Why Doesn\'t Instance Overriding Work for Special Methods Like `__repr__` in Python?

Understanding Instance Overrides for Special Methods

Consider the code below:

class A(object):
    pass

def __repr__(self):
    return "A"

from types import MethodType
a = A()
a

Surprisingly, repr(a) does not yield the intended result "A". Why is this, and can we rectify this behavior?

In Python, special methods (those with names surrounded by "__") are typically invoked on the class, rather than on the instance. This is an implementation detail of CPython, the standard interpreter. Therefore, there is no direct way to override __repr__() on an instance and expect it to work.

However, there is an indirect solution:

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

Now, to override __repr__() for an instance, simply replace it with _repr():

setattr(a, "__repr__", MethodType(_repr, a, a.__class__))

This approach proves effective because _repr() calls the original __repr__() method, which is implemented on the class level.

In contrast, consider this example, where we attempt to override a non-special method:

class A(object):
    def foo(self):
        return "foo"

def bar(self):
    return "bar"

a = A()
setattr(a, "foo", MethodType(bar, a, a.__class__))

In this case, the override works as expected, as foo() is a non-special method and is therefore called directly on the instance.

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