MethodType()
constructor (in the types module), and that the (undocumented) arguments
are the instance and the function.

So:

  foo.instance_function = MethodType(foo, my_function)

And here's a little test:

class InstanceMethodTest(TestCase):

    class Foo():
        class_attribute = 1
        def __init__(self):
            self.instance_attribute = 2
        def bar(self):
            return (self.class_attribute,
                    self.instance_attribute,
                    hasattr(self, 'baz'))

    def test_method(self):
        foo = self.Foo()
        assert foo.bar() == (1, 2, False)
        def my_baz(myself):
             return (myself.class_attribute,
                     myself.instance_attribute,
                     hasattr(myself, 'baz'))
        foo.baz = MethodType(my_baz, foo)
        assert foo.baz() == (1, 2, True)
        assert foo.bar() == (1, 2, True)


And finally here's similar code for earlier Python:
http://code.activestate.com/recipes/223613/

Andrew

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