Why Doesn\'t Overriding `__repr__` on an Instance Work in Python, and How Can It Be Effectively Achieved?

Overriding Special Methods on an Instance

Consider the code snippet below:

class A(object):
    pass

def __repr__(self):
    return "A"

from types import MethodType
a = A()

print(a) # <__main__.A object at 0x00AC6990>
print(repr(a)) # '<__main__.A object at 0x00AC6990>'

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

print(a) # <__main__.A object at 0x00AC6990>
print(repr(a)) # '<__main__.A object at 0x00AC6990>'

As observed, the custom __repr__ method is not called on the instance a. To understand this behavior, it's essential to note that:

In CPython (the standard Python interpreter), special methods with names surrounded by __ are typically not invoked on instances but rather on classes. As a result, attempting to override __repr__ directly on an instance will not yield the expected outcome.

To work around this limitation, one can leverage the following strategy:

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

Here, __repr__ calls a helper method _repr, which then calls the default __repr__ implementation for the object class. By overriding _repr, we can effectively override __repr__ on an instance.

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