Why Do Python Methods Behave Differently in Reference Equality Compared to Functions?

Unveiling the Enigma: Method Reference Equality in Python

In the realm of Python programming, the concept of reference equality can be both alluring and perplexing. Surprisingly, methods, unlike regular functions, exhibit a unique behavior when it comes to reference equality.

Why this striking difference? The answer lies in the nature of how methods are created and invoked. Unlike functions, which exist independently in the global scope, methods are dynamically generated each time they are accessed. This stems from the role of functions as descriptors in Python. When a function's .__get__ method is called, a method object is returned.

Consider the following example:

<code class="python">class What:
    def meth(self):
        pass

print(What.meth is What.meth)  # This is False in Python 2
print(inst.meth is inst.meth)  # False</code>

The above code will print False for both scenarios. This is because each access to What.meth results in a new method object being created. Therefore, these method objects cannot be considered reference-equal.

In contrast, regular functions are global entities, so when func is called, it always refers to the same object. This explains the True result in func is func.

However, Python 3.8 introduced a significant improvement. Now, two methods are considered equal if their .__self__ and .__func__ attributes are identical objects. This means that if two methods wrap the same function and are bound to the same instance, they will compare True using ==.

For Python versions prior to 3.8, method == behavior was inconsistent. Python methods and one type of C method compared self for equality, while another type of C method compared self by identity.

To test if two methods represent the same underlying function, it's recommended to compare their func attributes:

<code class="python">print(What.meth == What.meth)     # functions (or unbound methods in Python 2)
print(What().meth == What().meth) # bound methods with *different* instances
print(What().meth.__func__ == What().meth.__func__) # functions</code>

The above is the detailed content of Why Do Python Methods Behave Differently in Reference Equality Compared to Functions?. For more information, please follow other related articles on the PHP Chinese website!