How Can I Override Special Methods on a Python Instance?

Overriding a Special Method on an Instance

In Python, a common goal for developers is to override special methods on an instance. However, this is not typically possible as special methods are typically only called on the class itself.

Problem:

To illustrate this concept, consider the following code snippet:

class A(object):
    pass

def __repr__(self):
    return "A"

from types import MethodType
a = A()
a
repr(a)
setattr(a, "__repr__", MethodType(__repr__, a, a.__class__))
a
repr(a)

As seen above, overriding __repr__ directly on the instance doesn't yield the expected result. repr(a) returns the default class representation, not the overridden method's return value.

Solution:

To achieve overriding special methods on an instance, you can employ an indirect approach. Define the special method as a wrapper that calls an internal, overridden method:

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 overwriting _repr(). For example:

a = A()
setattr(a, "_repr", lambda self: "Overridden")
repr(a)

This updated code will output the overridden string representation.

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