Why Does `is` Fail to Compare Methods in Python, But Works for Functions?

Method Equality in Python: Unraveling the Reference Inequality Enigma

In Python, methods and functions exhibit a curious difference: while functions exhibit reference equality, methods do not. This can lead to unexpected bugs, particularly when relying on the is operator to compare methods.

Why Reference Equality Eludes Methods

Unlike functions, which are static entities, method objects are created dynamically each time they are accessed. This is because methods act as descriptors, returning a new method object when their .__get__ method is invoked. For instance, in the example provided:

class What:
    def meth(self):
        pass

The assignment What.meth returns a function object, which is different from the bound method object created later when inst.meth is called. This explains why What.meth is What.meth evaluates to False.

Is Equality in Python 3.8

In Python versions 3.8 and later, an alternative approach is available: using == for equality testing. Two methods are considered equal if their .__self__ and .__func__ attributes are identical objects, indicating that they wrap the same function and are bound to the same instance.

For Functions __and Unbound Methods

Regardless of Python version, functions and unbound methods (in Python 2) can be compared using the is operator, as they both represent the same underlying function reference.

Test for Underlying Function

If the requirement is to check if two methods represent the same underlying function, the .__func__ attribute can be compared:

if What().meth.__func__ == What().meth.__func__:
    print("Methods represent the same function")

This approach provides consistent and reliable equality checks for methods and functions, regardless of their binding status or Python version.

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