Why Can\'t I Override Special Methods Directly on Python Instances?

Overriding Special Methods on an Instance: An Exploration

Imagine a situation where you have an instance of a class and you desire to modify its special methods, such as __repr__(). Upon attempting to override these methods, you may encounter unexpected behavior.

To illustrate this, consider the following code:

class A(object):
    pass

def __repr__(self):
    return "A"

from types import MethodType
a = A()

When calling repr(a), you expect to obtain "A," given that repr has been overridden. However, the result is not as anticipated.

This discrepancy stems from Python's approach to invoking special methods. Typically, Python executes special methods only on the class itself, not on its instances. Consequently, overwriting repr directly on an instance is ineffective.

To overcome this limitation, consider the following approach:

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

In this modified code, repr on an instance can be overwritten by substituting _repr().

Contrasting this behavior with the following code snippet exemplifies the difference between overriding special methods and regular methods:

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 instance, the foo special method was successfully overridden on the instance. However, it is crucial to note that this ability is exclusive to regular methods.

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