Why is 'is None' preferred over '== None' in Python?

Understanding Python None Comparison: "is" vs. ==

Python developers often encounter the question of whether to use "is" or == when comparing a variable to None. While both options result in valid syntax, the preferred approach depends on the intended comparison.

Object Identity vs. Equality

The key distinction lies in the nature of the comparison: object identity or equality.

• Object identity (is): Compares whether two variables refer to the exact same object in memory.
• Equality (==): Compares the values of two objects.

In the case of None, there is only one such object in Python. Thus, my_var is None checks if my_var is referencing the same None object.

Why is is None Preferred?

While both is None and == None are valid syntax, is None is considered more explicit and less prone to errors.

Consider the following situation:

class Negator(object):
    def __eq__(self, other):
        return not other

thing = Negator()
print(thing == None)  # True
print(thing is None)  # False

In this example, the Negator class overrides the == operator to always return the opposite of the argument. As a result, thing == None evaluates to True, indicating equality in value. However, thing is None correctly evaluates to False, indicating that thing and None are not the same object.

Conclusion

For checking object identity, including comparisons to None, is is the preferred approach. It ensures clarity and prevents potential ambiguity caused by overridden equality operators. Remember, is checks for identity, while == checks for equality.

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