Python Language Creating user-defined method objects


Example

User-defined method objects may be created when getting an attribute of a class (perhaps via an instance of that class), if that attribute is a user-defined function object, an unbound user-defined method object, or a class method object.

class A(object):
    # func: A user-defined function object
    #
    # Note that func is a function object when it's defined,
    # and an unbound method object when it's retrieved.
    def func(self): 
        pass

    # classMethod: A class method
    @classmethod
    def classMethod(self):
        pass

class B(object):
    # unboundMeth: A unbound user-defined method object
    #
    # Parent.func is an unbound user-defined method object here,
    # because it's retrieved.
    unboundMeth = A.func

a = A()
b = B()

print A.func
# output: <unbound method A.func>
print a.func
# output: <bound method A.func of <__main__.A object at 0x10e9ab910>>
print B.unboundMeth
# output: <unbound method A.func>
print b.unboundMeth
# output: <unbound method A.func>
print A.classMethod
# output: <bound method type.classMethod of <class '__main__.A'>>
print a.classMethod
# output: <bound method type.classMethod of <class '__main__.A'>>

When the attribute is a user-defined method object, a new method object is only created if the class from which it is being retrieved is the same as, or a derived class of, the class stored in the original method object; otherwise, the original method object is used as it is.

# Parent: The class stored in the original method object
class Parent(object):
    # func: The underlying function of original method object
    def func(self): 
        pass
    func2 = func

# Child: A derived class of Parent
class Child(Parent):
    func = Parent.func

# AnotherClass: A different class, neither subclasses nor subclassed
class AnotherClass(object):
    func = Parent.func
    
print Parent.func is Parent.func                # False, new object created
print Parent.func2 is Parent.func2              # False, new object created
print Child.func is Child.func                  # False, new object created
print AnotherClass.func is AnotherClass.func    # True, original object used