Overriding special methods on an instance

Question

I hope someone can answer this that has a good deep understanding of Python :)

Consider the following code:

>>> class A(object): ...     pass ... >>> def __repr__(self): ...     return "A" ... >>> from types import MethodType >>> a = A() >>> a <__main__.A object at 0x00AC6990> >>> repr(a) '<__main__.A object at 0x00AC6990>' >>> setattr(a, "__repr__", MethodType(__repr__, a, a.__class__)) >>> a <__main__.A object at 0x00AC6990> >>> repr(a) '<__main__.A object at 0x00AC6990>' >>> 

Notice how repr(a) does not yield the expected result of "A" ? I want to know why this is the case and if there is a way to achieve this...

I contrast, the following example works however (Maybe because we're not trying to override a special method?):

>>> class A(object): ...     def foo(self): ...             return "foo" ... >>> def bar(self): ...     return "bar" ... >>> from types import MethodType >>> a = A() >>> a.foo() 'foo' >>> setattr(a, "foo", MethodType(bar, a, a.__class__)) >>> a.foo() 'bar' >>> 

Answer 1

Python usually doesn't call the special methods, those with name surrounded by __ on the instance, but only on the class. (Although this is an implementation detail, it's characteristic of CPython, the standard interpreter.) So there's no way to override __repr__() directly on an instance and make it work. Instead, you need to do something like so:

class A(object):     def __repr__(self):         return self._repr()     def _repr(self):         return object.__repr__(self) 

Now you can override __repr__() on an instance by substituting _repr().