Python

This is fun to play around and basically what Python does under the hood to implement classes. In Python2 it was even more obvious that classes are just fancy wrappers around a dict called, unsurprisingly, __dict__.

class Foo: 
    def __init__(self):
        self.__dict__["instance_method"] = lambda: "instance_method"
        self.__dict__["shadowed_class_method"] = lambda: "shadowed_class_method_from_instance"
        

Foo.__dict__["class_method"] = lambda cls: "class_method"
Foo.__dict__["shadowed_class_method"] = lambda cls: "shadowed_class_method_from_class"

f = Foo()
f.__dict__["dynamic_instance_method"] = lambda: "dynamic_instance_method"

print f.instance_method()
print f.dynamic_instance_method()
print f.class_method()
print f.shadowed_class_method()

OUTPUT:
instance_method
dynamic_instance_method
class_method
shadowed_class_method_from_instance

Note: this won't work in Python3 because the class.__dict__ becomes immutable at some point after declaring it, but the attribute name resolution stays the same. And it gets more interesting once you throw inheritance into the mix.